Efficacy of typhoid conjugate vaccine in Nepal: final results of a phase 3, randomised, controlled trial

Safety and immunogenicity of conjugate vaccine for typhoid (Vi-DT): Finding from an observer-blind, active-controlled, randomized, non-inferiority, phase III clinical trial among healthy volunteers

Typhoid fever is a significant public health concern with most of the sufferers between 15 and 25 y of age in Nepal. We undertook this study to demonstrate Vi polysaccharide conjugated with diphtheria toxoid (Vi-DT) conjugate vaccine which is non-inferior to Typbar typhoid conjugate vaccine, a Vi polysaccharide vaccine conjugated with tetanus toxoid (Vi-TT) with a focus on the adult population from Dhulikhel Hospital which was one of the total four sites in Nepal. In this study, we assigned the eligible participants in 1:1:1:1 ratio by block randomization, and stratified into three age groups (6 months to less than 2 y, 2 y to less than 18 y, and 18 y to 45 y), allotted to Group A, B, C, and D. Group A, B, and C received 25 μg (0.5 mL) of Vi-DT study vaccine and participants in Group D received 25 μg (0.5 mL) Vi-TT vaccine. We descriptively analyzed safety in all the participants receiving one dose of the investigational vaccine. The anti-Vi-IgG seroconversion rate in Vi-DT recipients was 99.71% (97.5% CI 98.04-99.96; 344 of 345 participants) and 99.13% (94.27-99.87; 114 of 115) in Vi-TT recipients which indicates that Vi-DT vaccine is non-inferior to Vi-TT vaccine. In safety aspect, 16.81% of total subject had at least one solicited adverse reaction and 22.61% of the Vi-TT participants experienced at least one solicited adverse reaction with most of them being local adverse reactions. None of the enrolled participants reported serious adverse events. Our study shows that a single dose of the Vi-DT vaccine is immunogenic, safe to administer and non-inferior to the Vi-TT vaccine four weeks after vaccination.

Environmental Surveillance for Salmonella Typhi and its Association With Typhoid Fever Incidence in India and Malawi

BACKGROUND

Environmental surveillance (ES) for Salmonella Typhi potentially offers a low-cost tool to identify communities with a high burden of typhoid fever.

METHODS

We developed standardized protocols for typhoid ES, including sampling site selection, validation, characterization; grab or trap sample collection, concentration; and quantitative PCR targeting Salmonella genes (ttr, staG, and tviB) and a marker of human fecal contamination (HF183). ES was implemented over 12 months in a historically high typhoid fever incidence setting (Vellore, India) and a lower incidence setting (Blantyre, Malawi) during 2021-2022.

RESULTS

S. Typhi prevalence in ES samples was higher in Vellore compared with Blantyre; 39/520 (7.5%; 95% confidence interval [CI], 4.4%-12.4%) vs 11/533 (2.1%; 95% CI, 1.1%-4.0%) in grab and 79/517 (15.3%; 95% CI, 9.8%-23.0%) vs 23/594 (3.9%; 95% CI, 1.9%-7.9%) in trap samples. Detection was clustered by ES site and correlated with site catchment population in Vellore but not Blantyre. Incidence of culture-confirmed typhoid in local hospitals was low during the study and zero some months in Vellore despite S. Typhi detection in ES.

CONCLUSIONS

ES describes the prevalence and distribution of S. Typhi even in the absence of typhoid cases and could inform vaccine introduction. Expanded implementation and comparison with clinical and serological surveillance will further establish its public health utility.

Safety and immunogenicity of Vi-diphtheria toxoid typhoid conjugate vaccine among children below 2 years: a systematic review and meta-analysis

Background

The Vi-diphtheria toxoid typhoid conjugate vaccine (Vi-DT) has shown promising results in preventing typhoid fever in children under 2 years of age. However, a thorough assessment of its safety and immunogenicity is required to inform vaccination strategies. This systematic review and meta-analysis aimed to determine the safety and immunogenicity of Vi-DT in children below 2 years.

Methods

We systematically searched multiple databases, including PubMed, Web of Science, and Scopus, for relevant studies published up to September 2023. We included studies reporting on the safety and immunogenicity outcomes of Vi-DT compared to the control or Vi-tetanus toxoid conjugated vaccine (Vi-TT) in children below 2 years. We applied a random-effects model for meta-analysis using RevMan 5.4. We expressed the results as risk ratio (RR) with a 95% confidence interval (95%CI).

Results

In this analysis, five studies were selected, encompassing 1,292 children under 2 years who received the Vi-DT vaccine. No significant difference in immediate reactions was observed within 30 min post-vaccination between Vi-DT and control groups (RR: 0.99 [95% CI: 0.19, 5.26]), nor between Vi-DT and Vi-TT groups. For solicited adverse events within 4 weeks, the VI-DT group showed no significant increase in adverse events compared to control (RR: 0.93 [95% CI: 0.78, 1.12]) or Vi-TT (RR: 0.86 [95% CI: 0.69, 1.07]). Similarly, within 7 days post-vaccination, risk ratios indicated no significant differences in adverse events between the groups. The 4-week seroconversion rate was significantly higher in the Vi-DT group compared to the control (RR: 1.99 [95% CI: 1.07, 3.69]), but no difference was found between Vi-DT and Vi-TT. Adverse events associated with typhoid conjugate vaccines were predominantly non-serious, including fever and injection site reactions. Serious adverse events were rare but included conditions like pneumonia and gastroenteritis.

Conclusion

This meta-analysis highlights Vi-DT safety and immunogenicity in six to 24-month-old children. The findings support the use of this Vi-DT to expand typhoid vaccination in endemic regions, in line with WHO's strategy.

Efficacy of typhoid vaccines against culture-confirmed Salmonella Typhi in typhoid endemic countries: a systematic review and meta-analysis

BACKGROUND

Typhoid is a serious public health threat in many low-income and middle-income countries. Several vaccines for typhoid have been recommended by WHO for typhoid prevention in endemic countries. This study aimed to review the efficacy of typhoid vaccines against culture-confirmed Salmonella enterica serovar Typhi.

METHODS

We searched the Cochrane Central Register of Controlled Trials, MEDLINE, and Embase for studies published in English between Jan 1, 1986 and Nov 2, 2023. We included randomised controlled trials (RCTs) comparing typhoid vaccines with a placebo or another vaccine. This meta-analysis evaluated the efficacy and safety of several typhoid vaccines, including live attenuated oral Ty21a vaccine, Vi capsular polysaccharide (Vi-PS), Vi polysaccharide conjugated to recombinant Pseudomonas aeruginosa exotoxin A vaccine (Vi-rEPA), and Vi-tetanus toxoid conjugate vaccine (TCV). The certainty of evidence for key outcomes was evaluated using Grading of Recommendations, Assessment, Development, and Evaluations methodology. The outcome of interest was typhoid fever confirmed by the isolation of Salmonella enterica serovar Typhi in blood and adverse events following immunisation. This study is registered with PROSPERO (CRD42021241043).

FINDINGS

We included 14 RCTs assessing four different vaccines (Ty21a: four trials; Vi-PS: five trials; Vi-rEPA: one trial; TCV: four trials) involving 585 253 participants. All trials were conducted in typhoid endemic countries and the age of participants ranged from 6 months to 50 years. The pooled efficacy against typhoid fever was 45% (95% CI 33-55%; four trials; 247 649 participants; I2 59%; moderate certainty) for Ty21a and 58% (44-69%; five trials; 214 456 participants; I2 34%; moderate certainty) for polysaccharide Vi-PS. The cumulative efficacy of two doses of Vi-rEPA vaccine at 2 years was 91% (88-96%; one trial; 12 008 participants; moderate certainty). The pooled efficacy of a single shot of TCV at 2 years post-immunisation was 83% (77-87%; four trials; 111 130 participants; I2 0%; moderate certainty). All vaccines were safe, with no serious adverse effects reported in the trials.

INTERPRETATION

The existing data from included trials provide promising results regarding the efficacy and safety of the four recommended typhoid vaccines. TCV and Vi-rEPA were found to have the highest efficacy at 2 years post-immunisation. However, follow-up data for Vi-rEPA are scarce and only TCV is pre-qualified by WHO. Therefore, roll-out of TCV into routine immunisation programmes in typhoid endemic settings is highly recommended.

FUNDING

There was no funding source for this study.

Monophosphoryl Lipid A-based Adjuvant to Promote the Immunogenicity of Multivalent Meningococcal Polysaccharide Conjugate Vaccines

Activation of the adaptive immune system requires the engagement of costimulatory pathways in addition to B and T cell Ag receptor signaling, and adjuvants play a central role in this process. Many Gram-negative bacterial polysaccharide vaccines, including the tetravalent meningococcal conjugate vaccines (MCV4) and typhoid Vi polysaccharide vaccines, do not incorporate adjuvants. The immunogenicity of typhoid vaccines is due to the presence of associated TLR4 ligands in these vaccines. Because the immunogenicity of MCV4 is poor and requires boosters, I hypothesized that TLR4 ligands are absent in MCV4 and that incorporation of a TLR4 ligand-based adjuvant would improve their immunogenicity. Consistent with this hypothesis, two Food and Drug Administration-approved MCV4 vaccines, MENVEO and MenQuadfi, lack TLR4 ligands. Admixing monophosphoryl lipid A, a TLR4 ligand-based adjuvant formulation named "Turbo" with MCV4 induced significantly improved IgM and IgG responses to all four meningococcal serogroup polysaccharides in adult and aged mice after a single immunization. Furthermore, in infant mice, a single booster was sufficient to promote a robust IgG response and 100% seroconversion when MCV4 was adjuvanted with Turbo. Turbo upregulated the expression of the costimulatory molecules CD40 and CD86 on B cells, and Turbo-driven adjuvanticity is lost in mice deficient in CD40 and CD86. These data suggest that Turbo induces the required costimulatory molecules for its adjuvant activity and that incorporation of Turbo could make bacterial polysaccharide vaccines more immunogenic, minimize booster requirements, and be cost-effective, particularly for those individuals in low- and middle-income and disease-endemic countries.

Immune responses to typhoid conjugate vaccine in a two dose schedule among Nepalese children <2 years of age

BACKGROUND

Previously, the Vi-typhoid conjugate vaccine (Vi-TT) was found to be highly efficacious in Nepalese children under 16 years of age. We assessed the immunogenicity of Vi-TT at 9 and 12 months of age and response to a booster dose at 15 months of age.

METHODS

Infants were recruited at Patan Hospital, Kathmandu and received an initial dose of Vi-TT at 9 or 12 months of age with a booster dose at 15 months of age. Blood was taken at four timepoints, and antibody titres were measured using a commercial ELISA kit. The primary study outcome was seroconversion (4-fold rise in antibody titre) of IgG one month after both the doses.

FINDINGS

Fifty children were recruited to each study group.Some visits were disrupted by the COVID19 pandemic and occurred out of protocol windows.Both the study groups attained 100 % IgG seroconversion after the initial dose. IgG seroconversion in the 9-month group was significantly higher than in the 12-month group (68.42 % vs 25.8 %, p < 0.001). Among individuals who attended visits per protocol, IgG seroconversion after the first dose occurred in 100 % of individuals (n = 27/27 in 9-month and n = 32/32 in 12-month group). However, seroconversion rates after the second dose were 80 % in the 9-month and 0 % in the shorter dose-interval 12-month group (p < 0.001) (n = 16/20 and n = 0/8, respectively).

INTERPRETATION

Vi-TT is highly immunogenic at both 9 and 12 months of age. Stronger response to a booster in the 9-month group is likely due to the longer interval between doses.

Typhoid Conjugate Vaccine: A Boon for Endemic Regions

Typhoid fever has the highest disease burden in countries in low- and middle-income countries, primarily located in Asia and Sub-Saharan Africa. Previous typhoid vaccines such as the live attenuated typhoid (Ty21a) vaccine and Vi (virulence) capsular polysaccharide vaccine had the limitation that they could not be administered with other standard childhood immunizations and were ineffective in children under two years of age. To address these shortcomings of the previous vaccines, typhoid conjugate vaccines (TCVs) were developed and prequalified by the World Health Organization. Cross-reacting material and tetanus toxoid are widely used as carrier proteins in TCVs. According to various studies, TCV has higher efficacy, has a more extended protection period, and is safe and immunogenic in infants as young as six months. This review article aims to comprehensively appraise the data available on TCVs' efficacy, duration of protection, safety, and immunogenicity in endemic regions.

Intranasal immunization of mice with chimera of Salmonella Typhi protein elicits protective intestinal immunity

Development of safe, highly effective and affordable enteric fever vaccines is a global health priority. Live, oral typhoid vaccines induce strong mucosal immunity and long-term protection, but safety remains a concern. In contrast, efficacy wears off rapidly for injectable, polysaccharide-based vaccines, which elicit poor mucosal response. We previously reported Salmonella Typhi outer membrane protein, T2544 as a potential candidate for bivalent (S. Typhi and S. Paratyphi A) vaccine development. Here, we show that intranasal immunization with a subunit vaccine (chimera of T2544 and cholera toxin B subunit) induced strong systemic and intestinal mucosal immunity and protection from S. Typhi challenge in a mouse model. CTB-T2544 augmented gut-homing receptor expression on lymphocytes that produced Th1 and Th17 cytokines, secretory IgA in stool that inhibited bacterial motility and epithelial attachment, antibody recall response and affinity maturation with increased number of follicular helper T cells and CD4+ central and effector memory cells.

Efficacy of typhoid conjugate vaccine: final analysis of a 4-year, phase 3, randomised controlled trial in Malawian children

BACKGROUND

Randomised controlled trials of typhoid conjugate vaccines among children in Africa and Asia have shown high short-term efficacy. Data on the durability of protection beyond 2 years are sparse. We present the final analysis of a randomised controlled trial in Malawi, encompassing more than 4 years of follow-up, with the aim of investigating vaccine efficacy over time and by age group.

METHODS

In this phase 3, double-blind, randomised controlled efficacy trial in Blantyre, Malawi, healthy children aged 9 months to 12 years were randomly assigned (1:1) by an unmasked statistician to receive a single dose of Vi polysaccharide conjugated to tetanus toxoid vaccine (Vi-TT) or meningococcal capsular group A conjugate (MenA) vaccine. Children had to have no previous history of typhoid vaccination and reside in the study areas for inclusion and were recruited from government schools and health centres. Participants, their parents or guardians, and the study team were masked to vaccine allocation. Nurses administering vaccines were unmasked. We did surveillance for febrile illness from vaccination until follow-up completion. The primary outcome was first occurrence of blood culture-confirmed typhoid fever. Eligible children who were randomly assigned and vaccinated were included in the intention-to-treat analyses. This trial is registered at ClinicalTrials.gov, NCT03299426.

FINDINGS

Between Feb 21, 2018, and Sept 27, 2018, 28 130 children were vaccinated; 14 069 were assigned to receive Vi-TT and 14 061 to receive MenA. After a median follow-up of 4·3 years (IQR 4·2-4·5), 24 (39·7 cases per 100 000 person-years) children in the Vi-TT group and 110 (182·7 cases per 100 000 person-years) children in the MenA group were diagnosed with a first episode of blood culture-confirmed typhoid fever. In the intention-to-treat population, efficacy of Vi-TT was 78·3% (95% CI 66·3-86·1), and 163 (129-222) children needed to be vaccinated to prevent one case. Efficacies by age group were 70·6% (6·4-93·0) for children aged 9 months to 2 years; 79·6% (45·8-93·9) for children aged 2-4 years; and 79·3% (63·5-89·0) for children aged 5-12 years.

INTERPRETATION

A single dose of Vi-TT is durably efficacious for at least 4 years among children aged 9 months to 12 years and shows efficacy in all age groups, including children younger than 2 years. These results support current WHO recommendations in typhoid-endemic areas for mass campaigns among children aged 9 months to 15 years, followed by routine introduction in the first 2 years of life.

FUNDING

Bill & Melinda Gates Foundation.

Ethical approval for controlled human infectious model clinical trial protocols - A workshop report

Controlled Human Infectious Model studies (CHIM) involve deliberately exposing volunteers to pathogens. To discuss ethical issues related to CHIM, the European Vaccine Initiative and the International Alliance for Biological Standardization organised the workshop "Ethical Approval for CHIM Clinical Trial Protocols", which took place on May 30-31, 2023, in Brussels, Belgium. The event allowed CHIM researchers, regulators, ethics committee (EC) members, and ethicists to examine the ethical criteria for CHIM and the role(s) of CHIM in pharmaceutical development. The discussions led to several recommendations, including continued assurance that routine ethical requirements are met, assurance that participants are well-informed, and that preparation of study documents must be both ethically and scientifically sound from an early stage. Study applications must clearly state the rationale for the challenge compared to alternative study designs. ECs need to have clear guidance and procedures for evaluating social value and assessing third-party risks. Among other things, public trust in research requires minimisation of harm to healthy volunteers and third-party risk. Other important considerations include appropriate stakeholder engagement, public education, and access to health care for participants after the study.

A candidate glycoconjugate vaccine induces protective antibodies in the serum and intestinal secretions, antibody recall response and memory T cells and protects against both typhoidal and non-typhoidal Salmonella serovars

Human Salmonella infections pose significant public health challenges globally, primarily due to low diagnostic yield of systemic infections, emerging and expanding antibiotic resistance of both the typhoidal and non-typhoidal Salmonella strains and the development of asymptomatic carrier state that functions as a reservoir of infection in the community. The limited long-term efficacy of the currently licensed typhoid vaccines, especially in smaller children and non-availability of vaccines against other Salmonella serovars necessitate active research towards developing a multivalent vaccine with wider coverage of protection against pathogenic Salmonella serovars. We had earlier reported immunogenicity and protective efficacy of a subunit vaccine containing a recombinant outer membrane protein (T2544) of Salmonella Typhi in a mouse model. This was achieved through the robust induction of serum IgG, mucosal secretory IgA and Salmonella-specific cytotoxic T cells as well as memory B and T cell response. Here, we report the development of a glycoconjugate vaccine, containing high molecular weight complexes of Salmonella Typhimurium O-specific polysaccharide (OSP) and recombinant T2544 that conferred simultaneous protection against S. Typhi, S. Paratyphi, S. Typhimurium and cross-protection against S. enteritidis in mice. Our findings corroborate with the published studies that suggested the potential of Salmonella OSP as a vaccine antigen. The role of serum antibodies in vaccine-mediated protection is suggested by rapid seroconversion with high titers of serum IgG and IgA, persistently elevated titers after primary immunization along with a strong antibody recall response with higher avidity serum IgG against both OSP and T2544 and significantly raised SBA titers of both primary and secondary antibodies against different Salmonella serovars. Elevated intestinal secretory IgA and bacterial motility inhibition by the secretory antibodies supported their role as well in vaccine-induced protection. Finally, robust induction of T effector memory response indicates long term efficacy of the candidate vaccine. The above findings coupled with protection of vaccinated animals against multiple clinical isolates confirm the suitability of OSP-rT2544 as a broad-spectrum candidate subunit vaccine against human infection due to typhoidal and non-typhoidal Salmonella serovars.

TLR4 Ligands in Typhoid Vi Polysaccharide Subunit Vaccines Contribute to Immunogenicity

Activation of B cells and T cells requires the engagement of costimulatory signaling pathways in addition to Ag receptor signaling for efficient immune responses. None of the typhoid Vi polysaccharide (ViPS) subunit vaccines contains adjuvants that could activate costimulatory signaling pathways, yet these vaccines are very immunogenic. I hypothesized that residual TLR ligands present in the ViPS preparation used for making typhoid subunit vaccines account for the robust immune response generated by these vaccines. I show the presence of endotoxin, a potent agonist of TLR4, in ViPS preparations and ViPS vaccines. Furthermore, I found that ViPS obtained from various sources induces the production of proinflammatory cytokines such as IL-6 from mouse peritoneal exudate cells. Unconjugated and tetanus toxoid-conjugated ViPS vaccines activate human and mouse TLR4. Mice deficient in TLR4 or the signaling adaptors MyD88 and Trif (Toll/IL-1R domain-containing adapter inducing IFN-β) are severely impaired in generating anti-ViPS responses to these vaccines. Elimination of the TLR4 agonist in ViPS preparation resulted in the loss of immunogenicity, and addition of lipid A, a known TLR4 agonist, restored the immunogenicity. These data highlight the importance of associated TLR ligands in the immunogenicity of ViPS subunit vaccines.

A randomized, observer-blind, controlled phase III clinical trial assessing safety and immunological non-inferiority of Vi-diphtheria toxoid versus Vi-tetanus toxoid typhoid conjugate vaccine in healthy volunteers in eastern Nepal

Typhoid remains one of the major serious health concerns for children in developing countries. With extremely drug-resistant cases emerging, preventative measures like sanitation and vaccination, including typhoid conjugate vaccines (TCV) remain the mainstay in its prevention and control. Different types of TCVs are being developed to meet the global demand. This report outlines the results from a study done to assess the immunogenicity and safety of Vi-Diphtheria toxoid (Vi-DT) TCV in Nepal. The study was a randomized, active-controlled, immunological non-inferiority and safety study. Eligible participants from Sunsari and Morang districts of eastern Nepal were randomized into 4 study groups (A-D) within 3 age strata (6 months to <2 years, 2 to <18 years, and 18 to 45 years). Groups A to C received a single dose (25 μg) of Vi-DT test vaccine from any of the 3 lots, while group D received the comparator, Typbar-TCV®, Vi-tetanus toxoid (Vi-TT) vaccine (25 μg) in 1:1:1:1 ratio and evaluated at 4 weeks postvaccination with 6 months follow-up. Amongst 400 randomized participants, anti-Vi-IgG seroconversion rates for all age strata in Vi-DT pooled groups (A+B+C) were 100.00% (97.5% CI 98.34-100.00) vs 98.99% (97.5% CI 93.99-99.85) in Vi-TT group (D) at 4 weeks. Comparable safety events were reported between the groups. Three serious adverse events (1 in Vi-DT; 2 in Vi-TT group) were reported during the 6 months follow-up, none being related to the investigational product. Thus, Vi-DT vaccine is safe, immunogenic, and immunologically non-inferior to Vi-TT when analyzed at 4 weeks postvaccination.

Vaccination to Reduce Antimicrobial Resistance Burden-Data Gaps and Future Research

Antimicrobial resistance (AMR) poses an immediate danger to global health. If unaddressed, the current upsurge in AMR threatens to reverse the achievements in reducing the infectious disease-associated mortality and morbidity associated with antimicrobial treatment. Consequently, there is an urgent need for strategies to prevent or slow the progress of AMR. Vaccines potentially contribute both directly and indirectly to combating AMR. Modeling studies have indicated significant gains from vaccination in reducing AMR burdens for specific pathogens, reducing mortality/morbidity, and economic loss. However, quantifying the real impact of vaccines in these reductions is challenging because many of the study designs used to evaluate the contribution of vaccination programs are affected by significant background confounding, and potential selection and information bias. Here, we discuss challenges in assessing vaccine impact to reduce AMR burdens and suggest potential approaches for vaccine impact evaluation nested in vaccine trials.

Typhoid fever

Typhoid fever is an invasive bacterial disease associated with bloodstream infection that causes a high burden of disease in Africa and Asia. Typhoid primarily affects individuals ranging from infants through to young adults. The causative organism, Salmonella enterica subsp. enterica serovar Typhi is transmitted via the faecal-oral route, crossing the intestinal epithelium and disseminating to systemic and intracellular sites, causing an undifferentiated febrile illness. Blood culture remains the practical reference standard for diagnosis of typhoid fever, where culture testing is available, but novel diagnostic modalities are an important priority under investigation. Since 2017, remarkable progress has been made in defining the global burden of both typhoid fever and antimicrobial resistance; in understanding disease pathogenesis and immunological protection through the use of controlled human infection; and in advancing effective vaccination programmes through strategic multipartner collaboration and targeted clinical trials in multiple high-incidence priority settings. This Primer thus offers a timely update of progress and perspective on future priorities for the global scientific community.

Strategic and scientific contributions of human challenge trials for vaccine development: facts versus fantasy

The unprecedented speed of delivery of SARS-CoV-2 pandemic vaccines has redefined the limits for all vaccine development. Beyond the aspirational 100-day timeline for tomorrow's hypothetical pandemic vaccines, there is a sense of optimism that development of other high priority vaccines can be accelerated. Early in the COVID-19 pandemic, an intense and polarised academic and public discourse arose concerning the role of human challenge trials for vaccine development. A case was made for human challenge trials as a powerful tool to establish early proof-of-concept of vaccine efficacy in humans, inform vaccine down selection, and address crucial knowledge gaps regarding transmission, pathogenesis, and immune protection. We review the track record of human challenge trials contributing to the development of vaccines for 19 different pathogens and discuss relevant limitations, barriers, and pitfalls. This Review also highlights opportunities for efforts to broaden the scope and boost the effects of human challenge trials, to accelerate all vaccine development.

Environmental sampling for typhoidal Salmonellas in household and surface waters in Nepal identifies potential transmission pathways

INTRODUCTION

Salmonella Typhi and Salmonella Paratyphi, fecal-oral transmitted bacterium, have temporally and geographically heterogeneous pathways of transmission. Previous work in Kathmandu, Nepal implicated stone waterspouts as a dominant transmission pathway after 77% of samples tested positive for Salmonella Typhi and 70% for Salmonella Paratyphi. Due to a falling water table, these spouts no longer provide drinking water, but typhoid fever persists, and the question of the disease's dominant pathway of transmission remains unanswered.

METHODS

We used environmental surveillance to detect Salmonella Typhi and Salmonella Paratyphi A DNA from potential sources of transmission. We collected 370, 1L drinking water samples from a population-based random sample of households in the Kathmandu and Kavre Districts of Nepal between February and October 2019. Between November 2019 and July 2021, we collected 380, 50mL river water samples from 19 sentinel sites on a monthly interval along the rivers leading through the Kathmandu and Kavre Districts. We processed drinking water samples using a single qPCR and processed river water samples using differential centrifugation and qPCR at 0 and after 16 hours of liquid culture enrichment. A 3-cycle threshold (Ct) decrease of Salmonella Typhi or Salmonella Paratyphi, pre- and post-enrichment, was used as evidence of growth. We also performed structured observations of human-environment interactions to understand pathways of potential exposure.

RESULTS

Among 370 drinking water samples, Salmonella Typhi was detected in 7 samples (1.8%) and Salmonella Paratyphi A was detected in 4 (1.0%) samples. Among 380 river water samples, Salmonella Typhi was detected in 171 (45%) and Salmonella Paratyphi A was detected in 152 (42%) samples. Samples located upstream of the Kathmandu city center were positive for Salmonella Typhi 12% of the time while samples from locations in and downstream were positive 58% and 67% of the time respectively. Individuals were observed bathing, washing clothes, and washing vegetables in the rivers.

IMPLICATIONS

These results suggest that drinking water was not the dominant pathway of transmission of Salmonella Typhi and Salmonella Paratyphi A in the Kathmandu Valley in 2019. The high degree of river water contamination and its use for washing vegetables raises the possibility that river systems represent an important source of typhoid exposure in Kathmandu.

Durable Anti-Vi IgG and IgA Antibody Responses in 15-Month-Old Children Vaccinated With Typhoid Conjugate Vaccine in Burkina Faso

We assessed anti-Vi IgG/IgA responses to typhoid conjugate vaccine (TCV) in children enrolled in a double-blind randomized controlled, phase 2 trial in Burkina Faso. Anti-Vi IgG seroconversion and anti-Vi IgA titers were higher in TCV than control recipients at 30-35 months post-vaccination. TCV induces durable immunity in Burkinabe children vaccinated at 15 months.

Molecular correlates of vaccine-induced protection against typhoid fever

BACKGROUNDTyphoid fever is caused by the Gram-negative bacterium Salmonella enterica serovar Typhi and poses a substantial public health burden worldwide. Vaccines have been developed based on the surface Vi-capsular polysaccharide of S. Typhi; these include a plain-polysaccharide-based vaccine, ViPS, and a glycoconjugate vaccine, ViTT. To understand immune responses to these vaccines and their vaccine-induced immunological protection, molecular signatures were analyzed using bioinformatic approaches.METHODSBulk RNA-Seq data were generated from blood samples obtained from adult human volunteers enrolled in a vaccine trial, who were then challenged with S. Typhi in a controlled human infection model (CHIM). These data were used to conduct differential gene expression analyses, gene set and modular analyses, B cell repertoire analyses, and time-course analyses at various post-vaccination and post-challenge time points between participants receiving ViTT, ViPS, or a control meningococcal vaccine.RESULTSTranscriptomic responses revealed strong differential molecular signatures between the 2 typhoid vaccines, mostly driven by the upregulation in humoral immune signatures, including selective usage of immunoglobulin heavy chain variable region (IGHV) genes and more polarized clonal expansions. We describe several molecular correlates of protection against S. Typhi infection, including clusters of B cell receptor (BCR) clonotypes associated with protection, with known binders of Vi-polysaccharide among these.CONCLUSIONThe study reports a series of contemporary analyses that reveal the transcriptomic signatures after vaccination and infectious challenge, while identifying molecular correlates of protection that may inform future vaccine design and assessment.TRIAL REGISTRATIONClinicalTrials.gov NCT02324751.

Development of Shigella conjugate vaccines targeting Shigella flexneri 2a and S. flexneri 3a using a simple platform-approach conjugation by squaric acid chemistry

There is a need for vaccines effective against shigella infection in young children in resource-limited areas. Protective immunity against shigella infection targets the O-specific polysaccharide (OSP) component of lipopolysaccharide. Inducing immune responses to polysaccharides in young children can be problematic, but high level and durable responses can be induced by presenting polysaccharides conjugated to carrier proteins. An effective shigella vaccine will need to be multivalent, targeting the most common global species and serotypes such as Shigella flexneri 2a, S. flexneri 3a, S. flexneri 6, and S. sonnei. Here we report the development of shigella conjugate vaccines (SCV) targeting S. flexneri 2a (SCV-Sf2a) and 3a (SCV-Sf3a) using squaric acid chemistry to result in single point sun-burst type display of OSP from carrier protein rTTHc, a 52 kDa recombinant protein fragment of the heavy chain of tetanus toxoid. We confirmed structure and demonstrated that these conjugates were recognized by serotype-specific monoclonal antibodies and convalescent sera of humans recovering from shigellosis in Bangladesh, suggesting correct immunological display of OSP. We vaccinated mice and found induction of serotype-specific OSP and LPS IgG responses, as well as rTTHc-specific IgG responses. Vaccination induced serotype-specific bactericidal antibody responses against S. flexneri, and vaccinated animals were protected against keratoconjunctivitis (Sereny test) and intraperitoneal challenge with virulent S. flexneri 2a and 3a, respectively. Our results support further development of this platform conjugation technology in the development of shigella conjugate vaccines for use in resource-limited settings.

Accelerating clinical development of a live attenuated vaccine against Salmonella Paratyphi A (VASP): study protocol for an observer-participant-blind randomised control trial of a novel oral vaccine using a human challenge model of Salmonella Paratyphi A infection in healthy adult volunteers

INTRODUCTION

This is the first efficacy study of an oral live attenuated vaccine against Salmonella Paratyphi A using a human challenge model of paratyphoid infection. S. Paratyphi A is responsible for 3.3 million cases of enteric fever every year, with over 19 000 deaths. Although improvements to sanitation and access to clean water are vital to reduce the burden of this condition, vaccination offers a cost-effective, medium-term solution. Efficacy trials of potential S. Paratyphi vaccine candidates in the field are unlikely to be feasible given the large number of participants required. Human challenge models therefore offer a unique, cost-effective solution to test efficacy of such vaccines.

METHODS AND ANALYSIS

This is an observer-blind, randomised, placebo-controlled trial phase I/II of the oral live-attenuated vaccine against S. Paratyphi A, CVD 1902. Volunteers will be randomised 1:1 to receive two doses of CVD 1902 or placebo, 14 days apart. One month following second vaccination all volunteers will ingest S. Paratyphi A bacteria with a bicarbonate buffer solution. They will be reviewed daily in the following 14 days and diagnosed with paratyphoid infection if the predefined microbiological or clinical diagnostic criteria are met. All participants will be treated with antibiotics on diagnosis, or at day 14 postchallenge if not diagnosed. The vaccine efficacy will be determined by comparing the relative attack rate, that is, the proportion of those diagnosed with paratyphoid infection, in the vaccine and placebo groups.

ETHICS AND DISSEMINATION

Ethical approval for this study has been obtained from the Berkshire Medical Research Ethics Committee (REC ref 21/SC/0330). The results will be disseminated via publication in a peer-reviewed journal and presentation at international conferences.

TRIAL REGISTRATION NUMBER

ISRCTN15485902.

Seroepidemiology for Enteric Fever: Emerging Approaches and Opportunities

Safe and effective typhoid conjugate vaccines (TCVs) are available, but many countries lack the high-resolution data needed to prioritize TCV introduction to the highest-risk communities. Here we discuss seroepidemiology-an approach using antibody response data to characterize infection burden-as a potential tool to fill this data gap. Serologic tests for typhoid have existed for over a hundred years, but only recently were antigens identified that were sensitive and specific enough to use as epidemiologic markers. These antigens, coupled with new methodological developments, permit estimating seroincidence-the rate at which new infections occur in a population-from cross-sectional serosurveys. These new tools open up many possible applications for enteric fever seroepidemiology, including generating high-resolution surveillance data, monitoring vaccine impact, and integrating with other serosurveillance initiatives. Challenges remain, including distinguishing Salmonella Typhi from Salmonella Paratyphi infections and accounting for reinfections. Enteric fever seroepidemiology can be conducted at a fraction of the cost, time, and sample size of surveillance blood culture studies and may enable more efficient and scalable surveillance for this important infectious disease.

Taking on Typhoid: Eliminating Typhoid Fever as a Global Health Problem

Typhoid fever is a significant global health problem that impacts people living in areas without access to clean water and sanitation. However, collaborative international partnerships and new research have improved both knowledge of the burden in countries with endemic disease and the tools for improved surveillance, including environmental surveillance. Two typhoid conjugate vaccines (TCVs) have achieved World Health Organization prequalification, with several more in the development pipeline. Despite hurdles posed by the coronavirus disease 2019 pandemic, multiple TCV efficacy trials have been conducted in high-burden countries, and data indicate that TCVs provide a high degree of protection from typhoid fever, are safe to use in young children, provide lasting protection, and have the potential to combat typhoid antimicrobial resistance. Now is the time to double down on typhoid control and elimination by sustaining progress made through water, sanitation, and hygiene improvements and accelerating TCV introduction in high-burden locations.

Progress in the Typhoid Conjugate Vaccine Program Rollout Supported by Gavi During the COVID-19 Pandemic and the Path Forward

Gavi supports countries to introduce typhoid conjugate vaccine (TCV) with catch-up campaigns. Available TCVs are highly efficacious, equity-focused, and critical to curbing the expansion of antimicrobial resistance. Four Gavi-supported countries have introduced TCVs since 2018. In the wake of the COVID-19 emergency, momentum is building to scale up TCV introduction worldwide, supported by global partners and Gavi's funding for improved typhoid diagnostics.

Typhoid Control in an Era of Antimicrobial Resistance: Challenges and Opportunities

Historically, typhoid control has been achieved with water and sanitation interventions. Today, in an era of rising antimicrobial resistance (AMR), two World Health Organization-prequalified vaccines are available to accelerate control in the shorter term. Meanwhile, water and sanitation interventions could be implemented in the longer term to sustainably prevent typhoid in low- and middle-income countries. This article first approaches typhoid control from a historical perspective, subsequently presents how vaccination could complement water and sanitation activities, and finally discusses the challenges and opportunities for impactful control of typhoid infection. It also addresses data blind spots and knowledge gaps to focus on for typhoid control and to ultimately progress towards elimination. This article presents a synthesis of discussions held in December 2021 during a roundtable session at the "12th International Conference on Typhoid and Other Invasive Salmonelloses".

Using Typhoid Conjugate Vaccines to Prevent Disease, Promote Health Equity, and Counter Drug-Resistant Typhoid Fever

Typhoid fever is a serious disease that disproportionately impacts children in low-resource settings in sub-Saharan Africa, South and Southeast Asia, and the Western Pacific. The prevalence of antimicrobial-resistant strains of S. Typhi continue to increase worldwide. Two safe, effective, and cost-effective typhoid conjugate vaccines (TCVs) are World Health Organization-prequalified for the prevention of typhoid fever in children as young as 6 months. Typhoid conjugate vaccines have proven effectiveness in preventing drug-resistant S. Typhi and have been deployed successfully in outbreak response and routine immunization scenarios. Broad and equitable distribution of TCVs is essential to combat the spread and potentially devastating consequences of typhoid fever. It is vital to empower decision-makers in typhoid-endemic countries to introduce TCVs and for leaders to embrace this critical tool to prevent typhoid fever, slow the spread of drug-resistant S. Typhi strains, promote health equity, and save lives.

Immune responses in children after vaccination with a typhoid Vi-tetanus toxoid conjugate vaccine in Bangladesh

A cluster-randomized trial of Vi-TT was conducted in Dhaka, Bangladesh, using JE vaccine as the control. A subset of 1,500 children were randomly selected on 2:1 basis (Vi-TT vs JE) to assess immune response. Blood was collected before vaccination, and on days 28, 545 and 730 post-vaccination and plasma anti-Vi-IgG response was measured. A robust, persistent antibody response was induced after single dose of Vi-TT, even after 2 years of vaccination. While there is no accepted serological antibody threshold of protection, analyzing the antibodies of children who received Vi-TT provides evidence that may later be useful in predicting population protection.

Assessment of vaccine herd protection in a cluster-randomised trial of Vi conjugate vaccine against typhoid fever: results of further analysis

Background

A cluster-randomised trial of Vi-tetanus toxoid (Vi-TT) conjugate vaccine conducted in urban Bangladeshi children found a high level of direct protection by Vi-TT but no significant vaccine herd protection. We reassessed the trial using a "fried egg" analysis to evaluate whether herd protection might have been obscured by transmission of typhoid into the clusters from the outside.

Methods

A participant- and observer-blind, cluster-randomised trial was conducted between February 14, 2018 and August 12, 2019 in three wards of Mirpur, a densely populated urban area of Dhaka, Bangladesh. Children 9 months to under 16 years of age in 150 geographic clusters, which had a total of 311,289 persons present at baseline or entering during follow-up, were randomised by cluster to a single-dose of Vi-TT or Japanese encephalitis (JE) vaccine. Vi-TT protection against typhoid fever, detected at 8 treatment centres serving the study population, was compared in the original clusters for the trial, and for progressively more central subclusters ("yolks" of the "fried egg") of the cluster residents. If transmission of typhoid into the clusters had diluted observed vaccine herd protection, we hypothesised that analysis of the innermost "yolks" would reveal vaccine herd protection that was not evident in analysis of the entire clusters. The trial is registered at www.isrctn.com as ISRCTN11643110.

Findings

At ≤18 months of follow-up, total vaccine effectiveness (protection of Vi-TT recipients relative to JE vaccine recipients) was 85% (95% CI: 76%, 90%); indirect effectiveness (protection of non-Vi-TT recipients in Vi-TT clusters relative to non-JE vaccine recipients in JE vaccine clusters) was 17% (95% CI: -13%, 40%); and overall effectiveness (protection of all residents in the Vi-TT clusters relative to all residents of the JE vaccine clusters) was 57% (95% CI: 44%, 66%). Analyses of subpopulations in inner 75%, 50% and 25% "yolks" of the clusters failed to reveal significant changes in any of these estimates.

Interpretation

Our analysis did not reveal Vi-TT herd protection in the trial. Consideration should be given to exploring whether targeting adults as well as children with Vi-TT yields appreciable levels of vaccine herd protection.

Funding

Bill & Melinda Gates Foundation (OPP1151153, INV-025388).

Cost-effectiveness analysis of typhoid conjugate vaccines in an outbreak setting: a modeling study

BACKGROUND

Several prolonged typhoid fever epidemics have been reported since 2010 throughout eastern and southern Africa, including Malawi, caused by multidrug-resistant Salmonella Typhi. The World Health Organization recommends the use of typhoid conjugate vaccines (TCVs) in outbreak settings; however, current data are limited on how and when TCVs might be introduced in response to outbreaks.

METHODOLOGY

We developed a stochastic model of typhoid transmission fitted to data from Queen Elizabeth Central Hospital in Blantyre, Malawi from January 1996 to February 2015. We used the model to evaluate the cost-effectiveness of vaccination strategies over a 10-year time horizon in three scenarios: (1) when an outbreak is likely to occur; (2) when an outbreak is unlikely to occur within the next ten years; and (3) when an outbreak has already occurred and is unlikely to occur again. We considered three vaccination strategies compared to the status quo of no vaccination: (a) preventative routine vaccination at 9 months of age; (b) preventative routine vaccination plus a catch-up campaign to 15 years of age; and (c) reactive vaccination with a catch-up campaign to age 15 (for Scenario 1). We also explored variations in outbreak definitions, delays in implementation of reactive vaccination, and the timing of preventive vaccination relative to the outbreak.

RESULTS

Assuming an outbreak occurs within 10 years, we estimated that the various vaccination strategies would prevent a median of 15-60% of disability-adjusted life-years (DALYs). Reactive vaccination was the preferred strategy for WTP values of $0-300 per DALY averted. For WTP values > $300, introduction of preventative routine TCV immunization with a catch-up campaign was the preferred strategy. Routine vaccination with a catch-up campaign was cost-effective for WTP values above $890 per DALY averted if no outbreak occurs and > $140 per DALY averted if implemented after the outbreak has already occurred.

CONCLUSIONS

Countries for which the spread of antimicrobial resistance is likely to lead to outbreaks of typhoid fever should consider TCV introduction. Reactive vaccination can be a cost-effective strategy, but only if delays in vaccine deployment are minimal; otherwise, introduction of preventive routine immunization with a catch-up campaign is the preferred strategy.

Chronic Salmonella Typhi carriage at sites other than the gallbladder

Typhoid fever caused by infection with Salmonella enterica subspecies enterica serotype Typhi (S. Typhi), an important public health problem in many low- and middle-income countries, is transmitted by ingestion of water or food contaminated by feces or urine from individuals with acute or chronic S. Typhi infection. Most chronic S. Typhi carriers (shedding for ≥12 months) harbor infection in their gallbladder wherein preexisting pathologies, particularly cholelithiasis, provide an environment that fosters persistence. Much less appreciated is the existence of non-gallbladder hepatobiliary chronic S. Typhi carriers and urinary carriers. The former includes parasitic liver flukes as a chronic carriage risk factor. Chronic urinary carriers typically have pathology of their urinary tract, with or without renal or bladder stones. Even as the prevalence of multidrug-resistant and extensively drug-resistant S. Typhi strains is rising, global implementation of highly effective typhoid vaccines is increasing. There is also renewed interest in identifying, monitoring, and (where possible) treating chronic carriers who comprise the long-term reservoir of S. Typhi.

Comparison of model predictions of typhoid conjugate vaccine public health impact and cost-effectiveness

Models are useful to inform policy decisions on typhoid conjugate vaccine (TCV) deployment in endemic settings. However, methodological choices can influence model-predicted outcomes. To provide robust estimates for the potential public health impact of TCVs that account for structural model differences, we compared four dynamic and one static mathematical model of typhoid transmission and vaccine impact. All models were fitted to a common dataset of age-specific typhoid fever cases in Kolkata, India. We evaluated three TCV strategies: no vaccination, routine vaccination at 9 months of age, and routine vaccination at 9 months with a one-time catch-up campaign (ages 9 months to 15 years). The primary outcome was the predicted percent reduction in symptomatic typhoid cases over 10 years after vaccine introduction. For three models with economic analyses (Models A-C), we also compared the incremental cost-effectiveness ratios (ICERs), calculated as the incremental cost (US$) per disability-adjusted life-year (DALY) averted. Routine vaccination was predicted to reduce symptomatic cases by 10-46 % over a 10-year time horizon under an optimistic scenario (95 % initial vaccine efficacy and 19-year mean duration of protection), and by 2-16 % under a pessimistic scenario (82 % initial efficacy and 6-year mean protection). Adding a catch-up campaign predicted a reduction in incidence of 36-90 % and 6-35 % in the optimistic and pessimistic scenarios, respectively. Vaccine impact was predicted to decrease as the relative contribution of chronic carriers to transmission increased. Models A-C all predicted routine vaccination with or without a catch-up campaign to be cost-effective compared to no vaccination, with ICERs varying from $95-789 per DALY averted; two models predicted the ICER of routine vaccination alone to be greater than with the addition of catch-up campaign. Despite differences in model-predicted vaccine impact and cost-effectiveness, routine vaccination plus a catch-up campaign is likely to be impactful and cost-effective in high incidence settings such as Kolkata.

Typhoid conjugate vaccine effectiveness in Malawi: evaluation of a test-negative design using randomised, controlled clinical trial data

BACKGROUND

Typhoid conjugate vaccines are being introduced in low-income and middle-income countries to prevent typhoid illness in children. Vaccine effectiveness studies assess vaccine performance after introduction. The test-negative design is a commonly used method to estimate vaccine effectiveness that has not been applied to typhoid vaccines because of concerns over blood culture insensitivity. The overall aim of the study was to evaluate the appropriateness of using a test-negative design to assess typhoid Vi polysaccharide-tetanus toxoid conjugate vaccine (Vi-TT) effectiveness using a gold standard randomised controlled trial database.

METHODS

Using blood culture data from a randomised controlled trial of Vi-TT in Malawi, we simulated a test-negative design to derive vaccine effectiveness estimates using three different approaches and compared these to randomised trial efficacy results. In the randomised trial, 27 882 children aged 9 months to 12 years were randomly assigned (1:1) to receive a single dose of Vi-TT or meningococcal capsular group A conjugate vaccine between Feb 21 and Sept 27, 2018, and were followed up for blood culture-confirmed typhoid fever until Sept 30, 2021.

FINDINGS

For all three test-negative design approaches, vaccine effectiveness estimates (test-negative design A, 80·3% [95% CI 66·2 to 88·5] vs test-negative design B, 80·5% [66·5 to 88·6] vs test-negative design C, 80·4% [66·9 to 88·4]) were almost identical to the randomised trial results (80·4% [95% CI 66·4 to 88·5]). Receipt of Vi-TT did not affect the risk of non-typhoid fever (vaccine efficacy against non-typhoid fever -0·4% [95% CI -4·9 to 3·9] vs -1% [-5·6 to 3·3] vs -2·5% [-6·4 to 1·3] for test-negative design A, test-negative design B, and test-negative design C, respectively).

INTERPRETATION

This study validates the test-negative design core assumption for typhoid vaccine effectiveness estimation and shows the accuracy and precision of the estimates compared with the randomised controlled trial. These results show that the test-negative design is suitable for assessing typhoid conjugate vaccine effectiveness in post-introduction studies using blood culture surveillance.

FUNDING

Bill & Melinda Gates Foundation.

The genomic characterization of Salmonella Paratyphi A from an outbreak of enteric fever in Vadodara, India

Salmonella enterica Typhi (S. Typhi) and Paratyphi A (S. Paratyphi A) are the causative agents of enteric fever, a systemic human disease with a burden of 300 000 cases per year in India. The majority of enteric fever cases are associated with S. Typhi, resulting in a paucity of data regarding S. Paratyphi A, specifically with respect to genomic surveillance and antimicrobial resistance (AMR). Here, we exploited whole-genome sequencing (WGS) to identify S. Paratyphi A genotypes and AMR determinants associated with an outbreak of S. Paratyphi A in Vadodara, India, from December 2018 to December 2019. In total 117 S. Paratyphi A were isolated and genome sequenced, most were genotype 2.4.2 (72.6 % of all cases), which is the globally dominant genotype. The remainder were genotype 2.3 (25.6 %), while only two isolates belonged to genotype 2.4.1. A single base-pair mutation in gyrA, associated with reduced susceptibility to fluoroquinolones, was present in all of the outbreak isolates; with 74.35 % of isolates having a S83F substitution and the remainder having an S83Y substitution. Our surveillance study suggests that S. Paratyphi A is an emergent pathogen in South Asia, which may become increasingly relevant with the introduction of Vi conjugate vaccines.

A Bivalent MAPS Vaccine Induces Protective Antibody Responses against Salmonella Typhi and Paratyphi A

Infections by Salmonella Typhi and Paratyphi A strain are still a major cause of morbidity and mortality in developing countries. Generation of antibodies against the Vi capsular polysaccharide of S. Typhi via either pure polysaccharide or protein-polysaccharide conjugate is a very effective way to protect against S. Typhi. To date, there is no commercially available vaccine against S. Paratyphi A. The O-specific polysaccharide (OSP) has been generally considered a good vaccine target for Paratyphi A. Here, a bivalent vaccine against Vi and OSP was generated using the Multiple Antigen Presenting System (MAPS). Three different protein constructs, including CRM197, rEPA of Pseudomonas, and a pneumococcal fusion protein SP1500-SP0785, were fused to Rhizavidin (Rhavi) and evaluated their impact on immunogenicity when incorporated as fusion proteins affinity-bound to the two polysaccharides. We compared the antibody responses, antibody avidity, and cidal activity of sera post-immunization with monovalent vs. combination vaccines. We also wished to evaluate the generation of Vi-specific memory B cells in mice. We found little interference when combination vaccine was compared to monovalent vaccines with respect to antibody concentration and cidal activity of sera. Significant affinity maturation was noted for both Vi and OSP antigens. Thus, our preclinical results with a combination Vi- and OSP-MAPS vaccine strongly support the feasibility of this approach and its application of this approach to other important salmonella and Shigella species.

Safety, immunogenicity and non-interference of concomitant Typhoid Vi capsular polysaccharide-tetanus toxoid conjugate vaccine (Typbar-TCV®) and measles or measles-mumps-rubella vaccines in 8-9 months-old Indian children

We evaluated safety, reactogenicity, and immunogenicity when the WHO-prequalified single-dose Typhoid Vi-polysaccharide conjugate vaccine, Typbar-TCV®, was administered concomitantly with measles (MV) or measles-mumps-rubella (MMR) vaccines in 8- or 9-month-old children. We enrolled 493 children who were randomized 2:1:1:1 to four groups to receive either TCV (0.5 mL intramuscularly) and MV (0.5 ml subcutaneously) concomitantly at 9 months of age (Group 1) with two subgroups given TCV booster 28 days (Group 1A) or 180 days (Group 1B) later, or MV on Day 0 and TCV on Day 28 (Group 2); or TCV at 8 months of age and MV 28 days later (Group 3), or MV only at 9 months of age (Group 4). All children received MMR at 15 months of age. We observed no statistically significant differences between group rates of solicited or unsolicited adverse events assessed throughout the study. Seroconversion rates for measles, mumps, and rubella antibodies were unaffected by concomitant administration with TCV, being similar in Groups 1, 2, and 3 and comparable to Group 4 (Control). IgG anti-Vi antibody titers were similar in all groups after primary Typbar-TCV® vaccination and were not increased by a second dose 28 days later. A small response to a booster dose of Typbar-TCV® given at 180 days did not achieve the high titers observed after the first dose, suggesting that booster vaccination may be more effective after a longer interval than 6 months. Typbar-TCV® can be safely co-administered with measles and MMR vaccines in children aged ≥9 months.Clinical trial registration number: CTRI/2014/04/004532.

Public engagement during a typhoid conjugate vaccine trial in Lalitpur, Nepal- experience, challenges and lessons learnt

Typhoid is a public health problem in Nepal. To generate evidence on the impact of Typhoid Conjugate Vaccine (TCV), a phase 3, double-blind, randomized controlled trial was conducted in Lalitpur, Nepal. 20,000 children aged between 9 months and ≤16 years were vaccinated with a new TCV, or control vaccine. Participants were actively followed for safety and efficacy over 2 years through passive surveillance (PS) clinics. Several challenges were encountered during vaccination and PS stemming from misinformation, misconception, and fear around clinical trials in the community. Public engagement (PE) activities were conducted across various tiers moving from decision makers in the first tier; to elected local representatives in the second tier; ending with interaction in community with parents/guardians of the targeted population. Prior and during vaccination, engagement was conducted to inform about the study and discuss the importance of vaccination. Post-vaccination, engagement was conducted to inform about PS clinics, alleviate study concerns and share study updates. Direct and continuous interaction with community stakeholders, including parents/guardians of the targeted population contributed to build trust around the study and community willingness to be involved. It helped to raise awareness, drive away misconceptions, and allowed adaptation according to feedback from community members.

Treatment of enteric fever (typhoid and paratyphoid fever) with cephalosporins

BACKGROUND

Typhoid and paratyphoid (enteric fever) are febrile bacterial illnesses common in many low- and middle-income countries. The World Health Organization (WHO) currently recommends treatment with azithromycin, ciprofloxacin, or ceftriaxone due to widespread resistance to older, first-line antimicrobials. Resistance patterns vary in different locations and are changing over time. Fluoroquinolone resistance in South Asia often precludes the use of ciprofloxacin. Extensively drug-resistant strains of enteric fever have emerged in Pakistan. In some areas of the world, susceptibility to old first-line antimicrobials, such as chloramphenicol, has re-appeared. A Cochrane Review of the use of fluoroquinolones and azithromycin in the treatment of enteric fever has previously been undertaken, but the use of cephalosporins has not been systematically investigated and the optimal choice of drug and duration of treatment are uncertain.

OBJECTIVES

To evaluate the effectiveness of cephalosporins for treating enteric fever in children and adults compared to other antimicrobials.

SEARCH METHODS

We searched the Cochrane Infectious Diseases Group Specialized Register, CENTRAL, MEDLINE, Embase, LILACS, the WHO ICTRP and ClinicalTrials.gov up to 24 November 2021. We also searched reference lists of included trials, contacted researchers working in the field, and contacted relevant organizations.

SELECTION CRITERIA

We included randomized controlled trials (RCTs) in adults and children with enteric fever that compared a cephalosporin to another antimicrobial, a different cephalosporin, or a different treatment duration of the intervention cephalosporin. Enteric fever was diagnosed on the basis of blood culture, bone marrow culture, or molecular tests.

DATA COLLECTION AND ANALYSIS

We used standard Cochrane methods. Our primary outcomes were clinical failure, microbiological failure and relapse. Our secondary outcomes were time to defervescence, duration of hospital admission, convalescent faecal carriage, and adverse effects. We used the GRADE approach to assess certainty of evidence for each outcome.

MAIN RESULTS

We included 27 RCTs with 2231 total participants published between 1986 and 2016 across Africa, Asia, Europe, the Middle East and the Caribbean, with comparisons between cephalosporins and other antimicrobials used for the treatment of enteric fever in children and adults. The main comparisons are between antimicrobials in most common clinical use, namely cephalosporins compared to a fluoroquinolone and cephalosporins compared to azithromycin. Cephalosporin (cefixime) versus fluoroquinolones Clinical failure, microbiological failure and relapse may be increased in patients treated with cefixime compared to fluoroquinolones in three small trials published over 14 years ago: clinical failure (risk ratio (RR) 13.39, 95% confidence interval (CI) 3.24 to 55.39; 2 trials, 240 participants; low-certainty evidence); microbiological failure (RR 4.07, 95% CI 0.46 to 36.41; 2 trials, 240 participants; low-certainty evidence); relapse (RR 4.45, 95% CI 1.11 to 17.84; 2 trials, 220 participants; low-certainty evidence). Time to defervescence in participants treated with cefixime may be longer compared to participants treated with fluoroquinolones (mean difference (MD) 1.74 days, 95% CI 0.50 to 2.98, 3 trials, 425 participants; low-certainty evidence). Cephalosporin (ceftriaxone) versus azithromycin Ceftriaxone may result in a decrease in clinical failure compared to azithromycin, and it is unclear whether ceftriaxone has an effect on microbiological failure compared to azithromycin in two small trials published over 18 years ago and in one more recent trial, all conducted in participants under 18 years of age: clinical failure (RR 0.42, 95% CI 0.11 to 1.57; 3 trials, 196 participants; low-certainty evidence); microbiological failure (RR 1.95, 95% CI 0.36 to 10.64, 3 trials, 196 participants; very low-certainty evidence). It is unclear whether ceftriaxone increases or decreases relapse compared to azithromycin (RR 10.05, 95% CI 1.93 to 52.38; 3 trials, 185 participants; very low-certainty evidence). Time to defervescence in participants treated with ceftriaxone may be shorter compared to participants treated with azithromycin (mean difference of -0.52 days, 95% CI -0.91 to -0.12; 3 trials, 196 participants; low-certainty evidence). Cephalosporin (ceftriaxone) versus fluoroquinolones It is unclear whether ceftriaxone has an effect on clinical failure, microbiological failure, relapse, and time to defervescence compared to fluoroquinolones in three trials published over 28 years ago and two more recent trials: clinical failure (RR 3.77, 95% CI 0.72 to 19.81; 4 trials, 359 participants; very low-certainty evidence); microbiological failure (RR 1.65, 95% CI 0.40 to 6.83; 3 trials, 316 participants; very low-certainty evidence); relapse (RR 0.95, 95% CI 0.31 to 2.92; 3 trials, 297 participants; very low-certainty evidence) and time to defervescence (MD 2.73 days, 95% CI -0.37 to 5.84; 3 trials, 285 participants; very low-certainty evidence). It is unclear whether ceftriaxone decreases convalescent faecal carriage compared to the fluoroquinolone gatifloxacin (RR 0.18, 95% CI 0.01 to 3.72; 1 trial, 73 participants; very low-certainty evidence) and length of hospital stay may be longer in participants treated with ceftriaxone compared to participants treated with the fluoroquinolone ofloxacin (mean of 12 days (range 7 to 23 days) in the ceftriaxone group compared to a mean of 9 days (range 6 to 13 days) in the ofloxacin group; 1 trial, 47 participants; low-certainty evidence).

AUTHORS' CONCLUSIONS

Based on very low- to low-certainty evidence, ceftriaxone is an effective treatment for adults and children with enteric fever, with few adverse effects. Trials suggest that there may be no difference in the performance of ceftriaxone compared with azithromycin, fluoroquinolones, or chloramphenicol. Cefixime can also be used for treatment of enteric fever but may not perform as well as fluoroquinolones.  We are unable to draw firm general conclusions on comparative contemporary effectiveness given that most trials were small and conducted over 20 years previously. Clinicians need to take into account current, local resistance patterns in addition to route of administration when choosing an antimicrobial.

Typhoid fever control in the 21st century: where are we now?

PURPOSE OF REVIEW

Momentum for achieving widespread control of typhoid fever has been growing over the past decade. Typhoid conjugate vaccines represent a potentially effective tool to reduce the burden of disease in the foreseeable future and new data have recently emerged to better frame their use-case.

RECENT FINDINGS

We describe how antibiotic resistance continues to pose a major challenge in the treatment of typhoid fever, as exemplified by the emergence of azithromycin resistance and the spread of Salmonella Typhi strains resistant to third-generation cephalosporins. We review efficacy and effectiveness data for TCVs, which have been shown to have high-level efficacy (≥80%) against typhoid fever in diverse field settings. Data from randomized controlled trials and observational studies of TCVs are reviewed herein. Finally, we review data from multicountry blood culture surveillance studies that have provided granular insights into typhoid fever epidemiology. These data are becoming increasingly important as countries decide how best to introduce TCVs into routine immunization schedules and determine the optimal delivery strategy.

SUMMARY

Continued advocacy is needed to address the ongoing challenge of typhoid fever to improve child health and tackle the rising challenge of antimicrobial resistance.

Controlled human infectious models, a path forward in uncovering immunological correlates of protection: Lessons from enteric fevers studies

Enteric infectious diseases account for more than a billion disease episodes yearly worldwide resulting in approximately 2 million deaths, with children under 5 years old and the elderly being disproportionally affected. Enteric pathogens comprise viruses, parasites, and bacteria; the latter including pathogens such as Salmonella [typhoidal (TS) and non-typhoidal (nTS)], cholera, Shigella and multiple pathotypes of Escherichia coli (E. coli). In addition, multi-drug resistant and extensively drug-resistant (XDR) strains (e.g., S. Typhi H58 strain) of enteric bacteria are emerging; thus, renewed efforts to tackle enteric diseases are required. Many of these entero-pathogens could be controlled by oral or parenteral vaccines; however, development of new, effective vaccines has been hampered by lack of known immunological correlates of protection (CoP) and limited knowledge of the factors contributing to protective responses. To fully comprehend the human response to enteric infections, an invaluable tool that has recently re-emerged is the use of controlled human infection models (CHIMs) in which participants are challenged with virulent wild-type (wt) organisms. CHIMs have the potential to uncover immune mechanisms and identify CoP to enteric pathogens, as well as to evaluate the efficacy of therapeutics and vaccines in humans. CHIMs have been used to provide invaluable insights in the pathogenesis, host-pathogen interaction and evaluation of vaccines. Recently, several Oxford typhoid CHIM studies have been performed to assess the role of multiple cell types (B cells, CD8+ T, Tregs, MAIT, Monocytes and DC) during S. Typhi infection. One of the key messages that emerged from these studies is that baseline antigen-specific responses are important in that they can correlate with clinical outcomes. Additionally, volunteers who develop typhoid disease (TD) exhibit higher levels and more activated cell types (e.g., DC and monocytes) which are nevertheless defective in discrete signaling pathways. Future critical aspects of this research will involve the study of immune responses to enteric infections at the site of entry, i.e., the intestinal mucosa. This review will describe our current knowledge of immunity to enteric fevers caused byS. Typhi and S. Paratyphi A, with emphasis on the contributions of CHIMs to uncover the complex immunological responses to these organisms and provide insights into the determinants of protective immunity.

The Burden of Typhoid Fever in Sub-Saharan Africa: A Perspective

While typhoid fever has largely been eliminated in high-income regions which have developed modern water, sanitation, and hygiene facilities, it remains a significant public health burden resulting in morbidity and mortality among millions of individuals in resource-constrained settings. Prevention and control efforts are needed that integrate several high-impact interventions targeting facilities and infrastructure, including those addressing improvements in sanitation, access to safe water, and planned urbanization, together with parallel efforts directed at effective strategies for use of typhoid conjugate vaccines (TCV). The use of TCVs is a critical tool with the potential of having a rapid impact on typhoid fever disease burden; their introduction will also serve as an important strategy to combat evolving antimicrobial resistance to currently available typhoid fever treatments. Well-designed epidemiological surveillance studies play a critical role in establishing the need for, and monitoring the impact of, typhoid fever control and prevention strategies implemented by public health authorities. Here, we present a perspective based on a narrative review of the impact of typhoid fever on morbidity and mortality in sub-Saharan Africa and discuss ongoing surveillance activities and the role of vaccination in prevention and control efforts.

Point-of-Care Ultrasound by Nonexpert Operators Demonstrates High Sensitivity and Specificity in Detecting Gallstones: Data from the Samoa Typhoid Fever Control Program

Approximately 90% of chronic typhoid carriers with persistent Salmonella enterica serovar Typhi (S. Typhi) gallbladder infection have gallstones. In Samoa, where typhoid fever has been endemic for many decades, risk factors predisposing to the development of gallstones are increasing among adults. The Samoa Typhoid Fever Control Program dispatches a "Typhoid Epidemiologic SWAT Team" to perform a household investigation of every blood culture-confirmed case of acute typhoid fever. Investigations include screening household contacts to detect chronic carriers. Following limited training, two nonexpert ultrasound operators performed point-of-care ultrasound (POCUS) on 120 Samoan adults from August to September 2019 to explore the feasibility of POCUS to detect individuals with gallstones during household investigations and community screenings. POCUS scans from 120 Samoan adults in three cohorts (28 food handlers, two typhoid cases and their 18 household contacts, and 72 attendees at an ambulatory clinic) were reviewed by a board-certified radiologist who deemed 96/120 scans (80%) to be interpretable. Compared with the radiologist (gold standard), the nonexpert operators successfully detected 6/7 Samoans with gallstones (85.7% sensitivity) and correctly identified 85/89 without gallstones (95.5% specificity). The proportion (24/120) of uninterpretable scans from this pilot that used minimally trained clinicians (who are neither radiologists nor ultrasound technicians) indicates the need for additional training of POCUS operators. Nevertheless, this pilot feasibility study engenders optimism that in the Samoan setting nonexperts can be trained to use POCUS to diagnose cholelithiasis, thereby helping (along with stool cultures and Vi serology) to identify possible chronic S. Typhi carriers.