Implications and measurement of herd protection (indirect effects) for enteric vaccine development

Indirect Effects of Universal Infant Rotavirus Vaccination: A Narrative Systematic Review

Background/Objective: Rotavirus is a major cause of acute gastroenteritis (AGE) in children <5 years. While rotavirus vaccines are effective in reducing AGE, limited data on their indirect effects exist. The aim of our narrative systematic review was to summarise the indirect effects of rotavirus vaccines on unvaccinated children and adults (PROSPERO: CRD42023418015). Methods: Peer-reviewed articles and conference abstracts were searched through Medline, Embase and PubMed on 8 December 2024. Observational studies of national/regional vaccine introduction were included. We included five outcomes: rotavirus-AGE inpatient admissions, rotavirus-AGE outpatient attendances, all-cause AGE inpatient admissions, all-cause AGE outpatient attendances, and stool rotavirus positivity. Outcome measures reported as percent reduction or individual incidence rates for the pre- and post-introduction periods were transformed to incidence rate ratios (IRRs). Median IRRs and interquartile ranges (IQRs) were calculated for each outcome by age group (<5, 5-19, and >18 years). Results: From an initial 757 articles, 44 studies including 9,327,974 participants were included. In unvaccinated children <5 years, there were reductions in rotavirus-AGE admissions (median IRR: 0.62, IQR: 0.40-0.82), rotavirus-AGE outpatient attendances (0.74, 0.16-0.98), all-cause AGE admissions (0.70, 0.56-0.86), and stool rotavirus positivity (0.42, 0.31-0.57), but not all-cause AGE outpatient attendances (0.92, 0.78-1.17). Few studies reported these outcomes for children and adolescents aged 5-19 years and adults >18 years. Indirect effects appeared to be greater in higher income and lower under-five mortality settings. Conclusions: Understanding these indirect benefits is crucial for evaluating the broader impact and cost-effectiveness of rotavirus immunisation programs.

Vaccine value profile for enterotoxigenic Escherichia coli (ETEC)

Enterotoxigenic Escherichia coli (ETEC) is one of the leading bacterial causes of diarrhoea, especially among children in low-resource settings, and travellers and military personnel from high-income countries. WHO's primary strategic goal for ETEC vaccine development is to develop a safe, effective, and affordable ETEC vaccine that reduces mortality and morbidity due to moderate-to-severe diarrhoeal disease in infants and children under 5 years of age in LMICs, as well as the long-term negative health impact on infant physical and cognitive development resulting from infection with this enteric pathogen. An effective ETEC vaccine will also likely reduce the need for antibiotic treatment and help limit the further emergence of antimicrobial resistance bacterial pathogens. The lead ETEC vaccine candidate, ETVAX, has shown field efficacy in travellers and has moved into field efficacy testing in LMIC infants and children. A Phase 3 efficacy study in LMIC infants is projected to start in 2024 and plans for a Phase 3 trial in travellers are under discussion with the U.S. FDA. Licensing for both travel and LMIC indications is projected to be feasible in the next 5-8 years. Given increasing recognition of its negative impact on child health and development in LMICs and predominance as the leading etiology of travellers' diarrhoea (TD), a standalone vaccine for ETEC is more cost-effective than vaccines targeting other TD pathogens, and a viable commercial market also exists. In contrast, combination of an ETEC vaccine with other vaccines for childhood pathogens in LMICs would maximize protection in a more cost-effective manner than a series of stand-alone vaccines. This 'Vaccine Value Profile' (VVP) for ETEC is intended to provide a high-level, holistic assessment of available data to inform the potential public health, economic and societal value of pipeline vaccines and vaccine-like products. This VVP was developed by a working group of subject matter experts from academia, non-profit organizations, public private partnerships, and multi-lateral organizations. All contributors have extensive expertise on various elements of the ETEC VVP and collectively aimed to identify current research and knowledge gaps. The VVP was developed using only existing and publicly available information.

Vaccine value profile for Shigella

Shigella is the leading bacterial cause of diarrhoea and the second leading cause of diarrhoeal mortality among all ages. It also exhibits increasing levels of antibiotic resistance. The greatest burden is among children under five in low- and middle-income countries (LMICs). As such, a priority strategic goal of the World Health Organization (WHO) is the development of a safe, effective and affordable vaccine to reduce morbidity and mortality from Shigella-attributable dysentery and diarrhea, including long term outcomes associated with chronic inflammation and growth faltering, in children under 5 years of age in LMICs. In addition, a safe and effective Shigella vaccine is of potential interest to travellers and military both to prevent acute disease and rarer, long-term sequelae. An effective Shigella vaccine is also anticipated to reduce antibiotic use and thereby help diminish further emergence of enteric pathogens resistant to antimicrobials. The most advanced vaccine candidates are multivalent, parenteral formulations in Phase 2 and Phase 3 clinical studies. They rely on O-antigen-polysaccharide protein conjugate technologies or, alternatively, outer membrane vesicles expressing penta-acylated lipopolysaccharide that has been detoxified. Other parenteral and oral formulations, many delivering a broader array of Shigella antigens, are at earlier stages of clinical development. These formulations are being assessed in alignment with the WHO Preferred Product Characteristics, which call for a 1 to 2 dose primary immunization series given during the first 12 months of life, ideally starting at 6 months of age. This 'Vaccine Value Profile' (VVP) for Shigella is intended to provide a high-level, holistic assessment of the information and data that are currently available to inform the potential public health, economic and societal value of pipeline vaccines and vaccine-like products. This VVP was developed by a working group of subject matter experts from academia, non-profit organizations, government agencies and multi-lateral organizations. All contributors have extensive expertise on various elements of the Shigella VVP and collectively aimed to identify current research and knowledge gaps. The VVP was developed using only existing and publicly available information.

The success of publicly funded rotavirus vaccine programs for preventing community- and hospital-acquired rotavirus infections in Canadian pediatric hospitals: an observational study

BACKGROUND

Canadian immunization programs for rotavirus started in 2011. We sought to determine their effect on the burden of community-acquired admissions and hospital-acquired rotavirus at pediatric hospitals.

METHODS

The Canadian Immunization Monitoring Program Active (IMPACT) network conducted active surveillance for rotavirus-positive hospital admissions between 2005 and 2020 at 12 pediatric hospitals. We used yearly rates of community-acquired rotavirus per 10 000 admissions and hospital-acquired rotavirus infections per 1000 patient-days to determine changes in the pre- and post-vaccine program periods.

RESULTS

During the 15-year study period, 5691 rotavirus hospital admissions and hospital-acquired infections were detected, including 4323 (76%) community-acquired infections and 1368 (24%) hospital-acquired infections. The average community-acquired rate in the pre-vaccine period was 60.3 (95% confidence interval [CI] 53.7-68.3) per 10 000 admissions, with a decline to 11.0 (95% CI 7.5-15.1) per 10 000 admissions in the post-vaccine period, resulting in an average reduction of 81.7% (95% CI 74.4%-87.8%). The rate of hospital-acquired rotavirus declined from 0.35 (95% CI 0.29-0.41) per 1000 patient-days in the pre-vaccine period to 0.05 (95% CI 0.03-0.07) per 1000 patient-days in the post-vaccine period, resulting in an 85.3% (95% CI 77.7%-91.9%) average decline. Herd protection was present among children aged 2-16 years.

INTERPRETATION

Although start dates of rotavirus vaccine programs across provinces varied, there was around an 80% average decrease in both community-acquired and hospital-acquired rotavirus infections at pediatric hospitals in Canada in the 1- to 9-year interval after implementation of rotavirus vaccine programs. Herd protection is an important aspect of rotavirus vaccines for other children who are not vaccine eligible, and rotavirus vaccines continue to provide important benefits both for children and health care systems.

Etiology and Incidence of Moderate-to-Severe Diarrhea in Young Children in Niger

BACKGROUND

High-resolution data on the etiology of childhood diarrhea in countries with the highest burden and mortality remain sparse and are needed to inform burden estimates and prioritize interventions.

METHODS

We tested stool specimens collected between October 2014 and December 2017 from children under 2 years of age from the per-protocol population of a placebo-controlled clinical trial of a bovine rotavirus pentavalent vaccine (Rotasiil) in Niger. We tested 1729 episodes of moderate-to-severe diarrhea (Vesikari score ≥ 7) using quantitative PCR and estimated pathogen-specific burdens by age, season, severity, and trial intervention arm.

RESULTS

The 4 pathogens with the highest attributable incidence of diarrhea were Shigella (7.2 attributable episodes per 100 child-years; 95% confidence interval: 5.2, 9.7), Cryptosporidium (6.5; 5.8, 7.2), rotavirus (6.4; 5.9, 6.7), and heat-stabile toxin-producing enterotoxigenic Escherichia coli (ST-ETEC) (6.2; 3.1, 7.7). Cryptosporidium was the leading etiology of severe diarrhea (Vesikari score ≥ 11) and diarrhea requiring hospitalization. Shigella was the leading etiology of diarrhea in children 12-23 months of age but also had a substantial burden in the first year of life, with 60.5% of episodes of severe shigellosis occurring in infants. Shigella, Cryptosporidium, and ST-ETEC incidence peaked during the warmer and wetter period and coincided with peak all-cause diarrhea incidence.

CONCLUSIONS

In this high-burden setting, the leading diarrheal pathogens were Shigella, Cryptosporidium, rotavirus, and ST-ETEC, and each was disproportionately seen in infants. Vaccine development should target these pathogens, and the impact of vaccine schedule on diarrhea burden in the youngest children will need to be considered.