Evaluation of trivalent, live, cold-adapted (CAIV-T) and inactivated (TIV) influenza vaccines in prevention of virus infection and illness following challenge of adults with wild-type influenza A (H1N1), A (H3N2), and B viruses

Mucosal vaccines for viral diseases: Status and prospects

Virus-associated infectious diseases are highly detrimental to human health and animal husbandry. Among all countermeasures against infectious diseases, prophylactic vaccines, which developed through traditional or novel approaches, offer potential benefits. More recently, mucosal vaccines attract attention for their extraordinary characteristics compared to conventional parenteral vaccines, particularly for mucosal-related pathogens. Representatively, coronavirus disease 2019 (COVID-19), a respiratory disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), further accelerated the research and development efforts for mucosal vaccines by thoroughly investigating existing strategies or involving novel techniques. While several vaccine candidates achieved positive progresses, thus far, part of the current COVID-19 mucosal vaccines have shown poor performance, which underline the need for next-generation mucosal vaccines and corresponding platforms. In this review, we summarized the typical mucosal vaccines approved for humans or animals and sought to elucidate the underlying mechanisms of these successful cases. In addition, mucosal vaccines against COVID-19 that are in human clinical trials were reviewed in detail since this public health event mobilized all advanced technologies for possible solutions. Finally, the gaps in developing mucosal vaccines, potential solutions and prospects were discussed. Overall, rational application of mucosal vaccines would facilitate the establishing of mucosal immunity and block the transmission of viral diseases.

Intranasal influenza-vectored COVID-19 vaccines confer broad protection against SARS-CoV-2 XBB variants in hamsters

The XBB.1.5 subvariant has garnered significant attention due to its exceptional immune evasion and transmissibility. Significantly, the evolutionary trajectory of SARS-CoV-2 has shown continual progression, with a recent global shift observed from XBB to BA.2.86, exemplified by the emergence of the predominant JN.1 subvariant. This phenomenon highlights the need for vaccines that can provide broad-spectrum antigenic coverage. In this study, we utilized a NS1-deleted (dNS1) influenza viral vector to engineer an updated live-attenuated vectored vaccine called dNS1-XBB-RBD. This vaccine encodes the receptor-binding domain (RBD) protein of the XBB.1.5 strain. Our findings demonstrate that the dNS1-XBB-RBD vaccine elicits a similar systemic and mucosal immune response compared to its prototypic form, dNS1-RBD. In hamsters, the dNS1-XBB-RBD vaccine provided robust protection against the SARS-CoV-2 immune-evasive strains XBB.1.9.2.1 and Beta. Remarkably, nasal vaccination with dNS1-RBD, which encodes the ancestor RBD gene, also effectively protected hamsters against both the XBB.1.9.2.1 and Beta strains. These results provide valuable insights about nasal influenza-vectored vaccine and present a promising strategy for the development of a broad-spectrum vaccine against COVID-19 in the future.

Progress and challenges in the clinical evaluation of immune responses to respiratory mucosal vaccines

INTRODUCTION

Following the coronavirus disease pandemic, respiratory mucosal vaccines that elicit both mucosal and systemic immune responses have garnered increasing attention. However, human physiological characteristics pose significant challenges in the evaluation of mucosal immunity, which directly impedes the development and application of respiratory mucosal vaccines.

AREAS COVERED

This study summarizes the characteristics of immune responses in the respiratory mucosa and reviews the current status and challenges in evaluating immune response to respiratory mucosal vaccines.

EXPERT OPINION

Secretory Immunoglobulin A (S-IgA) is a major effector molecule at mucosal sites and a commonly used indicator for evaluating respiratory mucosal vaccines. However, the unique physiological structure of the respiratory tract pose significant challenges for the clinical collection and detection of S-IgA. Therefore, it is imperative to develop a sampling method with high collection efficiency and acceptance, a sensitive detection method, reference materials for mucosal antibodies, and to establish a threshold for S-IgA that correlates with clinical protection. Sample collection is even more challenging when evaluating mucosal cell immunity. Therefore, a mucosal cell sampling method with high operability and high tolerance should be established. Targets of the circulatory system capable of reflecting mucosal cellular immunity should also be explored.

Leveraging human immune organoids for rational vaccine design

Current influenza A and B virus (IABV) vaccines provide suboptimal protection and efforts are underway to develop a universal IABV vaccine. Blood neutralizing antibodies are the current gold standard for protection, but many processes that regulate human IABV-specific immunity occur in mucosal and lymphoid tissues. We need an improved mechanistic understanding of how immune cells respond within these tissues to advance our current (slow and expensive) vaccine testing model. We posit that advanced in vitro models of human adaptive immunity can bridge some of the gaps between vaccine design, animal models, and human clinical trials. Here, we highlight how they can be integrated into current practices and play a role in reverse translating the defined features of protective vaccines to rationally design new candidates.

Safety and efficacy of the intranasal spray SARS-CoV-2 vaccine dNS1-RBD: a multicentre, randomised, double-blind, placebo-controlled, phase 3 trial

BACKGROUND

The live-attenuated influenza virus vector-based intranasal SARS-CoV-2 vaccine (dNS1-RBD, Pneucolin; Beijing Wantai Biological Pharmacy Enterprise, Beijing, China) confers long-lasting and broad protection in animal models and is, to our knowledge, the first COVID-19 mucosal vaccine to enter into human trials, but its efficacy is still unknown. We aimed to assess the safety and efficacy (but not the immunogenicity) of dNS1-RBD against COVID-19.

METHODS

We did a multicentre, randomised, double-blind, placebo-controlled, adaptive design, phase 3 trial at 33 centres (private or public hospitals, clinical research centres, or Centre for Disease Control and Prevention) in four countries (Colombia, Philippines, South Africa, and Viet Nam). Men and non-pregnant women (aged ≥18 years) were eligible if they had never been infected with SARS-CoV-2, and if they did not have a SARS-CoV-2 vaccination history at screening or if they had received at least one dose of other SARS-CoV-2 vaccines 6 months or longer before enrolment. Eligible adults were randomly assigned (1:1) to receive two intranasal doses of dNS1-RBD or placebo administered 14 days apart (0·2 mL per dose; 0·1 mL per nasal cavity), with block randomisation via an interactive web-response system, stratified by centre, age group (18-59 years or ≥60 years), and SARS-CoV-2 vaccination history. All participants, investigators, and laboratory staff were masked to treatment allocation. The primary outcomes were safety of dNS1-RBD in the safety population (ie, those who had received at least one dose of dNS1-RBD or placebo) and efficacy against symptomatic SARS-CoV-2 infection confirmed by RT-PCR occurring 15 days or longer after the second dose in the per-protocol population (ie, those who received two doses, were followed up for 15 days or longer after the second dose, and had no major protocol deviations). The success criterion was predefined as vaccine efficacy of more than 30%. This trial is registered with the Chinese Clinical Trial Registry (ChiCTR2100051391) and is completed.

FINDINGS

Between Dec 16, 2021, and May 31, 2022, 41 620 participants were screened for eligibility and 31 038 participants were enrolled and randomly assigned (15 517 in the vaccine group and 15 521 in the placebo group). 30 990 participants who received at least one dose (15 496 vaccine and 15 494 placebo) were included in the safety analysis. The results showed a favourable safety profile, with the most common local adverse reaction being rhinorrhoea (578 [3·7%] of 15 500 vaccine recipients and 546 [3·5%] of 15 490 placebo recipients) and the most common systemic reaction being headache (829 [5·3%] vaccine recipients and 797 [5·1%] placebo recipients). We found no differences in the incidences of adverse reactions between participants in the vaccine and placebo groups. No vaccination-related serious adverse events or deaths were observed. Among 30 290 participants who received two doses, 25 742 were included in the per-protocol efficacy analysis (12 840 vaccine and 12 902 placebo). The incidence of confirmed symptomatic SARS-CoV-2 infection caused by omicron variants regardless of immunisation history was 1·6% in the vaccine group and 2·3% in the placebo group, resulting in an overall vaccine efficacy of 28·2% (95% CI 3·4-46·6), with a median follow-up duration of 161 days.

INTERPRETATION

Although this trial did not meet the predefined efficacy criteria for success, dNS1-RBD was well tolerated and protective against omicron variants, both as a primary immunisation and as a heterologous booster.

FUNDING

Beijing Wantai Biological Pharmacy Enterprise, National Science and Technology Major Project, National Natural Science Foundation of China, Fujian Provincial Science and Technology Plan Project, Natural Science Foundation of Fujian Province, Xiamen Science and Technology Plan Special Project, Bill & Melinda Gates Foundation, the Ministry of Education of China, Xiamen University, and Fieldwork Funds of Xiamen University.

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.

Novel Influenza Vaccines: From Research and Development (R&D) Challenges to Regulatory Responses

Influenza vaccines faced significant challenges in achieving sufficient protective efficacy and production efficiency in the past. In recent decades, novel influenza vaccines, characterized by efficient and scalable production, advanced platforms, and new adjuvant technologies, have overcome some of these weaknesses and have been widely licensed. Furthermore, researchers are actively pursuing the development of next-generation and universal influenza vaccines to provide comprehensive protection against potential pandemic subtypes or strains. However, new challenges have emerged as these novel vaccines undergo evaluation and authorization. In this review, we primarily outline the critical challenges and advancements in research and development (R&D) and highlight the improvements in regulatory responses for influenza vaccines.

Influenza vaccine format mediates distinct cellular and antibody responses in human immune organoids

Highly effective vaccines elicit specific, robust, and durable adaptive immune responses. To advance informed vaccine design, it is critical that we understand the cellular dynamics underlying responses to different antigen formats. Here, we sought to understand how antigen-specific B and T cells were activated and participated in adaptive immune responses within the mucosal site. Using a human tonsil organoid model, we tracked the differentiation and kinetics of the adaptive immune response to influenza vaccine and virus modalities. Each antigen format elicited distinct B and T cell responses, including differences in their magnitude, diversity, phenotype, function, and breadth. These differences culminated in substantial changes in the corresponding antibody response. A major source of antigen format-related variability was the ability to recruit naive vs. memory B and T cells to the response. These findings have important implications for vaccine design and the generation of protective immune responses in the upper respiratory tract.

Cost-effectiveness of routine annual influenza vaccination by age and risk status

BACKGROUND

The epidemiology of circulating seasonal influenza strains changed following the 2009 pandemic influenza A(H1N1). A universal influenza vaccination recommendation has been implemented and new vaccine types have become available post-2009. The objective of this study was to evaluate the cost-effectiveness of routine annual influenza vaccination in the context of this new evidence.

METHODS

A state transition simulation model was constructed to estimate the health and economic outcomes of influenza vaccination compared to no vaccination for hypothetical US cohorts stratified by age and risk status. Model input parameters were derived from multiple sources, including post-2009 vaccine effectiveness data from the US Flu Vaccine Effectiveness Network. The analysis used societal and healthcare sector perspectives and a one-year time horizon, except permanent outcomes were also included. The primary outcome was the incremental cost-effectiveness ratio (ICER) in dollars per quality-adjusted life years (QALYs) gained.

RESULTS

Compared to no vaccination, vaccination yielded ICERs lower than $95,000/QALY for all age and risk groups, except for non-high-risk adults 18-49 years ($194,000/QALY). Vaccination was cost-saving for adults ≥50 years at higher risk for influenza-related complications. Results were most sensitive to changes in the probability of influenza illness. Performing the analysis from the healthcare sector perspective, excluding vaccination time costs, delivering vaccinations in lower-cost settings, and including productivity losses improved the cost-effectiveness of vaccination. Sensitivity analysis revealed that vaccination remains below $100,000/QALY for older persons ≥65 years at vaccine effectiveness estimates as low as 4 %.

CONCLUSIONS

Cost-effectiveness of influenza vaccination varied by age and risk status and was less than $95,000/QALY for all subgroups, except for non-high-risk working-age adults. Results were sensitive to the probability of influenza illness and vaccination was more favorable under certain scenarios. Vaccination for higher risk subgroups resulted in ICERs below $100,000/QALY even at low levels of vaccine effectiveness or circulating virus.

Viral vectored vaccines: design, development, preventive and therapeutic applications in human diseases

Human diseases, particularly infectious diseases and cancers, pose unprecedented challenges to public health security and the global economy. The development and distribution of novel prophylactic and therapeutic vaccines are the prioritized countermeasures of human disease. Among all vaccine platforms, viral vector vaccines offer distinguished advantages and represent prominent choices for pathogens that have hampered control efforts based on conventional vaccine approaches. Currently, viral vector vaccines remain one of the best strategies for induction of robust humoral and cellular immunity against human diseases. Numerous viruses of different families and origins, including vesicular stomatitis virus, rabies virus, parainfluenza virus, measles virus, Newcastle disease virus, influenza virus, adenovirus and poxvirus, are deemed to be prominent viral vectors that differ in structural characteristics, design strategy, antigen presentation capability, immunogenicity and protective efficacy. This review summarized the overall profile of the design strategies, progress in advance and steps taken to address barriers to the deployment of these viral vector vaccines, simultaneously highlighting their potential for mucosal delivery, therapeutic application in cancer as well as other key aspects concerning the rational application of these viral vector vaccines. Appropriate and accurate technological advances in viral vector vaccines would consolidate their position as a leading approach to accelerate breakthroughs in novel vaccines and facilitate a rapid response to public health emergencies.

Nasal vaccines: solutions for respiratory infectious diseases

Nasal vaccines induce pathogen-specific dual protective immunity at mucosal surfaces and systemically throughout the body. Consequently, nasal vaccines both prevent pathogen invasion and reduce disease severity. Because of these features, nasal vaccines are considered to be a next-generation tool for preventing respiratory infectious diseases, including COVID-19. However, nasal vaccines must overcome key safety concerns given the anatomic proximity of the central nervous system (CNS) via the olfactory bulbs which lie next to the nasal cavity. This review summarizes current efforts to develop safe and effective nasal vaccines and delivery systems, as well as their clinical applications for the prevention of respiratory infections. We also discuss various concerns regarding the safety of nasal vaccines and introduce a system for evaluating them.

Revisiting live attenuated influenza vaccine efficacy among children in developing countries

Seasonal influenza epidemics cause significant pediatric mortality and morbidity worldwide. Live attenuated influenza vaccines (LAIVs) can be administered intranasally, induce a broad and robust immune response, demonstrate higher yields during manufacturing as compared to inactivated influenza vaccines (IIVs), and thereby represent an attractive possibility for young children in developing countries. We summarize recent pediatric studies evaluating LAIV efficacy in developing countries where a large proportion of the influenza-virus-associated respiratory disease burden occurs. Recently, two randomized controlled trials (RCTs) assessing Russian-backbone trivalent LAIV in children reported contradictory results; vaccine efficacy varied between Bangladesh (41 %) and Senegal (0.0 %) against all influenza viral strains. Prior to 2013, Ann Arbor-based LAIV demonstrated superior efficacy as compared to IIV. However, due to low effectiveness of the Ann Arbor-based LAIV against influenza A(H1N1)pdm09-like viruses, the CDC Advisory Committee on Immunization Practices (ACIP) recommended against the use of LAIV during the 2016-17 and 2017-18 influenza seasons. Reduced replicative fitness of the A(H1N1)pdm09 LAIV strains is thought to have led to the low effectiveness of the Ann-Arbor-based LAIV. Once the A(H1N1)pdm09 component was updated, the ACIP reintroduced the Ann-Arbor-based LAIV as a vaccine choice for the 2018-19 influenza season. In 2021, results from a 2-year RCT evaluating the Russian-backbone trivalent LAIV in rural north India reported that LAIV demonstrated significantly lower efficacy compared to IIV, but in Year 2, the vaccine efficacy for LAIV and IIV was comparable. A profounder understanding of the mechanisms underlying varied efficacy of LAIV in developing countries is warranted. Assessing replicative fitness, in addition to antigenicity, when selecting annual A(H1N1)pdm09 components in the Russian-backbone trivalent LAIVs is essential and may ultimately, enable widespread utility in resource-poor settings.

Intranasal M2SR (M2-Deficient Single Replication) H3N2 Influenza Vaccine Provides Enhanced Mucosal and Serum Antibodies in Adults

BACKGROUND

We previously demonstrated that an intranasal dose of 108 50% tissue culture infectious dose (TCID50) M2-deficient single replication (M2SR) influenza vaccine protected against highly drifted H3N2 influenza challenge in a subset of subjects who demonstrated ≥2-fold increase in microneutralization (MN) antibodies to Belgium2015 (the challenge strain) after vaccination. Here, we describe a phase 1b, observer-blinded, dose-escalation study demonstrating an increased proportion of responders with this signal of immune protection.

METHODS

Serosusceptible subjects aged 18-49 years were randomized to receive 2 doses (108-109 TCID50) of M2SR or placebo administered 28 days apart. Clinical specimens were collected before and after each dose. The primary objective was to demonstrate safety of M2SR vaccines.

RESULTS

The vaccine was well tolerated at all dose levels. Against Belgium2015, ≥ 2-fold increases in MN antibodies were noted among 40% (95% confidence interval [CI], 24.9%-56.7%) of subjects following a single 108 TCID50 M2SR dose and among 80.6% (95% CI, 61.4%-92.3%) after 109 dose (P < .001). A single 109 TCID50 dose of M2SR generated ≥4-fold hemagglutination inhibition antibody seroconversion against the vaccine strain in 71% (95% CI, 52.0%-85.8%) of recipients. Mucosal and cellular immune responses were also induced.

CONCLUSIONS

These results indicate that M2SR may provide substantial protection against infection with highly drifted strains of H3N2 influenza.

CLINICAL TRIALS REGISTRATION

NCT03999554.

HA and M2 sequences alter the replication of 2013-16 H1 live attenuated influenza vaccine infection in human nasal epithelial cell cultures

From 2013 to 2016, the H1N1 component of live, attenuated influenza vaccine (LAIV) performed very poorly in contrast to the inactivated influenza vaccine. We utilized a primary, differentiated human nasal epithelial cell (hNEC) culture system to assess the replication differences between isogenic LAIVs containing the HA segment from either A/Bolivia/559/2013 (rBol), which showed poor vaccine efficacy, and A/Slovenia/2903/2015 (rSlov), which had reasonable vaccine efficacy. There were minimal differences in infectious virus production in Madin-Darby Canine Kidney (MDCK) cells, but the rSlov LAIV showed markedly improved replication in hNEC cultures at both 32 °C and 37 °C, demonstrating that the HA segment alone could impact LAIV replication in physiologically relevant systems. The rSlov-infected hNEC cultures showed stronger production of interferon and proinflammatory chemokines which might also be contributing to the increased overall vaccine effectiveness through enhanced recruitment and activation of immune cells. An M2-S86A mutation had no positive effects on H1 LAIV replication in hNEC cultures, in contrast to the increased infectious virus production seen in an H3 LAIV. No obvious defects in viral RNA packaging were detected, suggesting that HA function, rather than defective particle production, may be driving the differential infectious virus production in hNEC cultures. Overall, we have shown that not all H1 HA segments can be successfully used in LAIV, and this phenotype cannot be fully explained by segment incompatibilities. Physiologically relevant temperatures and primary cell cultures should be used to demonstrate that candidate LAIVs can replicate efficiently, which is a necessary property for effective vaccines.

Safety and immunogenicity of a live-attenuated influenza virus vector-based intranasal SARS-CoV-2 vaccine in adults: randomised, double-blind, placebo-controlled, phase 1 and 2 trials

Background

All currently available SARS-CoV-2 vaccines are administered by intramuscular injection. We aimed to evaluate the safety and immunogenicity of a live-attenuated influenza virus vector-based SARS-CoV-2 vaccine (dNS1-RBD) administered by intranasal spray in healthy adults.

Methods

We did double-blind, randomised, placebo-controlled phase 1 and 2 trials, followed by a phase 2 extension trial, at a single centre in Jiangsu, China. Healthy adults (≥18 years) who had negative serum or fingertip blood total antibody tests for SARS-CoV-2 (in phases 1 and 2), with no prevalent SARS-CoV-2 infection or history of infection and no SARS-CoV-2 vaccination history (in all three trials reported here), were enrolled. Participants were randomly allocated (4:1 in phase 1, 2:1 in phase 2, and 1:1 in the extension trial) to receive two intranasal doses of the dNS1-RBD vaccine or placebo on days 0 and 14 or, for half of the participants in phase 2, on days 0 and 21. To avoid cross-contamination during administration, vaccine and placebo recipients were vaccinated in separate rooms in the extension trial. The phase 1 primary outcome was safety (adverse events recorded on days 0–44; serious adverse events recorded from day 0 until 12 months after the second dose). In the phase 2 and extension trials, the primary immunogenicity outcomes were SARS-CoV-2-specific T-cell response in peripheral blood (measured by IFN-γ ELISpot), proportion of participants with positive conversion for SARS-CoV-2 receptor-binding domain (RBD)-specific IgG and secretory IgA (s-IgA) antibodies, and concentration of SARS-CoV-2 RBD IgG in serum and SARS-CoV-2 RBD s-IgA in the nasopharynx (measured by ELISA) at 1 month after the second dose in the per-protocol set for immunogenicity. χ² test and Fisher's exact test were used to analyse categorical data, and t test and Wilcoxon rank sum test to compare the measurement data between groups. These trials were registered with the Chinese Clinical Trial Registry (ChiCTR2000037782, ChiCTR2000039715, and ChiCTR2100048316).

Findings

Between Sept 1, 2020, and July 4, 2021, 63, 724, and 297 participants without a history of SARS-CoV-2 vaccination were enrolled in the phase 1, phase 2, and extension trials, respectively. At least one adverse reaction after vaccination was reported in 133 (19%) of 684 participants in the vaccine groups. Most adverse reactions were mild. No vaccine-related serious adverse event was noted. Specific T-cell immune responses were observed in 211 (46% [95% CI 42–51]) of 455 vaccine recipients in the phase 2 trial, and in 48 (40% [31–49]) of 120 vaccine recipients compared with one (1% [0–5]) of 111 placebo recipients (p<0·0001) in the extension trial. Seroconversion for RBD-specific IgG was observed in 48 (10% [95% CI 8–13]) of 466 vaccine recipients in the phase 2 trial (geometric mean titre [GMT] 3·8 [95% CI 3·4–4·3] in responders), and in 31 (22% [15–29]) of 143 vaccine recipients (GMT 4·4 [3·3–5·8]) and zero (0% [0–2]) of 147 placebo recipients (p<0·0001) in the extension trial. 57 (12% [95% CI 9–16]) of 466 vaccine recipients had positive conversion for RBD-specific s-IgA (GMT 3·8 [95% CI 3·5–4·1] in responders) in the phase 2 trial, as did 18 (13% [8–19]) of 143 vaccine recipients (GMT 5·2 [4·0–6·8]) and zero (0% [0–2]) of 147 placebo recipients (p<0·0001) in the extension trial.

Interpretation

dNS1-RBD was well tolerated in adults. Weak T-cell immunity in peripheral blood, as well as weak humoral and mucosal immune responses against SARS-CoV-2, were detected in vaccine recipients. Further studies are warranted to verify the safety and efficacy of intranasal vaccines as a potential supplement to current intramuscular SARS-CoV-2 vaccine pools. Steps should be taken in future studies to reduce the potential for cross-contamination caused by the vaccine strain aerosol during administration.

Funding

National Key Research and Development Program of China, National Science, Fujian Provincial Science, CAMS Innovation Fund for Medical Sciences, and Beijing Wantai Biological Pharmacy Enterprise.

A live attenuated virus-based intranasal COVID-19 vaccine provides rapid, prolonged, and broad protection against SARS-CoV-2

Remarkable progress has been made in developing intramuscular vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2); however, they are limited with respect to eliciting local immunity in the respiratory tract, which is the primary infection site for SARS-CoV-2. To overcome the limitations of intramuscular vaccines, we constructed a nasal vaccine candidate based on an influenza vector by inserting a gene encoding the receptor-binding domain (RBD) of the spike protein of SARS-CoV-2, named CA4-dNS1-nCoV-RBD (dNS1-RBD). A preclinical study showed that in hamsters challenged 1 d after single-dose vaccination or 9 months after booster vaccination, dNS1-RBD largely mitigated lung pathology, with no loss of body weight. Moreover, such cellular immunity is relatively unimpaired for the most concerning SARS-CoV-2 variants, especially for the latest Omicron variant. In addition, this vaccine also provides cross-protection against H1N1 and H5N1 influenza viruses. The protective immune mechanism of dNS1-RBD could be attributed to the innate immune response in the nasal epithelium, local RBD-specific T cell response in the lung, and RBD-specific IgA and IgG response. Thus, this study demonstrates that the intranasally delivered dNS1-RBD vaccine candidate may offer an important addition to the fight against the ongoing coronavirus disease 2019 pandemic and influenza infection, compensating limitations of current intramuscular vaccines.

Safety and Efficacy of Spray Intranasal Live Attenuated Influenza Vaccine: Systematic Review and Meta-Analysis

Although influenza is a major public health concern, little is known about the use of spray live attenuated influenza vaccine (LAIV) among adults. For this reason, we conducted a systematic review and meta-analysis to investigate the efficacy and safety of LAIV, especially in adults with/without clinical conditions and children <2 years, with the final aim of possibly extending the clinical indications. PubMed/MEDLINE and Scopus were the two databases consulted through February 2021. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines were followed. A critical appraisal was conducted. Analyses were performed by using ProMeta3 software. Twenty-two studies were included, showing that LAIV was associated with a higher probability of seroconversion when compared with a placebo and considering the A/H1N1 serotype (pooled OR = 2.26 (95% CI = 1.12–4.54), p-value = 0.022; based on 488 participants, without heterogeneity (I2 = 0.0%)). The meta-analysis also confirmed no significant association with systemic adverse events. Only rhinorrhea, nasal congestion, and sore throat were significantly associated with LAIV compared to the placebo. Despite limited available evidence, LAIV has proved to be a safe and effective flu vaccination, also due to its very low invasiveness, and our review’s results can be considered a starting point for guiding future research and shaping forthcoming vaccination campaigns.

Single Replication M2SR Influenza Vaccine Induced Immune Responses Associated with Protection Against Human Challenge with Highly Drifted H3N2 Influenza Strain

Background

Current influenza vaccines are strain specific and demonstrate low vaccine efficacy against H3N2 influenza disease, especially when vaccine is mismatched to circulating virus. The novel influenza vaccine candidate, M2-deficient single replication (M2SR), induces a broad, multi-effector immune response.

Methods

A phase 2 challenge study was conducted to assess the efficacy of an M2SR vaccine expressing hemagglutinin and neuraminidase from A/Brisbane/10/2007 (Bris2007 M2SR H3N2; clade 1). Four weeks after vaccination, recipients were challenged with antigenically distinct H3N2 virus (A/Belgium/4217/2015, clade 3C.3b) and assessed for infection and clinical symptoms.

Results

Adverse events after vaccination were mild and similar in frequency for placebo and M2SR recipients. A single dose of Bris2007 M2SR induced neutralizing antibody to the vaccine (48% of recipients) and challenge strain (27% of recipients). Overall, 54% of M2SR recipients were infected after challenge, compared with 71% of placebo recipients. The subset of M2SR recipients with a vaccine-induced microneutralization response against the challenge virus had reduced rates of infection after challenge (38% vs 71% of placebo recipients; P = .050) and reduced illness.

Conclusions

Study participants with vaccine-induced neutralizing antibodies were protected against infection and illness after challenge with an antigenically distinct virus. This is the first demonstration of vaccine-induced protection against a highly drifted H3N2 challenge virus.

Viewpoint of a WHO Advisory Group Tasked to Consider Establishing a Closely-monitored Challenge Model of Coronavirus Disease 2019 (COVID-19) in Healthy Volunteers

WHO convened an Advisory Group (AG) to consider the feasibility, potential value, and limitations of establishing a closely-monitored challenge model of experimental severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and coronavirus disease 2019 (COVID-19) in healthy adult volunteers. The AG included experts in design, establishment, and performance of challenges. This report summarizes issues that render a COVID-19 model daunting to establish (the potential of SARS-CoV-2 to cause severe/fatal illness, its high transmissibility, and lack of a "rescue treatment" to prevent progression from mild/moderate to severe clinical illness) and it proffers prudent strategies for stepwise model development, challenge virus selection, guidelines for manufacturing challenge doses, and ways to contain SARS-CoV-2 and prevent transmission to household/community contacts. A COVID-19 model could demonstrate protection against virus shedding and/or illness induced by prior SARS-CoV-2 challenge or vaccination. A limitation of the model is that vaccine efficacy in experimentally challenged healthy young adults cannot per se be extrapolated to predict efficacy in elderly/high-risk adults.

Exploring Risks of Human Challenge Trials For COVID-19

Human challenge trials (HCTs) are a potential method to accelerate development of vaccines and therapeutics. However, HCTs for COVID-19 pose ethical and practical challenges, in part due to the unclear and developing risks. In this article , we introduce an interactive model for exploring some risks of a severe acute respiratory syndrome coronavirus-2 (SARS-COV-2) dosing study, a prerequisite for any COVID-19 challenge trials. The risk estimates we use are based on a Bayesian evidence synthesis model which can incorporate new data on infection fatality risks (IFRs) to patients, and infer rates of hospitalization. The model estimates individual risk, which we then extrapolate to overall mortality and hospitalization risk in a dosing study. We provide a web tool to explore risk under different study designs. Based on the Bayesian model, IFR for someone between 20 and 30 years of age is 15.1 in 100,000, with a 95% uncertainty interval from 11.8 to 19.2, while risk of hospitalization is 130 per 100,000 (100-160). However, risk will be reduced in an HCT via screening for comorbidities, selecting lower-risk population, and providing treatment. Accounting for this with stronger assumptions, we project the fatality risk to be as low as 2.5 per 100,000 (1.6-3.9) and the hospitalization risk to be 22.0 per 100,000 (14.0-33.7). We therefore find a 50-person dosing trial has a 99.74% (99.8-99.9%) chance of no fatalities, and a 98.9% (98.3-99.3%) probability of no cases requiring hospitalization.

Review of infective dose, routes of transmission and outcome of COVID-19 caused by the SARS-COV-2: comparison with other respiratory viruses

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is pandemic. Prevention and control strategies require an improved understanding of SARS-CoV-2 dynamics. We did a rapid review of the literature on SARS-CoV-2 viral dynamics with a focus on infective dose. We sought comparisons of SARS-CoV-2 with other respiratory viruses including SARS-CoV-1 and Middle East respiratory syndrome coronavirus. We examined laboratory animal and human studies. The literature on infective dose, transmission and routes of exposure was limited specially in humans, and varying endpoints were used for measurement of infection. Despite variability in animal studies, there was some evidence that increased dose at exposure correlated with higher viral load clinically, and severe symptoms. Higher viral load measures did not reflect coronavirus disease 2019 severity. Aerosol transmission seemed to raise the risk of more severe respiratory complications in animals. An accurate quantitative estimate of the infective dose of SARS-CoV-2 in humans is not currently feasible and needs further research. Our review suggests that it is small, perhaps about 100 particles. Further work is also required on the relationship between routes of transmission, infective dose, co-infection and outcomes.

Prevention of Respiratory Syncytial Virus Infection in Healthy Adults by a Single Immunization of Ad26.RSV.preF in a Human Challenge Study

BACKGROUND

Respiratory syncytial virus (RSV) is a significant cause of severe lower respiratory tract disease in children and older adults, but has no approved vaccine. This study assessed the potential of Ad26.RSV.preF to protect against RSV infection and disease in an RSV human challenge model.

METHODS

In this double-blind, placebo-controlled study, healthy adults aged 18-50 years were randomized 1:1 to receive 1x1011 vp Ad26.RSV.preF or placebo intramuscularly. Twenty-eight days post-immunization, volunteers were challenged intranasally with RSV-A (Memphis 37b). Assesments included viral load (VL), RSV infections, clinical symptom score (CSS), safety and immunogenicity.

RESULTS

Post-challenge, VL, RSV infections and disease severity were lower in Ad26.RSV.preF (n=27) versus placebo (n=26) recipients: median VL-AUC (area under the curve) qRT-PCR: 0.0 versus 236.0 (P=.012; predefined primary endpoint); median VL-AUC quantitative culture: 0.0 versus 109; RSV infections 11 (40.7%) versus 17 (65.4%); median RSV AUC-CSS 35 versus 167, respectively. From baseline to 28 days post-immunization, geometric mean fold-increases in RSV A2 neutralizing antibody titers of 5.8 and 0.9 were observed in Ad26.RSV.preF and placebo, respectively. Ad26.RSV.preF was well tolerated.

CONCLUSIONS

Ad26.RSV.preF demonstrated protection from RSV infection through immunization in a human challenge model, and therefore could potentially protect against natural RSV infection and disease.

CLINICAL TRIALS REGISTRATION

NCT03334695.

Influenza in Children

Influenza poses a significant disease burden on children worldwide, with high rates of hospitalization and substantial morbidity and mortality. Although the clinical presentation of influenza in children has similarities to that seen in adults, there are unique aspects to how children present with infection that are important to recognize. In addition, children play a significant role in viral transmission within communities. Growing evidence supports the idea that early influenza infection can uniquely establish lasting immunologic memory, making an understanding of how viral immunity develops in this population critical to better protect children from infection and to facilitate efforts to develop a more universally protective influenza vaccine.

A Mutated PB1 Residue 319 Synergizes with the PB2 N265S Mutation of the Live Attenuated Influenza Vaccine to Convey Temperature Sensitivity

Current influenza vaccines have modest efficacy. This is especially true for current live attenuated influenza vaccines (LAIV), which have been inferior to the inactivated versions in recent years. Therefore, a new generation of live vaccines may be needed. We previously showed that a mutation at PB1 residue 319 confers enhanced temperature sensitivity and attenuation in an LAIV constructed in the genetic background of the mouse-adapted Influenza A Virus (IAV) strain A/PR/8/34 (PR8). Here, we describe the origin/discovery of this unique mutation and demonstrate that, when combined with the PB2 N265S mutation of LAIV, it conveys an even greater level of temperature sensitivity and attenuation on PR8 than the complete set of attenuating mutations from LAIV. Furthermore, we show that the combined PB1 L319Q and PB2 N265S mutations confer temperature sensitivity on IAV polymerase activity in two different genetic backgrounds, PR8 and A/Cal/04/09. Collectively, these findings show that the PB2 LAIV mutation synergizes with a mutation in PB1 and may have potential utility for improving LAIVs.

A Dose-finding Study of a Wild-type Influenza A(H3N2) Virus in a Healthy Volunteer Human Challenge Model

BACKGROUND

The development of vaccines and therapeutics has relied on healthy volunteer influenza challenge studies. A validated human infection model with wild-type A(H1N1)pdm09 was reported previously. Our objective was to characterize a wild-type influenza A/Bethesda/MM1/H3N2 challenge virus in healthy volunteers.

METHODS

Participants received a single dose of a cell-based, reverse-genetics, Good Manufacturing Practices-produced wild-type influenza A(H3N2)2011 virus intranasally and were isolated at the National Institutes of Health Clinical Center for ≥9 days. Dose escalation was performed from 104 to 107 TCID50 (50% tissue culture infectious dose). Viral shedding and clinical disease were evaluated daily.

RESULTS

Of 37 participants challenged, 16 (43%) had viral shedding and 27 (73%) developed symptoms, with 12 (32%) participants experiencing mild to moderate influenza disease (MMID), defined as shedding and symptoms. Only participants receiving 106 and 107 TCID50 experienced MMID at 44% and 40%, respectively. Symptom severity peaked on day 3, whereas most viral shedding occurred 1-2 days after challenge. Only 10 (29%) participants had a ≥4-fold rise in hemagglutination inhibition antibody titer after challenge.

CONCLUSIONS

The A/Bethesda/MM1/H3N2 challenge virus safely induced MMID in healthy volunteers, but caused less MMID than the A(H1N1)pdm09 challenge virus even at the highest dose. There was less detection of shedding though the incidence of symptoms was similar to A(H1N1)pdm09. Fewer serum anti-hemagglutinin (HA) antibody responses with less MMID indicate that preexisting immunity factors other than anti-HA antibody may limit shedding in healthy volunteers. This A/Bethesda/MM1/H3N2 challenge virus can be utilized in future studies to further explore pathogenesis and immunity and to evaluate vaccine candidates.

CLINICAL TRIALS REGISTRATION

NCT02594189.

Immune Imprinting in the Influenza Ferret Model

The initial exposure to influenza virus usually occurs during childhood. This imprinting has long-lasting effects on the immune responses to subsequent infections and vaccinations. Animal models that are used to investigate influenza pathogenesis and vaccination do recapitulate the pre-immune history in the human population. The establishment of influenza pre-immune ferret models is necessary for understanding infection and transmission and for designing efficacious vaccines.

Understanding Immunity in Children Vaccinated With Live Attenuated Influenza Vaccine

Live attenuated influenza vaccine (LAIV), or FluMist, was approved for use in the United States in 2003. This vaccine, administered intranasally, offers the advantage of stimulating immunity at the site of infection in the upper respiratory tract and, by mimicking natural infection, has the potential to elicit a multifaceted immune response. However, the development of immunity following LAIV administration requires viral replication, causing vaccine effectiveness to be impacted by both the replicative fitness of the attenuated viruses being administered and the degree of the host's preexisting immunity. In this review, we discuss the current state of knowledge regarding the mechanisms of protection elicited by LAIV in children, contrast this with immune protection that develops upon vaccination with inactivated influenza vaccines, and briefly discuss both the potential advantages as well as challenges offered by this vaccination platform.

Efficacy, immunogenicity, and safety of an oral influenza vaccine: a placebo-controlled and active-controlled phase 2 human challenge study

BACKGROUND

Influenza is an important public health problem and existing vaccines are not completely protective. New vaccines that protect by alternative mechanisms are needed to improve efficacy of influenza vaccines. In 2015, we did a phase 1 trial of an oral influenza vaccine, VXA-A1.1. A favourable safety profile and robust immunogenicity results in that trial supported progression of the vaccine to the current phase 2 trial. The aim of this study was to evaluate efficacy of the vaccine in a human influenza challenge model.

METHODS

We did a single-site, placebo-controlled and active-controlled, phase 2 study at WCCT Global, Costa Mesa, CA, USA. Eligible individuals had an initial A/California/H1N1 haemagglutination inhibition titre of less than 20 and were aged 18-49 years and in good health. Individuals were randomly assigned (2:2:1) to receive a single immunisation of either 10¹¹ infectious units of VXA-A1.1 (a monovalent tablet vaccine) orally, a full human dose of quadrivalent inactivated influenza vaccine (IIV) via intramuscular injection, or matched placebo. Randomisation was done by computer-generated assignments with block size of five. An unmasked pharmacist provided the appropriate vaccines and placebos to the administrating nurse. Individuals receiving the treatments, investigators, and staff were all masked to group assignments. 90 days after immunisation, individuals without clinically significant symptoms or signs of influenza, an oral temperature of higher than 37·9°C, a positive result for respiratory viral shedding on a Biofire test, and any investigator-assessed contraindications were challenged intranasally with 0·5 mL wild-type A/CA/like(H1N1)pdm09 influenza virus. The primary outcomes were safety, which was assessed in all immunised participants through 365 days, and influenza-positive illness after viral challenge, which was assessed in individuals that received the viral challenge and the required number of assessments post viral challenge. This trial is registered with ClinicalTrials.gov, number NCT02918006.

RESULTS

Between Aug 31, 2016, and Jan 23, 2017, 374 individuals were assessed for eligibility, of whom 179 were randomly assigned to receive either VXA-A1.1 (n=71 [one individual did not provide a diary card, thus the solicited events were assessed in 70 individuals]), IIV (n=72), or placebo (n=36). Between Dec 2, 2016, and April 26, 2017, 143 eligible individuals (58 in the VXA-A1.1 group, 54 in the IIV group, and 31 in the placebo group) were challenged with influenza virus. VXA-A1.1 was well tolerated with no serious or medically significant adverse events. The most prevalent solicited adverse events for each of the treatment groups after immunisation were headache in the VXA-A1.1 (in five [7%] of 70 participants) and placebo (in seven [19%] of 36 participants) groups and tenderness at injection site in the IIV group (in 19 [26%] of 72 participants) Influenza-positive illness after challenge was detected in 17 (29%) of 58 individuals in the VXA-A1.1 group, 19 (35%) of 54 in the IIV group, and 15 (48%) of 31 in the placebo group.

INTERPRETATION

Orally administered VXA-A1.1 was well tolerated and generated protective immunity against virus shedding, similar to a licensed intramuscular IIV. These results represent a major step forward in developing a safe and effective oral influenza vaccine.

FUNDING

Department of Health and Human Services, Office of the Assistant Secretary for Preparedness and Response, and Biomedical Advanced Research and Development Authority.

Insights into current clinical research on the immunogenicity of live attenuated influenza vaccines

Introduction: Live attenuated influenza vaccines (LAIVs) have been in use for more than three decades and are now licensed in many countries. There is evidence that LAIVs can have greater efficacy than inactivated influenza vaccines, especially against mismatched influenza, however, in recent years, a number of trials have found a lack of LAIV efficacy, mainly in relation to the H1N1 virus.Areas covered: In this review, we summarize the results of clinical research published in the past 5 years on the immunogenicity of LAIVs, with special attention to the mechanisms of establishing protective immunity and some factors that may influence immunogenicity and efficacy.Expert opinion: A number of recent clinical studies confirmed that the immune responses to LAIVs are multifaceted, involving different immune mechanisms. These trials suggest that the intrinsic replicative properties of each LAIV component should be taken into account, and the precise effects of adding a fourth vaccine strain to trivalent LAIV formulations are still to be identified. In addition, new data are emerging regarding the impact of pre-vaccination conditions, such as preexisting immunity or concurrent asymptomatic viral and bacterial respiratory infections, on LAIV immunogenicity, suggesting the importance of monitoring them during clinical trials or vaccination campaigns.

How to assess the effectiveness of nasal influenza vaccines? Role and measurement of sIgA in mucosal secretions

Secretory IgAs (sIgA) constitute the principal isotype of antibodies present in nasal and mucosal secretions. They are secreted by plasma cells adjacent to the mucosal epithelial cells, the site where infection occurs, and are the main humoral mediator of mucosal immunity. Mucosally delivered vaccines, such as live attenuated influenza vaccine (LAIV), are able to mimic natural infection without causing disease or virus transmission and mainly elicit a local immune response. The measurement of sIgA concentrations in nasal swab/wash and saliva samples is therefore a valuable tool for evaluating their role in the effectiveness of such vaccines. Here, we describe two standardized assays (enzyme‐linked immunosorbent assay and microneutralization) available for the quantification of sIgA and discuss the advantages and limitations of their use.

The Future of Flu: A Review of the Human Challenge Model and Systems Biology for Advancement of Influenza Vaccinology

Objectives: Novel approaches to advance the field of vaccinology must be investigated, and are particularly of importance for influenza in order to produce a more effective vaccine. A systematic review of human challenge studies for influenza was performed, with the goal of assessing safety and ethics and determining how these studies have led to therapeutic and vaccine development. A systematic review of systems biology approaches for the study of influenza was also performed, with a focus on how this technology has been utilized for influenza vaccine development. Methods: The PubMed database was searched for influenza human challenge studies, and for systems biology studies that have addressed both influenza infection and immunological effects of vaccination. Results: Influenza human challenge studies have led to important advancements in therapeutics and influenza immunization, and can be performed safely and ethically if certain criteria are met. Many studies have investigated the use of systems biology for evaluating immune response to influenza vaccine, and several promising molecular signatures may help advance our understanding of pathogenesis and be used as targets for influenza interventions. Combining these methodologies has the potential to lead to significant advances in the field of influenza vaccinology and therapeutics. Conclusions: Human challenge studies and systems biology approaches are important tools that should be used in concert to advance our understanding of influenza infection and provide targets for novel therapeutics and immunizations.

Virus-Like Particles-Based Mucosal Nanovaccines

Virus-like particles (VLPs) are protein complexes that resemble a virus and constitute highly immunogenic entities as they mimic the pathogen at an important degree. Among nanovaccines, those based on VLPs are the most successful thus far with some formulations already commercialized (e.g., those against hepatitis B and E viruses and human papillomavirus). This chapter highlights the advantages of VLPs-based vaccines, describing approaches for their design and transmittance of the state of the art for mucosal VLPs-based vaccines development. Several candidates have been produced in insect cells, plants, and E. coli and mammalian cells; they have been mainly evaluated in i.n. and oral immunization schemes. i.n. vaccines against the influenza virus and the Norwalk virus are the most advanced applications. For the latter, i.n. formulations are under clinical evaluation. Perspectives for the field comprise the expansion of the use of low-cost platforms such as plants and bacteria, the development of multiepitopic/multivalent vaccines, and computationally designed VLPs. Mucosal VLPs-based vaccines stand as a major promising approach in vaccinology and the initiation of more clinical trials is envisaged in a short time.

Cross-Protective Efficacy of Monovalent Live Influenza B Vaccines against Genetically Different Lineages of B/Victoria and B/Yamagata in Ferrets

Background

Currently, two genetic lineages of influenza B virus, B/Victoria and B/Yamagata, are cocirculating in humans in various countries. This situation has raised a question regarding the possibility of cross-protection between B components of live attenuated influenza vaccine (LAIV) belonging to different lineages. This study aimed to assess in naïve ferrets the potential protective activity of monovalent B-LAIVs against challenge with homologous and heterologous wild-type (WT) influenza B viruses.

Methods

Groups of seronegative female ferrets 5-6 months of age were given one dose of monovalent LAIV based on B/Victoria or B/Yamagata lineage virus. Ferrets were challenged 21 days later with B/Victoria or B/Yamagata WT virus. Ferrets were monitored closely for clinical signs and morbidity outcomes including febrile response, body weight loss, nasal symptoms, and level of activity one week prior to vaccination and for three days following vaccination/challenge. Nasal washes were collected three days after vaccination/challenge. Samples of lung tissue were taken three days after challenge. All samples were analyzed for the presence of challenge virus by culturing in embryonated chicken eggs and real-time polymerase chain reaction. Antibody response to vaccination was assessed by routine hemagglutination inhibition assay and microneutralization test.

Results

Vaccination led to intensive production of specific neutralizing and antihemagglutinating antibodies to vaccine virus, protected ferrets from homologous challenge infection, and significantly reduced clinical signs and replication of homologous challenge virus. In contrast, cross-lineage serum antibodies were not detected. However, ferrets vaccinated with monovalent B-LAIV had a significantly lower level of heterologous challenge virus in the respiratory tract than those given challenge virus only.

Conclusions

Monovalent B-LAIV has the potential to be cross-protective against infection with genetically different influenza lineages. Further studies are required to confirm this effect.

H5N2 vaccine viruses on Russian and US live attenuated influenza virus backbones demonstrate similar infectivity, immunogenicity and protection in ferrets

The continued detection of zoonotic influenza infections, most notably due to the avian influenza A H5N1 and H7N9 subtypes, underscores the need for pandemic preparedness. Decades of experience with live attenuated influenza vaccines (LAIVs) for the control of seasonal influenza support the safety and effectiveness of this vaccine platform. All LAIV candidates are derived from one of two licensed master donor viruses (MDVs), cold-adapted (ca) A/Ann Arbor/6/60 or ca A/Leningrad/134/17/57. A number of LAIV candidates targeting avian H5 influenza viruses derived with each MDV have been evaluated in humans, but have differed in their infectivity and immunogenicity. To understand these differences, we generated four H5N2 candidate pandemic LAIVs (pLAIVs) derived from either MDV and compared their biological characteristics in vitro and in vivo. We demonstrate that all candidate pLAIVs, regardless of gene constellation and derivation, were comparable with respect to infectivity, immunogenicity, and protection from challenge in the ferret model of influenza. These observations suggest that differences in clinical performance of H5 pLAIVs may be due to factors other than inherent biological properties of the two MDVs.

Immune responses after live attenuated influenza vaccination

Since 2003 (US) and 2012 (Europe) the live attenuated influenza vaccine (LAIV) has been used as an alternative to the traditional inactivated influenza vaccines (IIV). The immune responses elicted by LAIV mimic natural infection and have been found to provide broader clinical protection in children compared to the IIVs. However, our knowledge of the detailed immunological mechanisims induced by LAIV remain to be fully elucidated, and despite 14 years on the global market, there exists no correlate of protection. Recently, matters are further complicated by differing efficacy data from the US and Europe which are not understood. Better understanding of the immune responses after LAIV may aid in achieving the ultimate goal of a future "universal influenza vaccine". In this review we aim to cover the current understanding of the immune responses induced after LAIV.

Pan-Influenza A Protection by Prime-Boost Vaccination with Cold-Adapted Live-Attenuated Influenza Vaccine in a Mouse Model

Influenza virus infections continually pose a major public health threat with seasonal epidemics and sporadic pandemics worldwide. While currently licensed influenza vaccines provide only strain-specific protection, antigenic drift and shift occasionally render the viruses resistant to the host immune responses, which highlight the need for a vaccine that provides broad protection against multiple subtypes. In this study, we suggest a vaccination strategy using cold-adapted, live attenuated influenza vaccines (CAIVs) to provide a broad, potent, and safe cross-protection covering antigenically distinct hemagglutinin (HA) groups 1 and 2 influenza viruses. Using a mouse model, we tested different prime-boost combinations of CAIVs for their ability to induce humoral and T-cell responses, and protective efficacy against H1 and H5 (HA group 1) as well as H3 and H7 (HA group 2) influenza viruses. Notably, even in the absence of antibody-mediated neutralizing activity or HA inhibitory activity in vitro, CAIVs provided a potent protection against heterologous and heterosubtypic lethal challenges in vivo. Heterologous combination of prime (H1)-boost (H5) vaccine strains showed the most potent cross-protection efficacy. In vivo depletion experiments demonstrated not only that T cells and natural killer cells contributed to the cross-protection, but also the involvement of antibody-dependent mechanisms for the cross-protection. Vaccination-induced antibodies did not enhance the infectivity of heterologous viruses, and prime vaccination did not interfere with neutralizing antibody generation by the boost vaccination, allaying vaccine safety concerns associated with heterogeneity between the vaccines and challenge strains. Our data show that CAIV-based strategy can serve as a simple but powerful option for developing a "truly" universal influenza vaccine providing pan-influenza A protection, which has not been achieved yet by other vaccine strategies. The promising results of potency, breadth, and safety demonstrated in the mouse model support further studies in higher animal models for clinical relevance.

Perspectives from the Society for Pediatric Research: Decreased Effectiveness of the Live Attenuated Influenza Vaccine

The intranasal live attenuated influenza vaccine (LAIV), FluMist, has been widely appreciated by pediatricians, parents, and children alike for its ease of administration. However, concerns regarding lack of effectiveness in recent influenza seasons led to the CDC Advisory Committee on Immunization Practices (ACIP) recommendation to administer inactivated influenza vaccines (IIVs), and not LAIV, during the 2016-17 and 2017-18 seasons. Given that data from previous years demonstrated equivalent and even improved efficacy of LAIV compared with IIV, these recent data were surprising, raising many questions about the potential mechanisms underlying this change. This review seeks to summarize the history of LAIV studies and ACIP recommendations with a focus on the recent decrease in vaccine effectiveness (VE) and discordant results among studies performed in different countries. Decreased VE for A/H1N1pdm09 viruses represents the most consistent finding across studies, as VE has been low every season these viruses predominated since 2010-11. Potential explanations underlying diminished effectiveness include the hypothesis that prior vaccination, reduced thermostability of A/H1N1pdm09, addition of a fourth virus, or reduced replication fitness of A/H1N1pdm09 strains may have contributed to this phenomenon. Ongoing studies and potential alterations to LAIV formulations provide hope for a return of effective LAIV in future influenza seasons.

The M2 protein of live, attenuated influenza vaccine encodes a mutation that reduces replication in human nasal epithelial cells

The influenza A virus components of the live, attenuated influenza vaccine (LAIV) encode the HA and NA gene segments from a circulating virus strain and the remaining gene segments from the cold-adapted master donor virus, A/Ann Arbor/6/1960 (H2N2). The master donor virus imparts at least three phenotypes: temperature-sensitivity (ts), attenuation (att), and cold-adaption (ca). The genetic loci responsible for the att and ts phenotypes of LAIV were mapped to PB1, PB2, and NP by reverse genetics experiments using immortalized cell lines. However, some in vivo studies have demonstrated that the M segment, which acquired an alanine (Ala) to serine (Ser) mutation at M2 position 86 during cold adaption - a mutation found in no other influenza A virus strain - contributes to the att phenotype. Prior studies have shown this region of the M2 cytoplasmic tail to be critical for influenza virus replication. Using reverse genetics, we demonstrate that certain amino acid substitutions at M2 positions 83 and 86 alter the replication of influenza A/Udorn/307/72 (H3N2). Importantly, substitution of a Ser at M2 position 86 reduces A/Udorn/307/72 replication in differentiated primary human nasal epithelial cell (hNECs) cultures, but does not considerably affect replication in MDCK cells. When a Ser was substituted for Ala at M2 86 in LAIV, the virus replicated to higher titers and with faster kinetics in hNEC cultures, implicating this amino acid change as contributing to LAIV attenuation. Increased replication also resulted in increased production of IFN-λ. These data indicate the LAIV associated Ser mutation at M2 position 86 contributes to the att phenotype and is associated with a differential regulation of interferon in LAIV infection.

Novel Bivalent Viral-Vectored Vaccines Induce Potent Humoral and Cellular Immune Responses Conferring Protection against Stringent Influenza A Virus Challenge

Seasonal influenza viruses are a common cause of acute respiratory illness worldwide and generate a significant socioeconomic burden. Influenza viruses mutate rapidly, necessitating annual vaccine reformulation because traditional vaccines do not typically induce broad-spectrum immunity. In addition to seasonal infections, emerging pandemic influenza viruses present a continued threat to global public health. Pandemic influenza viruses have consistently higher attack rates and are typically associated with greater mortality compared with seasonal strains. Ongoing strategies to improve vaccine efficacy typically focus on providing broad-spectrum immunity; although B and T cells can mediate heterosubtypic responses, typical vaccine development will augment either humoral or cellular immunity. However, multipronged approaches that target several Ags may limit the generation of viral escape mutants. There are few vaccine platforms that can deliver multiple Ags and generate robust cellular and humoral immunity. In this article, we describe a novel vaccination strategy, tested preclinically in mice, for the delivery of novel bivalent viral-vectored vaccines. We show this strategy elicits potent T cell responses toward highly conserved internal Ags while simultaneously inducing high levels of Abs toward hemagglutinin. Importantly, these humoral responses generate long-lived plasma cells and generate Abs capable of neutralizing variant hemagglutinin-expressing pseudotyped lentiviruses. Significantly, these novel viral-vectored vaccines induce strong immune responses capable of conferring protection in a stringent influenza A virus challenge. Thus, this vaccination regimen induces lasting efficacy toward influenza. Importantly, the simultaneous delivery of dual Ags may alleviate the selective pressure that is thought to potentiate antigenic diversity in avian influenza viruses.

Intranasal Administration of Whole Inactivated Influenza Virus Vaccine as a Promising Influenza Vaccine Candidate

The effect of the current influenza vaccine, an inactivated virus vaccine administered by subcutaneous/intramuscular injection, is limited to reducing the morbidity and mortality associated with seasonal influenza outbreaks. Intranasal vaccination, by contrast, mimics natural infection and induces not only systemic IgG antibodies but also local secretory IgA (S-IgA) antibodies found on the surface of the mucosal epithelium in the upper respiratory tract. S-IgA antibodies are highly effective at preventing virus infection. Although the live attenuated influenza vaccine (LAIV) administered intranasally can induce local antibodies, this vaccine is restricted to healthy populations aged 2-49 years because of safety concerns associated with using live viruses in a vaccine. Instead of LAIV, an intranasal vaccine made with inactivated virus could be applied to high-risk populations, including infants and elderly adults. Normally, a mucosal adjuvant would be required to enhance the effect of intranasal vaccination with an inactivated influenza vaccine. However, we found that intranasal administration of a concentrated, whole inactivated influenza virus vaccine without any mucosal adjuvant was enough to induce local neutralizing S-IgA antibodies in the nasal epithelium of healthy individuals with some immunological memory for seasonal influenza viruses. This intranasal vaccine is a novel candidate that could improve on the current injectable vaccine or the LAIV for the prevention of seasonal influenza epidemics.

Formulation of a dry powder influenza vaccine for nasal delivery

The purpose of this research was to prepare a dry powder vaccine formulation containing whole inactivated influenza virus (VIIV) and a mucoadhesive compound suitable for nasal delivery. Powders containing WIIV and either lactose or trehalose were produced by lyophilization. A micro-ball mill was used to reduce the lyophilized cake to sizes suitable for nasal delivery. Chitosan flakes were reduced in size using a cryo-milling technique. Milled powders were sieved between 45 and 125 μm aggregate sizes and characterized for particle size and distribution, morphology, and flow properties. Powders were blended in the micro-ball mill without the ball. Lyophilization followed by milling produced irregularly shaped, polydisperse particles with a median primary particle diameter of ≈21 μm and a yield of ≈37% of particles in the 45 to 125 μm particle size range. Flow properties of lactose and trehalose powders after lyophilization followed by milling and sieving were similar. Cryo-milling produced a small yield of particles in the desired size range (<10%). Lyophilization followed by milling and sieving produced particles suitable for nasal delivery with different physicochemical properties as a function of processing conditions and components of the formulation. Further optimization of particle size and morphology is required for these powders to be suitable for clinical evaluation.

Alkyl polyglycoside, a highly promising adjuvant in intranasal split influenza vaccines

Influenza viral infections are significant global public health concerns due to the morbidity and mortality associated with acute respiratory disease, secondary complications, and pandemic threats; thus, continuous efforts have been made to develop potent influenza vaccines. In this study, 3 different mucosal adjuvants-alkyl polyglycoside (APG), gellan gum, and chitosan (CS)-were evaluated for their efficacy in intranasal A/H1N1 or B split influenza vaccines administered to BALB/c mice. Protective immunity was monitored by serum analysis for IgG, hemagglutination inhibition (HI), and neutralizing antibody levels, as well as mucosal IgA levels in nasal and pulmonary lavage fluids. Survival, body weight, lung viral titer, and pulmonary immunopathology were also examined following lethal influenza challenge. Notably, all adjuvants amplified the IgG and IgA immune responses (not detected in immunization of influenza B) and increased survival rate compared with controls administered adjuvant-free intranasal vaccines. Alternatively, intramuscular immunization stimulated IgG production, but had no effect on IgA levels. Our collective analysis identified that APG was the most effective intranasal adjuvant, as all mice survived influenza challenge with limited body weight loss, viral titer, and pulmonary pathology, similar to those observed with intramuscular vaccination. This evidence supports that APG can elicit both systemic and mucosal immunity, and may be an effective adjuvant in intranasal split influenza A/H1N1 and B vaccines.

Comparisons of the Humoral and Cellular Immune Responses Induced by Live Attenuated Influenza Vaccine and Inactivated Influenza Vaccine in Adults

Both live attenuated influenza vaccines (LAIV) and inactivated influenza vaccines (IIV) induce protective immunity against influenza. There is evidence that LAIV induces superior protection in children, whereas IIV may induce superior protection in adults. The immune mechanisms responsible for these differences have not been identified. We previously compared LAIV and IIV in young children of 6 to 36 months of age, and we demonstrated that while both induced similar hemagglutination inhibition (HAI) antibody responses, only LAIV induced significant increases in T cell responses. In the present study, 37 healthy adult subjects of 18 to 49 years of age were randomized to receive seasonal influenza vaccination with LAIV or IIV. Influenza virus-specific HAI, T cell, and secretory IgA (sIgA) responses were studied pre- and postvaccination. In contrast to the responses seen in young children, LAIV induced only minimal increases in serum HAI responses in adults, which were significantly lower than the responses induced by IIV. Both LAIV and IIV similarly induced only transient T cell responses to replication-competent whole virus in adults. In contrast, influenza virus-specific sIgA responses were induced more strongly by LAIV than by IIV. Our previous studies suggest that LAIV may be more protective than IIV in young children not previously exposed to influenza virus or influenza vaccines due to increased vaccine-induced T cell and/or sIgA responses. Our current work suggests that in adults with extensive and partially cross-reactive preexisting influenza immunity, LAIV boosting of sIgA responses to hemagglutinin (HA) and non-HA antigenic targets expressed by circulating influenza virus strains may be an important additional mechanism of vaccine-induced immunity.

Influenza and Memory T Cells: How to Awake the Force

Annual influenza vaccination is an effective way to prevent human influenza. Current vaccines are mainly focused on eliciting a strain-matched humoral immune response, requiring yearly updates, and do not provide protection for all vaccinated individuals. The past few years, the importance of cellular immunity, and especially memory T cells, in long-lived protection against influenza virus has become clear. To overcome the shortcomings of current influenza vaccines, eliciting both humoral and cellular immunity is imperative. Today, several new vaccines such as infection-permissive and recombinant T cell inducing vaccines, are being developed and show promising results. These vaccines will allow us to stay several steps ahead of the constantly evolving influenza virus.