Enhancing the reproducibility of serological methods used to evaluate immunogenicity of pandemic H1N1 influenza vaccines-an effective EU regulatory approach

Haemagglutination inhibition and virus microneutralisation serology assays: use of harmonised protocols and biological standards in seasonal influenza serology testing and their impact on inter-laboratory variation and assay correlation: A FLUCOP collaborative study

Introduction

The haemagglutination inhibition assay (HAI) and the virus microneutralisation assay (MN) are long-established methods for quantifying antibodies against influenza viruses. Despite their widespread use, both assays require standardisation to improve inter-laboratory agreement in testing. The FLUCOP consortium aims to develop a toolbox of standardised serology assays for seasonal influenza. Building upon previous collaborative studies to harmonise the HAI, in this study the FLUCOP consortium carried out a head-to-head comparison of harmonised HAI and MN protocols to better understand the relationship between HAI and MN titres, and the impact of assay harmonisation and standardisation on inter-laboratory variability and agreement between these methods.

Methods

In this paper, we present two large international collaborative studies testing harmonised HAI and MN protocols across 10 participating laboratories. In the first, we expanded on previously published work, carrying out HAI testing using egg and cell isolated and propagated wild-type (WT) viruses in addition to high-growth reassortants typically used influenza vaccines strains using HAI. In the second we tested two MN protocols: an overnight ELISA-based format and a 3-5 day format, using reassortant viruses and a WT H3N2 cell isolated virus. As serum panels tested in both studies included many overlapping samples, we were able to look at the correlation of HAI and MN titres across different methods and for different influenza subtypes.

Results

We showed that the overnight ELISA and 3-5 day MN formats are not comparable, with titre ratios varying across the dynamic range of the assay. However, the ELISA MN and HAI are comparable, and a conversion factor could possibly be calculated. In both studies, the impact of normalising using a study standard was investigated, and we showed that for almost every strain and assay format tested, normalisation significantly reduced inter-laboratory variation, supporting the continued development of antibody standards for seasonal influenza viruses. Normalisation had no impact on the correlation between overnight ELISA and 3-5 day MN formats.

An external quality assessment feasibility study; cross laboratory comparison of haemagglutination inhibition assay and microneutralization assay performance for seasonal influenza serology testing: A FLUCOP study

Introduction

External Quality Assessment (EQA) schemes are designed to provide a snapshot of laboratory proficiency, identifying issues and providing feedback to improve laboratory performance and inter-laboratory agreement in testing. Currently there are no international EQA schemes for seasonal influenza serology testing. Here we present a feasibility study for conducting an EQA scheme for influenza serology methods.

Methods

We invited participant laboratories from industry, contract research organizations (CROs), academia and public health institutions who regularly conduct hemagglutination inhibition (HAI) and microneutralization (MN) assays and have an interest in serology standardization. In total 16 laboratories returned data including 19 data sets for HAI assays and 9 data sets for MN assays.

Results

Within run analysis demonstrated good laboratory performance for HAI, with intrinsically higher levels of intra-assay variation for MN assays. Between run analysis showed laboratory and strain specific issues, particularly with B strains for HAI, whilst MN testing was consistently good across labs and strains. Inter-laboratory variability was higher for MN assays than HAI, however both assays showed a significant reduction in inter-laboratory variation when a human sera pool is used as a standard for normalization.

Discussion

This study has received positive feedback from participants, highlighting the benefit such an EQA scheme would have on improving laboratory performance, reducing inter laboratory variation and raising awareness of both harmonized protocol use and the benefit of biological standards for seasonal influenza serology testing.

Robustness of the Ferret Model for Influenza Risk Assessment Studies: a Cross-Laboratory Exercise

Past pandemic influenza viruses with sustained human-to-human transmissibility have emerged from animal influenza viruses. Employment of experimental models to assess the pandemic risk of emerging zoonotic influenza viruses provides critical information supporting public health efforts. Ferret transmission experiments have been utilized to predict the human-to-human transmission potential of novel influenza viruses. However, small sample sizes and a lack of standardized protocols can introduce interlaboratory variability, complicating interpretation of transmission experimental data. To assess the range of variation in ferret transmission experiments, a global exercise was conducted by 11 laboratories using two common stock H1N1 influenza viruses with different transmission characteristics in ferrets. Parameters known to affect transmission were standardized, including the inoculation route, dose, and volume, as well as a strict 1:1 donor/contact ratio for respiratory droplet transmission. Additional host and environmental parameters likely to affect influenza transmission kinetics were monitored and analyzed. The overall transmission outcomes for both viruses across 11 laboratories were concordant, suggesting the robustness of the ferret model for zoonotic influenza risk assessment. Among environmental parameters that varied across laboratories, donor-to-contact airflow directionality was associated with increased transmissibility. To attain high confidence in identifying viruses with moderate to high transmissibility or low transmissibility under a smaller number of participating laboratories, our analyses support the notion that as few as three but as many as five laboratories, respectively, would need to independently perform viral transmission experiments with concordant results. This exercise facilitates the development of a more homogenous protocol for ferret transmission experiments that are employed for the purposes of risk assessment.

Validation of a Harmonized Enzyme-Linked-Lectin-Assay (ELLA-NI) Based Neuraminidase Inhibition Assay Standard Operating Procedure (SOP) for Quantification of N1 Influenza Antibodies and the Use of a Calibrator to Improve the Reproducibility of the ELLA-NI With Reverse Genetics Viral and Recombinant Neuraminidase Antigens: A FLUCOP Collaborative Study

Current vaccination strategies against influenza focus on generating an antibody response against the viral haemagglutination surface protein, however there is increasing interest in neuraminidase (NA) as a target for vaccine development. A critical tool for development of vaccines that target NA or include an NA component is available validated serology assays for quantifying anti-NA antibodies. Additionally serology assays have a critical role in defining correlates of protection in vaccine development and licensure. Standardisation of these assays is important for consistent and accurate results. In this study we first validated a harmonized enzyme-linked lectin assay (ELLA)- Neuraminidase Inhibition (NI) SOP for N1 influenza antigen and demonstrated the assay was precise, linear, specific and robust within classical acceptance criteria for neutralization assays for vaccine testing. Secondly we tested this SOP with NA from influenza B viruses and showed the assay performed consistently with both influenza A and B antigens. Third, we demonstrated that recombinant NA (rNA) could be used as a source of antigen in ELLA-NI. In addition to validating a harmonized SOP we finally demonstrated a clear improvement in inter-laboratory agreement across several studies by using a calibrator. Importantly we showed that the use of a calibrator significantly improved agreement when using different sources of antigen in ELLA-NI, namely reverse genetics viruses and recombinant NA. We provide a freely available and detailed harmonized SOP for ELLA-NI. Our results add to the growing body of evidence in support of developing biological standards for influenza serology.

Assay Harmonization and Use of Biological Standards To Improve the Reproducibility of the Hemagglutination Inhibition Assay: a FLUCOP Collaborative Study

The hemagglutination inhibition (HAI) assay is an established technique for assessing influenza immunity, through measurement of antihemagglutinin antibodies. Improved reproducibility of this assay is required to provide meaningful data across different testing laboratories. This study assessed the impact of harmonizing the HAI assay protocol/reagents and using standards on interlaboratory variability. Human pre- and postvaccination sera from individuals (n = 30) vaccinated against influenza were tested across six laboratories. We used a design of experiment (DOE) method to evaluate the impact of assay parameters on interlaboratory HAI assay variability. Statistical and mathematical approaches were used for data analysis. We developed a consensus protocol and assessed its performance against in-house HAI testing. We additionally tested the performance of several potential biological standards. In-house testing with four reassortant viruses showed considerable interlaboratory variation (geometric coefficient of variation [GCV] range of 50% to 117%). The age, concentration of turkey red blood cells, incubation duration, and temperature were key assay parameters affecting variability. Use of a consensus protocol with common reagents, including viruses, significantly reduced GCV between laboratories to 22% to 54%. Pooled postvaccination human sera from different vaccination campaigns were effective as biological standards. Our results demonstrate that the harmonization of protocols and critical reagents is effective in reducing interlaboratory variability in HAI assay results and that pools of postvaccination human sera have potential as biological standards that can be used over multiple vaccination campaigns. Moreover, the use of standards together with in-house protocols is as potent as the use of common protocols and reagents in reducing interlaboratory variability.

IMPORTANCE The hemagglutination inhibition (HAI) assay is the most commonly used serology assay to detect antibodies from influenza vaccination or influenza virus infection. This assay has been used for decades but requires improved standardization of procedures to provide meaningful data. We designed a large study to assess selected parameters for their contribution to assay variability and developed a standard protocol to promote consistent HAI testing methods across laboratories. The use of this protocol and common reagents resulted in lower levels of variability in results between participating laboratories than achieved using in-house HAI testing. Human sera sourced from vaccination campaigns over several years, and thus including antibody to different influenza vaccine strains, served as effective assay standards. Based on our findings, we recommend the use of a common protocol and/or human serum standards, if available, for testing human sera for the presence of antibodies against seasonal influenza using turkey red blood cells.

Development and Assessment of a Pooled Serum as Candidate Standard to Measure Influenza A Virus Group 1 Hemagglutinin Stalk-Reactive Antibodies

The stalk domain of the hemagglutinin has been identified as a target for induction of protective antibody responses due to its high degree of conservation among numerous influenza subtypes and strains. However, current assays to measure stalk-based immunity are not standardized. Hence, harmonization of assay readouts would help to compare experiments conducted in different laboratories and increase confidence in results. Here, serum samples from healthy individuals (n = 110) were screened using a chimeric cH6/1 hemagglutinin enzyme-linked immunosorbent assay (ELISA) that measures stalk-reactive antibodies. We identified samples with moderate to high IgG anti-stalk antibody levels. Likewise, screening of the samples using the mini-hemagglutinin (HA) headless construct #4900 and analysis of the correlation between the two assays confirmed the presence and specificity of anti-stalk antibodies. Additionally, samples were characterized by a cH6/1N5 virus-based neutralization assay, an antibody-dependent cell-mediated cytotoxicity (ADCC) assay, and competition ELISAs, using the stalk-reactive monoclonal antibodies KB2 (mouse) and CR9114 (human). A “pooled serum” (PS) consisting of a mixture of selected serum samples was generated. The PS exhibited high levels of stalk-reactive antibodies, had a cH6/1N5-based neutralization titer of 320, and contained high levels of stalk-specific antibodies with ADCC activity. The PS, along with blinded samples of varying anti-stalk antibody titers, was distributed to multiple collaborators worldwide in a pilot collaborative study. The samples were subjected to different assays available in the different laboratories, to measure either binding or functional properties of the stalk-reactive antibodies contained in the serum. Results from binding and neutralization assays were analyzed to determine whether use of the PS as a standard could lead to better agreement between laboratories. The work presented here points the way towards the development of a serum standard for antibodies to the HA stalk domain of phylogenetic group 1.

Meeting report: Convening on the influenza human viral challenge model for universal influenza vaccines, Part 1: Value; challenge virus selection; regulatory, industry and ethical considerations; increasing standardization, access and capacity

In response to global interest in the development of a universal influenza vaccine, the Bill & Melinda Gates Foundation, PATH, and the Global Funders Consortium for Universal Influenza Vaccine Development convened a meeting of experts (London, UK, May 2018) to assess the role of a standardized controlled human influenza virus infection model (CHIVIM) towards the development of novel influenza vaccine candidates. This report (in two parts) summarizes those discussions and offers consensus recommendations. This article (Part 1) covers challenge virus selection, regulatory and ethical considerations, and issues concerning standardization, access, and capacity. Part 2 covers specific methodologic considerations.

Current methods for influenza vaccine development and licensure require large costly field trials. The CHIVIM requires fewer subjects and the controlled setting allows for better understanding of influenza transmission and host immunogenicity. The CHIVIM can be used to identify immune predictors of disease for at-risk populations and to measure efficacy of potential vaccines for further development.

Limitations to the CHIVIM include lack of standardization, limited access to challenge viruses and assays, lack of consensus regarding role of the CHIVIM in vaccine development pathway, and concerns regarding risk to study participants and community. To address these issues, the panel of experts recommended that WHO and other key stakeholders provide guidance on standardization, challenge virus selection, and risk management. A common repository of well-characterized challenge viruses, harmonized protocols, and standardized assays should be made available to researchers. A network of research institutions performing CHIVIM trials should be created, and more study sites are needed to increase capacity.

Experts agreed that a research network of institutions working with a standardized CHIVIM could contribute important data to support more rapid development and licensure of novel vaccines capable of providing long-lasting protection against seasonal and pandemic influenza strains.

Safety, tolerability, and immunogenicity of influenza vaccination with a high-density microarray patch: Results from a randomized, controlled phase I clinical trial

BACKGROUND

The Vaxxas high-density microarray patch (HD-MAP) consists of a high density of microprojections coated with vaccine for delivery into the skin. Microarray patches (MAPs) offer the possibility of improved vaccine thermostability as well as the potential to be safer, more acceptable, easier to use, and more cost-effective for the administration of vaccines than injection by needle and syringe (N&S). Here, we report a phase I trial using the Vaxxas HD-MAP to deliver a monovalent influenza vaccine that was to the best of our knowledge the first clinical trial to evaluate the safety, tolerability, and immunogenicity of lower doses of influenza vaccine delivered by MAPs.

METHODS AND FINDINGS

HD-MAPs were coated with a monovalent, split inactivated influenza virus vaccine containing A/Singapore/GP1908/2015 H1N1 haemagglutinin (HA). Between February 2018 and March 2018, 60 healthy adults (age 18-35 years) in Melbourne, Australia were enrolled into part A of the study and vaccinated with either: HD-MAPs delivering 15 μg of A/Singapore/GP1908/2015 H1N1 HA antigen (A-Sing) to the volar forearm (FA); uncoated HD-MAPs; intramuscular (IM) injection of commercially available quadrivalent influenza vaccine (QIV) containing A/Singapore/GP1908/2015 H1N1 HA (15 μg/dose); or IM injection of H1N1 HA antigen (15 μg/dose). After 22 days' follow-up and assessment of the safety data, a further 150 healthy adults were enrolled and randomly assigned to 1 of 9 treatment groups. Participants (20 per group) were vaccinated with HD-MAPs delivering doses of 15, 10, 5, 2.5, or 0 μg of HA to the FA or 15 μg HA to the upper arm (UA), or IM injection of QIV. The primary objectives of the study were safety and tolerability. Secondary objectives were to assess the immunogenicity of the influenza vaccine delivered by HD-MAP. Primary and secondary objectives were assessed for up to 60 days post-vaccination. Clinical staff and participants were blind as to which HD-MAP treatment was administered and to administration of IM-QIV-15 or IM-A/Sing-15. All laboratory investigators were blind to treatment and participant allocation. Two further groups in part B (5 participants per group), not included in the main safety and immunological analysis, received HD-MAPs delivering 15 μg HA or uncoated HD-MAPs applied to the forearm. Biopsies were taken on days 1 and 4 for analysis of the cellular composition from the HD-MAP application sites. The vaccine coated onto HD-MAPs was antigenically stable when stored at 40°C for at least 12 months. HD-MAP vaccination was safe and well tolerated; any systemic or local adverse events (AEs) were mild or moderate. Observed systemic AEs were mostly headache or myalgia, and local AEs were application-site reactions, usually erythema. HD-MAP administration of 2.5 μg HA induced haemagglutination inhibition (HAI) and microneutralisation (MN) titres that were not significantly different to those induced by 15 μg HA injected IM (IM-QIV-15). HD-MAP delivery resulted in enhanced humoral responses compared with IM injection with higher HAI geometric mean titres (GMTs) at day 8 in the MAP-UA-15 (GMT 242.5, 95% CI 133.2-441.5), MAP-FA-15 (GMT 218.6, 95% CI 111.9-427.0), and MAP-FA-10 (GMT 437.1, 95% CI 254.3-751.3) groups compared with IM-QIV-15 (GMT 82.8, 95% CI 42.4-161.8), p = 0.02, p = 0.04, p < 0.001 for MAP-UA-15, MAP-FA-15, and MAP-FA-10, respectively. Higher titres were also observed at day 22 in the MAP-FA-10 (GMT 485.0, 95% CI 301.5-780.2, p = 0.001) and MAP-UA-15 (367.6, 95% CI 197.9-682.7, p = 0.02) groups compared with the IM-QIV-15 group (GMT 139.3, 95% CI 79.3-244.5). Results from a panel of exploratory immunoassays (antibody-dependent cellular cytotoxicity, CD4+ T-cell cytokine production, memory B cell (MBC) activation, and recognition of non-vaccine strains) indicated that, overall, Vaxxas HD-MAP delivery induced immune responses that were similar to, or higher than, those induced by IM injection of QIV. The small group sizes and use of a monovalent influenza vaccine were limitations of the study.

CONCLUSIONS

Influenza vaccine coated onto the HD-MAP was stable stored at temperatures up to 40°C. Vaccination using the HD-MAP was safe and well tolerated and resulted in immune responses that were similar to or significantly enhanced compared with IM injection. Using the HD-MAP, a 2.5 μg dose (1/6 of the standard dose) induced HAI and MN titres similar to those induced by 15 μg HA injected IM.

TRIAL REGISTRATION

Australian New Zealand Clinical Trials Registry (ANZCTR.org.au), trial ID 108 ACTRN12618000112268/U1111-1207-3550.

Pandemic influenza A(H1N1pdm09) vaccine induced high levels of influenza-specific IgG and IgM antibodies as analyzed by enzyme immunoassay and dual-mode multiplex microarray immunoassay methods

Influenza A viruses continue to circulate throughout the world as yearly epidemics or occasional pandemics. Influenza infections can be prevented by seasonal multivalent or monovalent pandemic vaccines. In the present study, we describe a novel multiplex microarray immunoassay (MAIA) for simultaneous measurement of virus-specific IgG and IgM antibodies using Pandemrix-vaccinated adult sera collected at day 0 and 28 and 180 days after vaccination as the study material. MAIA showed excellent correlation with a conventional enzyme immunoassay (EIA) in both IgG and IgM anti-influenza A antibodies and good correlation with hemagglutination inhibition (HI) test. Pandemrix vaccine induced 5-30 fold increases in anti-H1N1pdm09 influenza antibodies as measured by HI, EIA or MAIA. A clear increase in virus-specific IgG antibodies was found in 93-97% of vaccinees by MAIA and EIA. Virus-specific IgM antibodies were found in 90-92% of vaccinees by MAIA and EIA, respectively and IgM antibodies persisted for up to 6 months after vaccination in 55-62% of the vaccinees. Pandemic influenza vaccine induced strong anti-influenza A IgG and IgM responses that persisted several months after vaccination. MAIA was demonstrated to be an excellent method for simultaneous measurement of antiviral IgG and IgM antibodies against multiple virus antigens. Thus the method is well suitable for large scale epidemiological and vaccine immunity studies.

Hemagglutination Inhibition Assay

The hemagglutination inhibition (HI) assay for influenza A virus has been used since the 1940s. The assay may be utilized to detect or quantify antibodies to influenza A viruses and can be used to characterize differences in antigenic reactivity between influenza isolates. In addition, data from HI assays are routinely used for antigenic cartography, influenza virus surveillance, epidemiology, and vaccine-seed strain selection. For antibody quantification, the HI assay is a fast and inexpensive method; other than a source of red blood cells, no expensive or unusual lab equipment is needed, and results can be obtained within a few hours. Historically, the HI assay has also served as a primary method of subtype identification and is still used widely. However, as gene sequencing technology has evolved to be cheaper and faster, it is replacing the HI assay for this purpose.

Meeting report and review: Immunological assays and correlates of protection for next-generation influenza vaccines

BACKGROUND

This report summarizes the discussions and conclusions from the "Immunological Assays and Correlates of Protection for Next-Generation Influenza Vaccines" meeting which took place in Siena, Italy, from March 31, 2019, to April 2, 2019.

CONCLUSIONS

Furthermore, we review current correlates of protection against influenza virus infection and disease and their usefulness for the development of next generation broadly protective and universal influenza virus vaccines.

Safety, tolerability, acceptability and immunogenicity of an influenza vaccine delivered to human skin by a novel high-density microprojection array patch (Nanopatch™)

BACKGROUND

Injection using needle and syringe (N&S) is the most widely used method for vaccination, but requires trained healthcare workers. Fear of needles, risk of needle-stick injury, and the need to reconstitute lyophilised vaccines, are also drawbacks. The Nanopatch (NP) is a microarray skin patch comprised of a high-density array of microprojections dry-coated with vaccine that is being developed to address these shortcomings. Here we report a randomised, partly-blinded, placebo-controlled trial that represents the first use in humans of the NP to deliver a vaccine.

METHODS

Healthy volunteers were vaccinated once with one of the following: (1) NPs coated with split inactivated influenza virus (A/California/07/2009 [H1N1], 15 µg haemagglutinin (HA) per dose), applied to the volar forearm (NP-HA/FA), n = 15; (2) NPs coated with split inactivated influenza virus (A/California/07/2009 [H1N1], 15 µg HA per dose), applied to the upper arm (NP-HA/UA), n = 15; (3) Fluvax® 2016 containing 15 µg of the same H1N1 HA antigen injected intramuscularly (IM) into the deltoid (IM-HA/D), n = 15; (4) NPs coated with excipients only, applied to the volar forearm (NP-placebo/FA), n = 5; (5) NPs coated with excipients only applied to the upper arm (NP-placebo/UA), n = 5; or (6) Saline injected IM into the deltoid (IM-placebo/D), n = 5. Antibody responses at days 0, 7, and 21 were measured by haemagglutination inhibition (HAI) and microneutralisation (MN) assays.

FINDINGS

NP vaccination was safe and acceptable; all adverse events were mild or moderate. Most subjects (55%) receiving patch vaccinations (HA or placebo) preferred the NP compared with their past experience of IM injection with N&S (preferred by 24%). The antigen-vaccinated groups had statistically higher HAI titres at day 7 and 21 compared with baseline (p < 0.0001), with no statistical differences between the treatment groups (p > 0.05), although the group sizes were small. The geometric mean HAI titres at day 21 for the NP-HA/FA, NP-HA/UA and IM-HA/D groups were: 335 (189-593 95% CI), 160 (74-345 95% CI), and 221 (129-380 95% CI) respectively. A similar pattern of responses was seen with the MN assays. Application site reactions were mild or moderate, and more marked with the influenza vaccine NPs than with the placebo or IM injection.

INTERPRETATION

Influenza vaccination using the NP appeared to be safe, and acceptable in this first time in humans study, and induced similar immune responses to vaccination by IM injection.

Workshop report: Immunoassay standardisation for "universal" influenza vaccines

The development of broadly reactive influenza vaccines raises the need to identify the most appropriate immunoassays that can be used for the evaluation of so-called universal influenza vaccines and to explore a path towards the standardisation of such assays. More than fifty experts from the global influenza vaccine research and development field met to initiate such discussion at a workshop co-organised by the EDUFLUVAC consortium, a European Union funded project coordinated by the European Vaccine Initiative, and the National Institutes of Health/National Institute of Allergy and Infectious Diseases, USA. The workshop audience agreed that it was not possible to establish a single immunoassay for "universal" influenza vaccines because the current approaches differ in the vaccines' nature and immunogenicity properties. Therefore, different scientific rationales for the immunoassay selection are required. To avoid dilution of efforts, the choice of the primary evaluation criteria (eg serological assays or T-cell assays) should drive the effort of harmonisation. However, at an early phase of clinical development, more efforts on exploratory assessments should be undertaken to better define the immune profile in response to immunisation with new vaccines. The workshop concluded that each laboratory should aim towards validation of the appropriate immunoassays used during the entire process of vaccine development from antigen discovery up to establishment of correlates of protection, including the different steps of quality control (eg potency assays), animal studies and human clinical development. Standardisation of the immunoassays is the ultimate goal, and there is a long way to go.

Weighing serological evidence of human exposure to animal influenza viruses - a literature review

Assessing influenza A virus strains circulating in animals and their potential to cross the species barrier and cause human infections is important to improve human influenza surveillance and preparedness. We reviewed studies describing serological evidence of human exposure to animal influenza viruses. Comparing serological data is difficult due to a lack of standardisation in study designs and in laboratory methods used in published reports. Therefore, we designed a scoring system to assess and weigh specificity of obtained serology results in the selected articles. Many studies report reliable evidence of antibodies to swine influenza viruses among persons occupationally exposed to pigs. Most avian influenza studies target H5, H7 and H9 subtypes and most serological evidence of human exposure to avian influenza viruses is reported for these subtypes. Avian influenza studies receiving a low grade in this review often reported higher seroprevalences in humans compared with studies with a high grade. Official surveillance systems mainly focus on avian H5 and H7 viruses. Swine influenza viruses and avian subtypes other than H5 and H7 (emphasising H9) should be additionally included in official surveillance systems. Surveillance efforts should also be directed towards understudied geographical areas, such as Africa and South America.

A review of the changes to the licensing of influenza vaccines in Europe

In 2014, the European Committee for Medicinal Products for Human Use (CHMP) published a draft regulatory guideline for the evaluation of influenza vaccines. Following a public consultation round, the final guidance will be published in the near future. Here, we highlight the main changes in the clinical section in this guideline and discuss the background to these changes and whether the new consolidated guidance document can be expected to achieve a better understanding of the performance of seasonal, zoonotic and pandemic influenza vaccines during the regulatory licensing process. The new influenza guideline reflects a changed approach to the regulatory assessment of influenza vaccines, resulting in the abolition of serological criteria, known as the CHMP criteria, which have been the mainstay for evaluating the influenza vaccine immunogenicity for several decades. The new guideline adopts a more diversified approach to the measurement and reporting of the immune response to influenza vaccines and sets a requirement to conduct clinical outcome trials in young children. Importantly, more emphasis is placed on the post‐licensure monitoring of the benefit risk of influenza vaccines, including a request for continuous monitoring of efficacy and enhanced safety surveillance. Despite the improvements these new requirements will expectedly bring to the regulatory assessment of influenza vaccines, major challenges remain which cannot be overcome by new guidance alone. Ongoing initiatives in which academia, manufacturers, public health institutes and regulators work together to address these challenges are central to the development of robust tools to evaluate and monitor performance of influenza vaccines in the future.

Evaluation of Antihemagglutinin and Antineuraminidase Antibodies as Correlates of Protection in an Influenza A/H1N1 Virus Healthy Human Challenge Model

Despite long-term investment, influenza continues to be a significant worldwide problem. The cornerstone of protection remains vaccination, and approved vaccines seek to elicit a hemagglutination inhibition (HAI) titer of ≥1:40 as the primary correlate of protection. However, recent poor vaccine performance raises questions regarding the protection afforded and whether other correlates of protection should be targeted. A healthy volunteer challenge study was performed with a wild-type 2009 A(H1N1)pdm influenza A challenge virus at the NIH Clinical Center to evaluate two groups of participants with HAI titers of ≥1:40 and <1:40. The primary objective was to determine whether participants with HAI titers of ≥1:40 were less likely to develop mild to moderate influenza disease (MMID) after intranasal inoculation. HAI titers of ≥1:40 were protective against MMID but did not reduce the incidence of symptoms alone. Although the baseline HAI titer correlated with some reduction in disease severity measures, overall, the baseline NAI titer correlated more significantly with all disease severity metrics and had a stronger independent effect on outcome. This study demonstrates the importance of examining other immunological correlates of protection rather than solely HAI titers. This challenge study confirms the importance of NAI titer as a correlate and for the first time establishes that it can be an independent predictor of reduction of all aspects of influenza disease. This suggests that NAI titer may play a more significant role than previously thought and that neuraminidase immunity should be considered when studying susceptibility after vaccination and as a critical target in future influenza vaccine platforms.

This study represents the first time the current gold standard for evaluating influenza vaccines as set by the U.S. Food and Drug Administration and the European Medicines Agency Committee for Medicinal Products for Human Use, a “protective” hemagglutination inhibition (HAI) titer of ≥1:40, has been evaluated in a well-controlled healthy volunteer challenge study since the cutoff was established. We used our established wild-type influenza A healthy volunteer human challenge model to evaluate how well this antibody titer predicts a reduction in influenza virus-induced disease. We demonstrate that although higher HAI titer is predictive of some protection, there is stronger evidence to suggest that neuraminidase inhibition (NAI) titer is more predictive of protection and reduced disease. This is the first time NAI titer has been clearly identified in a controlled trial of this type to be an independent predictor of a reduction in all aspects of influenza.

Influenza immunology evaluation and correlates of protection: a focus on vaccines

Vaccination is the most effective method of controlling seasonal influenza infections and preventing possible pandemic events. Although influenza vaccines have been licensed and used for decades, the potential correlates of protection induced by these vaccines are still a matter of discussion. Currently, inactivated vaccines are the most common and the haemagglutination inhibition antibody titer is regarded as an immunological correlate of protection and the best available parameter for predicting protection from influenza infection. However, the assay shows some limitations, such as its low sensitivity to B and avian strains and inter-laboratory variability. Additional assays and next-generation vaccines have been evaluated to overcome the limitations of the traditional serological techniques and to elicit broad immune responses, underlining the need to revise the current correlates of protection. The aim of this review is to provide an overview of the current scenario regarding the immunological evaluation and correlates of protection of influenza vaccines.

Standardization of Hemagglutination Inhibition Assay for Influenza Serology Allows for High Reproducibility between Laboratories

Standardization of the hemagglutination inhibition (HAI) assay for influenza serology is challenging. Poor reproducibility of HAI results from one laboratory to another is widely cited, limiting comparisons between candidate vaccines in different clinical trials and posing challenges for licensing authorities. In this study, we standardized HAI assay materials, methods, and interpretive criteria across five geographically dispersed laboratories of a multidisciplinary influenza research network and then evaluated intralaboratory and interlaboratory variations in HAI titers by repeatedly testing standardized panels of human serum samples. Duplicate precision and reproducibility from comparisons between assays within laboratories were 99.8% (99.2% to 100%) and 98.0% (93.3% to 100%), respectively. The results for 98.9% (95% to 100%) of the samples were within 2-fold of all-laboratory consensus titers, and the results for 94.3% (85% to 100%) of the samples were within 2-fold of our reference laboratory data. Low-titer samples showed the greatest variability in comparisons between assays and between sites. Classification of seroprotection (titer ≥ 40) was accurate in 93.6% or 89.5% of cases in comparison to the consensus or reference laboratory classification, respectively. This study showed that with carefully chosen standardization processes, high reproducibility of HAI results between laboratories is indeed achievable.

Report on the second WHO integrated meeting on development and clinical trials of influenza vaccines that induce broadly protective and long-lasting immune responses: Geneva, Switzerland, 5-7 May 2014

On 5-7 May 2014, the World Health Organization (WHO) convened the second integrated meeting on "influenza vaccines that induce broadly protective and long-lasting immune responses". Around 100 invited experts from academia, the vaccine industry, research and development funders, and regulatory and public health agencies attended the meeting. Areas covered included mechanisms of protection in natural influenza-virus infection and vaccine-induced immunity, new approaches to influenza-vaccine design and production, and novel routes of vaccine administration. A timely focus was on how this knowledge could be applied to both seasonal influenza and emerging viruses with pandemic potential such as influenza A (H7N9), currently circulating in China. Special attention was given to the development of possible universal influenza vaccines, given that the Global Vaccine Action Plan calls for at least one licensed universal influenza vaccine by 2020. This report highlights some of the topics discussed and provides an update on studies published since the report of the previous meeting.

Influenza Vaccination Strategies: Comparing Inactivated and Live Attenuated Influenza Vaccines

Influenza is a major respiratory pathogen causing annual outbreaks and occasional pandemics. Influenza vaccination is the major method of prophylaxis. Currently annual influenza vaccination is recommended for groups at high risk of complications from influenza infection such as pregnant women, young children, people with underlying disease and the elderly, along with occupational groups such a healthcare workers and farm workers. There are two main types of vaccines available: the parenteral inactivated influenza vaccine and the intranasal live attenuated influenza vaccine. The inactivated vaccines are licensed from 6 months of age and have been used for more than 50 years with a good safety profile. Inactivated vaccines are standardized according to the presence of the viral major surface glycoprotein hemagglutinin and protection is mediated by the induction of vaccine strain specific antibody responses. In contrast, the live attenuated vaccines are licensed in Europe for children from 2-17 years of age and provide a multifaceted immune response with local and systemic antibody and T cell responses but with no clear correlate of protection. Here we discuss the immunological immune responses elicited by the two vaccines and discuss future work to better define correlates of protection.

Immunological assessment of influenza vaccines and immune correlates of protection

Influenza vaccines remain the primary public health tool in reducing the ever-present burden of influenza and its complications. In seeking more immunogenic, more effective and more broadly cross-protective influenza vaccines, the landscape of influenza vaccines is rapidly expanding, both in near-term advances and next-generation vaccine design. Although the first influenza vaccines were licensed over 60 years ago, the hemagglutination-inhibition antibody titer is currently the only universally accepted immune correlate of protection against influenza. However, hemagglutination-inhibition titers appear to be less effective at predicting protection in populations at high risk for severe influenza disease; older adults, young children and those with certain medical conditions. The lack of knowledge and validated methods to measure alternate immune markers of protection against influenza remain a substantial barrier to the development of more immunogenic, broadly cross-reactive and effective influenza vaccines. Here, the authors review the knowledge of immune effectors of protection against influenza and discuss assessment methods for a broader range of immunological parameters that could be considered in the evaluation of traditional or new-generation influenza vaccines.

Unusual patterns of IgG avidity in some young children following two doses of the adjuvanted pandemic H1N1 (2009) influenza virus vaccine

During the 2009-2010 H1N1 influenza pandemic, an adjuvanted monovalent vaccine containing ∼25% of the normal antigen dose and AS03 adjuvant was widely used in Canada. This vaccine was found to be well-tolerated and immunogenic in young children (D. W. Scheifele et al., Pediatr. Infect. Dis. J. 30:402-407, 2011). We report here additional analyses to further characterize the humoral response to this vaccine. We measured standard hemagglutination inhibition (HAI) and microneutralization (MN) titers, as well as influenza virus-specific IgG avidity and subclass distribution by enzyme-linked immunosorbent assay in 73 subjects. Sera were collected before (day 0) and 3 weeks after each dose of vaccine (days 21 and 42). Most children (55/73) had undetectable HAI and MN titers at day 0 (presumed to be antigen naive) and mounted good responses at days 21 and 42. The majority of these children (43/55) had the expected pattern of an increasing IgG avidity index (AI) after each dose of vaccine (not detected [ND], 0.30, and 2.97 at days 0, 21, and 42, respectively). The avidity responses in the remaining children (12/55) were quite different, with AIs increasing abruptly after the first dose and then declining after the second dose of vaccine (ND, 8.83, and 7.15, respectively). These children also had higher concentrations of influenza virus-specific IgG1 and IgG3 antibodies at day 21. Although the antibody titers were similar, some antigen-naive children demonstrated an unusual pattern of avidity maturation after two immunizations with AS03-adjuvanted, low-dose influenza virus vaccine. These data suggest the presence of subtle differences in the quality of the antibodies produced by some subjects in response to this vaccine.

Correlates of protection to influenza virus, where do we go from here?

Influenza vaccination is the cornerstone of prophylaxis. The regulatory authorities currently annually license vaccines based on serum antibodies directed toward the major surface glycoprotein haemagglutinin (HA). The most commonly used serological test, the haemagglutination inhibition (HI) assay utilizes red blood cells, which show considerable biological variation. There is a need for validated, standardized assays to reduce laboratory variation steps that are currently being taken by the regulatory agencies. Here we examine the historical evidence for defining the HI titer ≥ 40 as a surrogate correlate of protection and examine alternative assays. Moreover, the immune response to influenza is multifacated and there are probably multiple correlates of protection. We conclude there is a need for detailed immunological analysis including kinetic studies and head to head comparison of vaccines by a range of immunological assays to further define correlates of protection.