Antigenically distinct MF59-adjuvanted vaccine to boost immunity to H5N1

Immunogenicity and safety of MF59-adjuvanted H5N1 influenza vaccine from infancy to adolescence

OBJECTIVE

This study evaluated the immunogenicity, safety, and tolerability of a MF59-adjuvanted H5N1 vaccine in a population 6 months through 17 years of age.

METHODS

Healthy subjects 6 to <36 months, 3 to <9 months, and 9 to <18 years of age were assigned randomly to receive 2 doses of either a MF59-adjuvanted H5N1 vaccine (7.5 μg/dose) or a MF59-adjuvanted trivalent seasonal influenza control vaccine (15 μg/dose for each antigen). Immunogenicity against the A/Vietnam/1194/2004-like vaccine strain was measured before and 3 weeks after the 2-dose primary series, through hemagglutination inhibition (HI), single radial hemolysis (SRH), and microneutralization. Local and systemic reactions were recorded.

RESULTS

A total of 335 subjects received the H5N1 vaccine, and 137 subjects received the seasonal vaccine. Rates of seroprotection (HI titer of ≥40) against the H5N1 vaccine antigen were 97% for children 6 to 36 months and 3 to 9 years of age and 89% for older children. All subjects seroconverted in the SRH assay. Microneutralization titers of ≥40 were achieved by 99% of subjects, and ≥98% of subjects, respectively. Local reactions, particularly injection site pain in older children, were common, generally mild to moderate in nature, and transient and resolved spontaneously. Up to 5% of participants. There were no vaccine-related serious adverse events in either group.

CONCLUSIONS

In this pediatric population, MF59-adjuvanted H5N1 vaccine was highly immunogenic, had a good safety profile, reactogenicity comparable with that of an adjuvanted seasonal influenza control vaccine.

Comparison of half and full doses of an MF59-adjuvanted cell culture-derived A/H1N1v vaccine in Japanese children

INTRODUCTION

The substantial pandemic (A/H1N1v) influenza disease burden in children highlights the need for effective vaccination. We report the results of modern cell culture technology, lower doses of antigen, and different doses of MF59(R) adjuvant (Novartis Vaccines, Marburg, Germany), on the immunogenicity and safety profile in a healthy Japanese pediatric population.

METHODS

A total of 123 children from 6 months to 19 years of age were randomly assigned in a 1:1 ratio to receive, at 21-day intervals, two doses of either 3.75 microg antigen with 50% of the standard MF59 dose (group A) or 7.5 microg antigen and 100% standard MF59 dose (group B). Antibody levels were measured by hemagglutinin inhibition (HI) and microneutralization assays on day 1 and on days 22 and 43 (3 weeks after the first and second vaccinations, respectively). Solicited adverse events were reported for 7 days after each injection and spontaneous events were reported throughout the study period.

RESULTS

At 3 weeks after the first vaccination, seroprotective HI antibodies (titers >or=40) were observed in 56% and 78% of subjects from groups A and B, respectively; 100% in both groups exhibited HI titers >or=40 after the second dose. The reactogenicity profile was acceptable, with local and systemic reactions described as mainly mild to moderate in severity. Five serious adverse events were reported, but none related to the study vaccine.

CONCLUSION

One dose of cell culture-derived A/H1N1v vaccine containing 7.5 microg antigen with the full MF59 adjuvant dose was immunogenic and well tolerated in healthy Japanese children, meeting all three European Union Committee for Medicinal Products for Human Use (EU CHMP) licensure criteria. Two doses of 3.75 microg antigen with 50% of the standard MF59 dose fulfilled these licensure criteria.

Vaccines with MF59 adjuvant expand the antibody repertoire to target protective sites of pandemic avian H5N1 influenza virus

Vaccines against influenza viruses with pandemic potential, including H5N1, are under development. Because of a lack of preexisting immunity to these viruses, adjuvants (immune potentiators or enhancers) are needed to improve immune responses, to conserve scarce vaccine, and for cross-protection against strains that have drifted evolutionarily from the original. Aluminum-based adjuvants do not improve vaccine immunogenicity for influenza subunit vaccines, whereas oil-in-water adjuvants are effective, especially with H5N1-inactivated vaccines. We used whole-genome-fragment phage display libraries followed by surface plasmon resonance (SPR) technologies to elucidate the effect of different adjuvants on the antibody repertoire against H5N1 vaccine in humans. The oil-in-water adjuvant MF59 induced epitope spreading from HA2 to HA1 in hemagglutinin (HA) and neuraminidase relative to unadjuvanted or aluminum-adjuvanted vaccines. Moreover, we observed an increase by a factor of 20 in the frequency of HA1-to-HA2-specific phage clones in sera after MF59-adjuvanted vaccine administration and a factor of 2 to 3 increase in the avidity of antibodies binding to properly folded HA1(28-319), as measured by SPR. The adjuvant-dependent increase in binding to conformational HA1 epitopes correlated with broadening of cross-clade neutralization and predicted improved in vivo protection. Thus, MF59 adjuvant improves the immune response to a H5N1 vaccine by inducing qualitative and quantitative expansion of the antibody repertoires with protective potential.

Avian influenza pandemic preparedness: developing prepandemic and pandemic vaccines against a moving target

The unprecedented global spread of highly pathogenic avian H5N1 influenza viruses within the past ten years and their extreme lethality to poultry and humans has underscored their potential to cause an influenza pandemic. Combating the threat of an impending H5N1 influenza pandemic will require a combination of pharmaceutical and nonpharmaceutical intervention strategies. The emergence of the H1N1 pandemic in 2009 emphasised the unpredictable nature of a pandemic influenza. Undoubtedly, vaccines offer the most viable means to combat a pandemic threat. Current egg-based influenza vaccine manufacturing strategies are unlikely to be able to cater to the huge, rapid global demand because of the anticipated scarcity of embryonated eggs in an avian influenza pandemic and other factors associated with the vaccine production process. Therefore, alternative, egg-independent vaccine manufacturing strategies should be evaluated to supplement the traditional egg-derived influenza vaccine manufacturing. Furthermore, evaluation of dose-sparing strategies that offer protection with a reduced antigen dose will be critical for pandemic influenza preparedness. Development of new antiviral therapeutics and other, nonpharmaceutical intervention strategies will further supplement pandemic preparedness. This review highlights the current status of egg-dependent and egg-independent strategies against an avian influenza pandemic.

Antibody response against heterogeneous circulating influenza virus strains elicited by MF59- and non-adjuvanted vaccines during seasons with good or partial matching between vaccine strain and clinical isolates

MF59 is already known to enhance the breadth of antibody response to mismatched influenza seasonal and avian strains. However, little is known on the effect of MF59 on immunogenicity of influenza vaccines when "apparent" good matching between circulating and vaccine strains exists. To this end, we compared the immune response elicited by MF59-adjuvanted or non-adjuvanted subunit vaccine, containing A/California/7/04(H3N2) strain, against circulating viruses isolated between 2004/2005 and 2006/2007 seasons, belonging to different clades. The advantage offered by MF59 in terms of higher immunogenicity, expressed as higher post-vaccination HI titres, is observable also against viruses showing antigenic and molecular pattern undistinguishable from vaccine strain, but it became even more evident as the antigenic and molecular distance between vaccine and circulating strains grew. These data show that seasonal influenza vaccine adjuvanted with MF59 can offer a stronger benefit as compared to non-adjuvanted vaccine in protecting against a broader range of virus strains circulating during the influenza season.

Report of the fourth meeting on 'Influenza vaccines that induce broad spectrum and long-lasting immune responses', World Health Organization and Wellcome Trust, London, United Kingdom, 9-10 November 2009

Current influenza vaccines are limited by the need for annual immunisation, frequent antigenic updating to match the evolution of circulating influenza virus strains, and reduced efficacy in elderly persons. On 9-10 November 2009, the Initiative for Vaccine Research of the World Health Organization convened jointly with the Wellcome Trust in London, United Kingdom, the fourth meeting on 'Influenza vaccines that induce broad spectrum and long-lasting immune responses'. Presentations were made by representatives from industry, academia, governmental and non-governmental organisations. The objectives of the meeting were to update the progress of research in the field of influenza vaccine strategies able to generate cross protection against divergent influenza virus strains. Improvements in existing strategies including live attenuated influenza vaccines and adjuvantation of inactivated vaccines were summarised. Developments in novel antigen production methods, new routes of vaccine delivery and administration, and vaccine approaches based on conserved virus antigens were explored. In addition, correlates of immune protection and regulatory issues for the evaluation and approval of future novel vaccine strategies were discussed.

Cross-clade protection against HPAI H5N1 influenza virus challenge in BALB/c mice intranasally administered adjuvant-combined influenza vaccine

The avian H5N1 influenza virus has the potential to cause a new pandemic. The increasing number of recent outbreaks of highly pathogenic avian influenza H5N1 in birds and humans emphasizes the urgent need to develop a potent H5N1 vaccine. Here, we studied the immunogenicity and protective effect of a vaccine prepared from H5N1 inactivated whole virus. This vaccine was intranasally co-administered in mice with phosphate buffered saline, recombinant cholera toxin B subunit (rCTB), cholera toxin (CT), rCTB containing a trace amount of holotoxin (rCTB/CT), polyinosinic:polycytidylic acid double-stranded RNA (polyI:C), or MF59 as an adjuvant. Intranasal administration of H5N1 inactivated whole virus vaccine with rCTB, CT, rCTB/CT, polyI:C, and MF59 elicited an immunological response with both secretory IgA (sIgA) in nasal, lung, and vaginal lavage, and IgG antibody in serum, showing protective immunity against lethal H5N1 infection. Cross-clade protection was also observed in animals immunized with a vaccine derived from Anhui/01/2005(H5N1) with rCTB, CT, rCTB/CT, polyI:C, or MF59 as adjuvants that were subsequently challenged with the A/OT/SZ/097/03 influenza strain.

Animal models for the preclinical evaluation of candidate influenza vaccines

At present, new influenza A (H1N1)2009 viruses of swine origin are responsible for the first influenza pandemic of the 21st Century. In addition, highly pathogenic avian influenza A/H5N1 viruses continue to cause outbreaks in poultry and, after zoonotic transmission, cause an ever-increasing number of human cases, of which 59% have a fatal clinical outcome. It is also feared that these viruses adapt to replication in humans and become transmissible from human to human. The development of effective vaccines against epidemic and (potentially) pandemic viruses is therefore considered a priority. In this review, we discuss animal models that are used for the preclinical evaluation of novel candidate influenza vaccines. In most cases, a tier of multiple animal models is used before the evaluation of vaccine candidates in clinical trials is considered. Commonly, vaccines are tested for safety and efficacy in mice, ferrets and/or macaques. The use of each of these species has its advantages and limitations, which are addressed here.

Effectiveness and cost-effectiveness of expanded antiviral prophylaxis and adjuvanted vaccination strategies for an influenza A (H5N1) pandemic

BACKGROUND

The pandemic potential of influenza A (H5N1) virus is a prominent public health concern of the 21st century.

OBJECTIVE

To estimate the effectiveness and cost-effectiveness of alternative pandemic (H5N1) mitigation and response strategies.

DESIGN

Compartmental epidemic model in conjunction with a Markov model of disease progression.

DATA SOURCES

Literature and expert opinion.

TARGET POPULATION

Residents of a U.S. metropolitan city with a population of 8.3 million.

TIME HORIZON

Lifetime.

PERSPECTIVE

Societal.

INTERVENTIONS

3 scenarios: 1) vaccination and antiviral pharmacotherapy in quantities similar to those currently available in the U.S. stockpile (stockpiled strategy), 2) stockpiled strategy but with expanded distribution of antiviral agents (expanded prophylaxis strategy), and 3) stockpiled strategy but with adjuvanted vaccine (expanded vaccination strategy). All scenarios assumed standard nonpharmaceutical interventions.

OUTCOME MEASURES

Infections and deaths averted, costs, quality-adjusted life-years (QALYs), and incremental cost-effectiveness.

RESULTS OF BASE-CASE ANALYSIS

Expanded vaccination was the most effective and cost-effective of the 3 strategies, averting 68% of infections and deaths and gaining 404 030 QALYs at $10 844 per QALY gained relative to the stockpiled strategy.

RESULTS OF SENSITIVITY ANALYSIS

Expanded vaccination remained incrementally cost-effective over a wide range of assumptions.

LIMITATIONS

The model assumed homogenous mixing of cases and contacts; heterogeneous mixing would result in faster initial spread, followed by slower spread. We did not model interventions for children or older adults; the model is not designed to target interventions to specific groups.

CONCLUSION

Expanded adjuvanted vaccination is an effective and cost-effective mitigation strategy for an influenza A (H5N1) pandemic. Expanded antiviral prophylaxis can help delay the pandemic while additional strategies are implemented.

PRIMARY FUNDING SOURCE

National Institutes of Health and Agency for Healthcare Research and Quality.

Pandemic influenza vaccines - the challenges

Recent years' enzootic spread of highly pathogenic H5N1 virus among poultry and the many lethal zoonoses in its wake has stimulated basic and applied pandemic vaccine research. The quest for an efficacious, affordable and timely accessible pandemic vaccine has been high on the agenda. When a variant H1N1 strain of swine origin emerged as a pandemic virus, it surprised many, as this subtype is well-known to man as a seasonal virus. This review will cover some difficult vaccine questions, such as the immunological challenges, the new production platforms, and the limited supply and global equity issues.

Single dose vaccination with AS03-adjuvanted H5N1 vaccines in a randomized trial induces strong and broad immune responsiveness to booster vaccination in adults

Priming a population with a pre-pandemic vaccine is being considered to maximize the response upon subsequent vaccination with a true pandemic vaccine more closely matched to the causative pandemic strain. The present study explored this prime-boost concept by evaluating different primary schedules with the pre-pandemic A/Vietnam/1194/2004(NIBRG-14) vaccine, containing 3.75 microg of HA, followed by a 6-month booster with a vaccine formulated with 3.75 microg HA of either the same strain or with A/Indonesia/05/2005(IBCDC-RG2), a heterologous strain from a different clade. In this multicentre, open, randomized study (NCT00430521) we measured immune responses in four groups (N = 48-60) of adults aged 18-60 years who received a single booster administration of either A/Indonesia/05/2005 or A/Vietnam/1194/2004 vaccine 6 months after a 1- or 2-dose (given 21 days apart) primary vaccination with A/Vietnam/1194/2004. All prime-boost schedules assessed induced early (7 days post-booster) humoral responses that met regulatory acceptance criteria. Two doses of A/Vietnam/1194/2004 given 6 months apart achieved equivalent homologous seroprotection after the second vaccination (89.6%), when compared to two doses given 21 days apart (92.7-93.2%). Remarkably, two doses of A/Vietnam/1194/2004 given 6 months apart induced a higher cross-reactive seroprotection against A/Indonesia/05/2005 (83.3%) when compared to two doses given 21 days apart (41.5-54.5%). A 6-month A/Indonesia/05/2005 booster dose after one primary dose of A/Vietnam/1194/2004 vaccine induced 92.5% seroprotection against A/Indonesia/05/2005 and 98.1% against A/Vietnam/1194/2004. Since a single booster 6 months after one primary dose of AS03-adjuvanted vaccine induces strong and rapid seroprotective immune response against both homologous and heterologous H5N1 strains, these results might have important implications for public health strategy aiming to organize vaccination campaigns with pre-pandemic vaccines.

Cross-reactive antibody responses to the 2009 pandemic H1N1 influenza virus

BACKGROUND

A new pandemic influenza A (H1N1) virus has emerged, causing illness globally, primarily in younger age groups. To assess the level of preexisting immunity in humans and to evaluate seasonal vaccine strategies, we measured the antibody response to the pandemic virus resulting from previous influenza infection or vaccination in different age groups.

METHODS

Using a microneutralization assay, we measured cross-reactive antibodies to pandemic H1N1 virus (2009 H1N1) in stored serum samples from persons who either donated blood or were vaccinated with recent seasonal or 1976 swine influenza vaccines.

RESULTS

A total of 4 of 107 persons (4%) who were born after 1980 had preexisting cross-reactive antibody titers of 40 or more against 2009 H1N1, whereas 39 of 115 persons (34%) born before 1950 had titers of 80 or more. Vaccination with seasonal trivalent inactivated influenza vaccines resulted in an increase in the level of cross-reactive antibody to 2009 H1N1 by a factor of four or more in none of 55 children between the ages of 6 months and 9 years, in 12 to 22% of 231 adults between the ages of 18 and 64 years, and in 5% or less of 113 adults 60 years of age or older. Seasonal vaccines that were formulated with adjuvant did not further enhance cross-reactive antibody responses. Vaccination with the A/New Jersey/1976 swine influenza vaccine substantially boosted cross-reactive antibodies to 2009 H1N1 in adults.

CONCLUSIONS

Vaccination with recent seasonal nonadjuvanted or adjuvanted influenza vaccines induced little or no cross-reactive antibody response to 2009 H1N1 in any age group. Persons under the age of 30 years had little evidence of cross-reactive antibodies to the pandemic virus. However, a proportion of older adults had preexisting cross-reactive antibodies.

The ABC of clinical and experimental adjuvants--a brief overview

Adjuvants are compounds that can increase and/or modulate the intrinsic immunogenicity of an antigen and elicit strong and long lasting immune responses. During the last 80 years many adjuvants have been used in experimental settings, but due to various shortcomings of most of them only aluminum compounds made it into regular clinical usage. However, during the last years promising candidates have arisen that may finally adjunct or displace aluminum substances as main adjuvant. This review summarizes information on adjuvants currently used in clinical as well as in experimental settings.

Priming with AS03 A-adjuvanted H5N1 influenza vaccine improves the kinetics, magnitude and durability of the immune response after a heterologous booster vaccination: an open non-randomised extension of a double-blind randomised primary study

An influenza vaccine with cross-immunogenic potential could play a key role in pandemic mitigation by promoting a rapid immune response to infection and/or subsequent vaccination with strains drifted from the primary vaccine strain. Here we assess the role of AS03(A) (an oil-in-water emulsion based Adjuvant System containing tocopherol) in this prime-boost concept using H5N1 as a model shift influenza antigen. In this open, non-randomised study (NCT00506350; an extension of an earlier randomised study) we assessed immunogenicity in nine groups of 35-50 volunteers aged 19-61 years following administration of AS03(A)-adjuvanted split-virion H5N1 vaccine containing 3.75mug of haemagglutinin (HA) from the A/Indonesia/5/2005(IBCDC-RG2) clade 2.1 strain. A single booster dose of vaccine was administered to four groups primed 14 months previously with different HA levels of AS03(A)-adjuvanted clade 1 A/Vietnam/1194/2004 H5N1 vaccine. Two booster doses (given 21 days apart) were administered to four groups primed 14 months previously with different HA levels of non-adjuvanted A/Vietnam/1194/2004 H5N1 vaccine and also to a control group of un-primed subjects. In individuals primed 14 months earlier with AS03(A)-adjuvanted A/Vietnam/1194/2004 vaccines, a single booster dose of AS03(A)-adjuvanted A/Indonesia/5/2005 induced rapid immune responses (licensure criteria met in 7-14 days) comparable to that observed in the un-primed control group following two doses of adjuvanted vaccine. In contrast, individuals primed with non-adjuvanted formulations exhibited minimal immune responses which, even after two doses, were unexpectedly much lower than that observed in un-primed subjects. AS03(A) enhances the initial priming effect of pandemic influenza vaccination enabling a rapid humoral response to single dose boosting with a heterologous strain at 14 months. In contrast, priming without adjuvant appears to inhibit the response to subsequent vaccination with a heterologous strain. These findings should guide the development of vaccines to combat the present influenza A/H1N1 pandemic.

Strategies for mitigating an influenza pandemic with pre-pandemic H5N1 vaccines

The recent worldwide spread of the swine-origin H1N1 2009 influenza outbreak has resulted in its designation as a pandemic by the World Health Organization. While it appears to result in mild symptoms, concern still exists that a more severe influenza pandemic with a high case fatality ratio might arise by reassortment or mutation of the currently circulating avian influenza (H5N1) virus. Given that recently developed candidate pre-pandemic H5N1 vaccines have shown potential for cross-strain protection, we investigated alternative vaccination strategies that exploit such vaccines using an agent-based simulation model of an actual community of approximately 30 000 people in a developed country. Assuming that a two-dose vaccination regimen would be required, we examined three vaccination strategies: pre-emptive, with vaccination applied prior to emergence of human-transmissible H5N1 influenza; reactive, where vaccination was initiated immediately after the first cases in the community were diagnosed; and a 'split' strategy where the first dose was administered pre-emptively during the pre-pandemic phase, with the second dose administered reactively. We showed that by effectively moving the delay between first and second doses into the pre-pandemic period, the split vaccination strategy achieved a substantially better attack rate reduction than the reactive strategy. Our results for an influenza strain with a reproduction number of 1.5 suggest reactive vaccination strategies that may be applicable to the current H1N1 2009 pandemic.

Pandemic influenza vaccines: meeting the supply, distribution and deployment challenges

An influenza pandemic will place an enormous strain on the world's vaccine production, distribution and administration systems. Following a pandemic declaration, industry's priority will be to deliver as much vaccine in as short a timeframe as possible. In respect to this challenge, manufacturers have successfully developed antigen-sparing strategies and significantly increased production capacity, with further growth planned assuming ongoing rising demand for seasonal vaccines. The combination of these factors has the potential to closer meet global needs for vaccine supply than ever before through increased availability of pandemic and pre-pandemic vaccines. The demonstration of cross-clade reactivity with H5N1 viruses makes the concept of pre-pandemic stockpiling and vaccination a reality for this subtype. Ensuring these vaccines are made available in a timely fashion to those who need them will present significant challenges. For local authorities, national governments and international organisations this means defining vaccine allocation and procurement processes as well as strengthening, and where necessary establishing, the critical health systems and infrastructure required for vaccine deployment. For vaccine producers this means addressing the technical and logistical issues associated with supply. This includes working with regulators to streamline key procedures, including generic labelling and batch release, while establishing flexibility in supply formats, including bulk and finished products, to maximise the speed of delivery. Similarly, the deployment of large quantities of vaccines in an emergency situation requires appropriate transport infrastructure and the distribution of associated medical supplies. As well as addressing these issues, specific consideration must be given to the logistics and storage aspects associated with stockpiling pre-pandemic vaccines. Finally, mutually agreed contractual arrangements between manufacturers and governments or international institutions represent the best approach toward addressing supply challenges and assisting vaccine producers meet national and international demand. To be effective, these contracts should be based on accurate forecasts, clearly defined vaccination strategies and the capabilities of public health infrastructure.

Pseudoparticle neutralization is a reliable assay to measure immunity and cross-reactivity to H5N1 influenza viruses

The standard serological methods present limitations for the measurement of immunity against H5N1 influenza strains. The hemagglutination inhibition (HI) assay lacks sensitivity and requires standardization, while the viral micro-neutralization (MN) assay needs handling of live virus. We produced pseudoparticles expressing hemagglutinin from clades 1 or 2 H5N1 in order to measure neutralizing antibodies in human sera after prime-boost vaccination with plain or MF59-adjuvanted H5N1 clade 1 subunit vaccines. Titers measured by pseudoparticle neutralization (PPN) assay significantly correlated with those measured by HI, single radial haemolysis or MN, with a PPN titer of 1:357 corresponding to an MN titer of 1:80. Notably, results from the PPN assay, confirm that MF59-H5N1 vaccine induces potent and long-lasting neutralizing antibody responses not only against the vaccine strain, but also against several heterologous clade 2 strains. Overall, the PPN assay represents a valid alternative to conventional serological methods for the evaluation of H5N1 vaccine immunogenicity.

Emergence and pandemic potential of swine-origin H1N1 influenza virus

Influenza viruses cause annual epidemics and occasional pandemics that have claimed the lives of millions. The emergence of new strains will continue to pose challenges to public health and the scientific communities. A prime example is the recent emergence of swine-origin H1N1 viruses that have transmitted to and spread among humans, resulting in outbreaks internationally. Efforts to control these outbreaks and real-time monitoring of the evolution of this virus should provide us with invaluable information to direct infectious disease control programmes and to improve understanding of the factors that determine viral pathogenicity and/or transmissibility.

MF59-adjuvanted vaccines for seasonal and pandemic influenza prophylaxis

Abstract  Influenza is a major cause of worldwide morbidity and mortality through frequent seasonal epidemics and infrequent pandemics. Morbidity and mortality rates from seasonal influenza are highest in the most frail, such as the elderly, those with underlying chronic conditions and very young children. Antigenic mismatch between strains recommended for vaccine formulation and circulating viruses can further reduce vaccine efficacy in these populations. Seasonal influenza vaccines with enhanced, cross‐reactive immunogenicity are needed to address these problems and can confer a better immune protection, particularly in seasons were antigenic mismatch occurs. A related issue for vaccine development is the growing threat of pandemic influenza caused by H5N1 avian strains. Vaccines against strains with pandemic potential offer the best approach for reducing the potential impact of a pandemic. However, current non‐adjuvanted pre‐pandemic vaccines offer suboptimal immunogenicity against H5N1. For both seasonal and pre‐pandemic vaccines, the addition of adjuvants may be the best approach for providing enhanced cross‐reactive immunogenicity. MF59^®, the first oil‐in‐water emulsion licensed as an adjuvant for human use, can enhance vaccine immune responses through multiple mechanisms. A trivalent MF59‐adjuvanted seasonal influenza vaccine (Fluad^®) has shown to induce significantly higher immune responses to influenza vaccination in the elderly, compared with non‐adjuvanted vaccines, and to provide cross‐reactive immunity against divergent influenza strains. Similar results have been generated with a MF59‐adjuvanted H5N1 pre‐pandemic vaccine, which showed higher and broader immunogenicity compared with non‐adjuvanted pre‐pandemic vaccines.

Report of the 5th meeting on the evaluation of pandemic influenza prototype vaccines in clinical trials: World Health Organization, Geneva, Switzerland, 12-13 February 2009

Influenza vaccines are potentially the most efficacious means of mitigating the impact of influenza pandemic and might contribute to the rapid containment of an emerging pandemic virus. On the 12-13 February 2009, the Initiative For Vaccine Research (IVR) of the World Health Organisation convened the 5th meeting on the 'Evaluation of pandemic influenza prototype vaccines in clinical trials' in Geneva. This was a follow-up meeting to the 4th meeting held on 14-15 February 2008 [Girard M, Palkonyay L, Kieny MP. Report of the 4th meeting on the evaluation of pandemic influenza prototype vaccines in clinical trials. Vaccine 2008;26:4975-7], and presentations were made by representatives from industry, academia, and governmental organisations. This year's meeting aimed to update the progress made during the past year on H5N1 and other prototype pandemic vaccines that have undergone clinical trials. A number of vaccine types were covered, including classical egg-derived inactivated vaccines, cell-derived inactivated vaccines, live-attenuated vaccines (LAIV) and vaccines developed using new technologies. The effects of different adjuvants and prime-boosting schedules were important topics, and further data were presented to show that children mount vigorous antibody responses to several H5N1 vaccines. Other subjects presented and discussed were standardisation, and regulatory issues concerning pandemic vaccines.

Current status and progress of prepandemic and pandemic influenza vaccine development

H5N1 viruses are widely considered to be a probable cause of the next influenza pandemic. Influenza vaccines are considered to form the main prophylactic measure against pandemic influenza. The world's population is expected to have no pre-existing immunity against the pandemic virus strain and will need two vaccine doses to acquire protective immunity. A pandemic outbreak will spread much faster than it will take for pandemic vaccines to be produced and distributed. Therefore, increasing efforts are being made to develop prepandemic vaccines that can induce broad cross-protective responses and that can be administered as soon as a pandemic is declared or even before, in order to successfully prime the immune system and allow for a rapid and protective antibody response with one dose of the pandemic vaccine. Several vaccine manufacturers have developed candidate pandemic and prepandemic vaccines, predominantly based on reverse-genetics reference strains and have improved the immunogenicity by formulating these vaccines with different adjuvants. Clinical studies with inactivated split-virion or whole-virion vaccines based on H5N1 indicate that two immunizations appear necessary to elicit the level of immunity required to meet licensure criteria. A detailed overview is given of the most successful candidate vaccines developed by seven vaccine manufacturers.

Progress with human H5N1 vaccines: a perspective from industry

With the ongoing widespread circulation of avian H5N1 influenza viruses and the threat of pandemic influenza an ever-present reality, it is essential that those charged with protecting public health continue to focus intently on this urgent issue. As part of the response to this pandemic threat, research-based influenza vaccine manufacturers have made rapid progress with the research, development and testing of new vaccines and have greatly expanded their capacity to manufacture these products in the last few years. With the research and development phase now reaching completion, regulators have approved several prototype 'mock-up' pandemic vaccines and a number of 'prepandemic' H5N1 vaccines. As a consequence, the world is better prepared than it has ever been to counter an influenza pandemic. However, despite these advances, many policy, logistical and practical issues must be resolved for the population to benefit from these breakthroughs. In particular, the global community must establish the infrastructure required for population-wide immunization, secure appropriate vaccine supplies and undertake sufficient vaccine stockpiling to protect against the first wave of a pandemic.

Fast rise of broadly cross-reactive antibodies after boosting long-lived human memory B cells primed by an MF59 adjuvanted prepandemic vaccine

Proactive priming before the next pandemic could induce immune memory responses to novel influenza antigens. In an open-label study, we analyzed B cell memory and antibody responses of 54 adults who received 2 7.5-microg doses of MF59-adjuvanted A/Vietnam/1194/2004 clade 1 (H5N1) vaccine. Twenty-four subjects had been previously primed with MF59-adjuvanted or plain clade 0-like A/duck/Singapore/1997 (H5N3) vaccine during 1999-2001. The prevaccination frequency of circulating memory B cells reactive to A/Vietnam/1194/2004 was low in both primed and unprimed individuals. However, at day 21 after boosting, MF59-adjuvanted primed subjects displayed a higher frequency of H5N1-specific memory B cells than plain-primed or unprimed subjects. The immune memory was rapidly mobilized by a single vaccine administration and resulted in high titers of neutralizing antibodies to antigenically diverse clade 0, 1, and 2 H5N1 viruses already at day 7. In general, postvaccination antibody titers were significantly higher in primed subjects than in unprimed subjects. Subjects primed with MF59-adjuvanted vaccine responded significantly better than those primed with plain vaccine, most notably in early induction and duration of cross-reacting antibody responses. After 6 months, high titers of cross-reactive antibody remained detectable among MF59-primed subjects. We conclude that distant priming with clade 0-like H5N3 induces a pool of cross-reactive memory B cells that can be boosted rapidly years afterward by a mismatched MF59-adjuvanted vaccine to generate high titers of cross-reactive neutralizing antibodies rapidly. These results suggest that pre-pandemic vaccination strategies should be considered.

Vaccinia virus-based multivalent H5N1 avian influenza vaccines adjuvanted with IL-15 confer sterile cross-clade protection in mice

The potential for a global influenza pandemic remains significant with epidemiologic and ecologic indicators revealing the entrenchment of the highly pathogenic avian influenza A H5N1 in both wild bird populations and domestic poultry flocks in Asia and in many African and European countries. Indisputably, the single most effective public health intervention in mitigating the devastation such a pandemic could unleash is the availability of a safe and effective vaccine that can be rapidly deployed for pre-exposure vaccination of millions of people. We have developed two vaccinia-based influenza vaccines that are molecularly adjuvanted with the immune stimulatory cytokine IL-15. The pentavalent Wyeth/IL-15/5Flu vaccine expresses the hemagglutinin, neuraminidase, and nucleoprotein derived from the H5N1 influenza virus A/Vietnam/1203/2004 and the matrix proteins M1 and M2 from the H5N1 A/CK/Indonesia/PA/2003 virus on the backbone of a currently licensed smallpox vaccine. The bivalent MVA/IL-15/HA/NA vaccine expresses only the H5 hemagglutinin and N1 neuraminidase on the modified vaccinia virus Ankara (MVA) backbone. Both vaccines induced cross-neutralizing Abs and robust cellular immune responses in vaccinated mice and conferred sterile cross-clade protection when challenged with the H5N1 virus of a different clade. In addition to having potential as a universal influenza vaccine, in the event of an impending pandemic the Wyeth/IL-15/5Flu is also readily amenable to bulk production to cover the global population. For those individuals for whom the use of the Wyeth vaccine is contraindicated, our MVA/IL-15/HA/NA offers a substitute or a prevaccine to be used in a mass vaccination campaign similar to the smallpox eradication campaigns of few decades ago.

Willingness of healthcare workers to accept voluntary stockpiled H5N1 vaccine in advance of pandemic activity

Healthcare workers may be at risk during the next influenza pandemic. Priming with stockpiled vaccine may protect staff and reduce nosocomial transmission. Despite campaigns to increase seasonal influenza vaccine coverage, uptake among healthcare workers is generally low; creating uncertainty whether they would participate in pre-pandemic vaccine programmes. We conducted a cross-sectional questionnaire survey of healthcare workers in a UK hospital during, and 6 months after, a period of media reporting of an H5N1 outbreak at a commercial UK poultry farm. A total of 520 questionnaires were returned, representing 20% of frontline workforce. More respondents indicated willingness to accept stockpiled H5N1 vaccine during the period of media attention than after (166/262, 63.4% vs. 134/258, 51.9%; p=0.009). Following multivariate analysis, factors associated with willingness to accept H5N1 vaccine included: previous seasonal vaccine (OR 6.2, 95% CI 3.0-12.8, p<0.0001), awareness of occupational seasonal vaccine campaigns (OR 2.2, 95% CI 1.4-3.5, p=0.001), belief that seasonal vaccine benefits themselves (OR 2.5, 95% CI 1.6-4.0, p<0.0001) or the hospital (OR 3.6, 95% CI 2.3-5.8, p<0.0001), belief that pandemic risk is high/moderate (OR 14.1, 95% CI 7.6-26.1, p<0.0001) and would threaten healthcare workers (OR 2.9, 95% CI 1.8-4.5, p<0.0001). Those who would not accept vaccine (220 respondents, 42.7%) if offered before the pandemic do not perceive pandemic influenza as a serious threat, and have concerns regarding vaccine safety. A majority of healthcare workers are amenable to accept stockpiled H5N1 vaccine if offered in advance of pandemic activity.