Elsevier

Vaccine

Volume 29, Issue 11, 3 March 2011, Pages 2092-2099
Vaccine

Longevity of the protective immune response induced after vaccination with one or two doses of AS03A-adjuvanted split H5N1 vaccine in ferrets

https://doi.org/10.1016/j.vaccine.2010.12.128Get rights and content

Abstract

It is crucial that a safe and effective pandemic vaccine be rapidly available to combat a new pandemic threat. In this study we investigated the magnitude and persistence of the protective efficacy induced by one or two doses (3.75 μg HA/dose) of AS03A-adjuvanted H5N1 A/Indonesia/5/05 split vaccine in a lethal ferret challenge model. All ferrets that received at least one dose of adjuvanted vaccine 4 weeks before homologous challenge survived and showed reduced or undetectable virus replication in the lungs and the upper airways. Ferrets receiving two doses of adjuvanted vaccine 19 and 16 weeks before the challenge also showed high level of protection from replication in the lungs and the upper airways, albeit with only 83% survival. Animals in the control groups (non-adjuvanted vaccine or saline) and animals immunized with one dose of adjuvanted vaccine administered 10 or 16 weeks before challenge showed only 17–33% survival rate after challenge. In conclusion, our observations support the possibility that a single dose of AS03A-adjuvanted H5N1 split vaccine can offer a rapid and short term but partial protection against disease. A second dose of the adjuvanted vaccine, which can be given with a flexible injection schedule, was shown to be essential to induce appreciable levels of antibodies and long-term protection.

Introduction

The timely availability of a safe and effective pandemic vaccine will play a crucial role in the efforts to combat a new pandemic threat. The use of a pandemic vaccine before or soon after the onset of a pandemic can be highly effective in reducing the clinical attack rate [1], [2]. However, non-adjuvanted H5 vaccines are known to be poorly immunogenic in humans and animal models and require a high antigen dose as well as two immunizations to elicit sufficient levels of immunity [3], [4], which are not favorable characteristics for a pre-pandemic or pandemic vaccine. Indeed, vaccines must be rapidly available and they must utilize the lowest antigen dose that is capable of inducing protective immune responses to ensure sufficient supply for mass vaccination. Pre-pandemic vaccination is an essential component of a pandemic preparedness plan since it is the only strategy that can be proactively implemented before or in the early stages of a pandemic. It is thus regarded as the most effective intervention to prevent or attenuate pandemic influenza [5]. The WHO has called for the development of vaccines that use adjuvants, able to improve immunogenicity and allowing both antigen sparing and the induction of broadly cross-protective immunity [6]. Using an animal model to evaluate the efficacy of an adjuvanted vaccine, Govorkova et al. confirmed that a 2-dose regimen is preferable for vaccination of an immunologically naïve population against a novel H5N1 strain [7]. The second dose of vaccine, adjuvanted with alum, significantly increased serum antibody levels and conferred complete protection in ferrets against challenge with a high dose of the homologous H5N1 virus, suggesting that two doses of vaccine should be the optimal strategy for a pandemic influenza vaccine. Likewise, a study in humans using an MF59-adjuvanted avian influenza vaccine demonstrated that two doses are required to induce adequate titers of neutralizing antibodies broadly reactive to drifted H5 strains [8].

Animal studies are required to study the efficacy of pandemic vaccines and ferrets (Mustela putorius furo) are considered to be the best mammalian proxy for humans in the study of highly pathogenic avian influenza A/H5N1 vaccines [9], [10]. Recently, we documented the first ferret study in which protection against heterologous challenge was generated by PrepandRIX™, an AS03A-adjuvanted pre-pandemic H5N1 influenza vaccine [9]. Neutralizing antibodies specific for both the vaccine strain and cross-reactive with a heterologous H5N1 virus from a distant clade, were induced. This study demonstrated that two low doses of the AS03A-adjuvanted split H5N1 vaccine A/Vietnam/1194/2004 (clade 1) provided substantial protection against mortality following lethal homologous challenge and heterologous challenge with wild-type virus A/Indonesia/5/2005 (clade 2). Moreover, lung virus loads and viral shedding in the upper respiratory tract were reduced in vaccinated animals. Therefore, this study not only demonstrated cross-clade protection induced in ferrets by the adjuvanted H5N1 influenza vaccine against lethal H5N1 challenge, but also suggested that vaccination can markedly reduce viral shedding from the upper respiratory tract during an infection, thus reducing the risk of virus transmission [11].

On the basis of the ferret data and clinical trial data [12], [13], [14], it has been established that two vaccinations of AS03A-adjuvanted preparations are capable of inducing protective immunity in an immunologically naïve population. Indeed, two vaccinations in humans with doses as little as 3.75 μg of the AS03A-adjuvanted clade 1 H5N1 A/Vietnam/1194/2004 vaccine induced immune responses in adults aged 18–60 years, which complied with all US and European immunological licensure criteria. In addition, a pivotal observation demonstrated the ability of this vaccine to induce a cross-reactive seroprotective immune response in adult volunteers against recombinant strains derived from three recently emerged H5N1 viruses belonging to sub-clades 2.1, 2.2 and 2.3 [12], [14]. However, little is known about the levels of immune responses that provide protection from severe disease or death.

Recently, it has been reported that vaccination of ferrets with a low and single-dose of an Iscomatrix™-based vaccine provided increased survival and significantly reduced disease severity [15]. An efficient single immunization approach would reduce the time needed to achieve a protective immune status and require less antigen, resulting in increased vaccination coverage for the world population.

The present study in ferrets was designed to evaluate the longevity of protection induced by one or two doses of the AS03A-adjuvanted split virus vaccine based on the H5N1 A/Indonesia/5/05 strain against a homologous virus challenge. We also examined the effect of a short and long interval between prime and booster immunizations (3 versus 15 weeks) with a view to establishing flexibility in application of the two-dose vaccine regimen.

Section snippets

Animals

Forty-eight outbred female ferrets (Mustela putorius furo, Schimmel Farms, Uddel, The Netherlands) were used in this study. The animals were approximately 8 months old, weighed 0.8–1.5 kg at the beginning of the experiment and were serologically tested for Aleutian disease virus by using counter-immunoelectrophoresis [16] and for the presence of antibodies against circulating seasonal influenza virus strains (A/H1N1, A/H3N2 and B) and the challenge virus (H5N1 strain A/Indonesia/5/2005) by using

Humoral immune responses

Functional antibody responses specific for H5N1 A/Indonesia/5/2005 were determined by virus neutralization assay (Fig. 1A) and haemagglutination inhibition assay (Fig. 1B). On the day of the first immunization, all ferrets were tested seronegative for currently circulating and H5N1 influenza viruses (data not shown). Two doses of AS03A-adjuvanted Indonesia vaccine were required to induce robust antibody titers in the majority of ferrets. Such regimen induced high immune responses, compared with

Discussion

The objective of this ferret study was to evaluate the persistence of protective immunity to homologous H5N1 viral challenge induced by one or two doses of AS03A-adjuvanted split virus vaccine based on H5N1 A/Indonesia/5/2005 strain. In addition, we examined the effect of a short and long interval between prime and boost immunizations (3 versus 15 weeks).

In our study, high levels of protective efficacy against mortality, clinical signs and virus replication were achieved after two doses of the

Acknowledgments

The authors are very grateful to Cécile Dubois, Pascale Festraets, Véronique Gobin, Philippe Lancelot, Badiaa Bouzya, Virginie Francq, Anne Van Beneden, Romuald Mascolo and Stéphanie Marot (GSK Biologicals, Rixensart, Belgium) and W. van Aert, C. van Hagen, R. van Lavieren, R. Boom, V. Teeuwsen and P. van Run (ViroClinics Biosciences BV and Erasmus Medical Center, Rotterdam, The Netherlands) for their very precious technical support. Finally, we would like to thank Ulrike Krause, Markus Voges

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