Abstract
The continued spread of highly pathogenic H5N1 influenza viruses among poultry and wild birds, together with the emergence of drug-resistant variants and the possibility of human-to-human transmission, has spurred attempts to develop an effective vaccine. Inactivated subvirion or whole-virion H5N1 vaccines have shown promising immunogenicity in clinical trials, but their ability to elicit protective immunity in unprimed human populations remains unknown. A cold-adapted, live attenuated vaccine with the hemagglutinin (HA) and neuraminidase (NA) genes of an H5N1 virus A/VN/1203/2004 (clade 1) was protective against the pulmonary replication of homologous and heterologous wild-type H5N1 viruses in mice and ferrets. In this study, we used reverse genetics to produce a cold-adapted, live attenuated H5N1 vaccine (AH/AAca) that contains HA and NA genes from a recent H5N1 isolate, A/Anhui/2/05 virus (AH/05) (clade 2.3), and the backbone of the cold-adapted influenza H2N2 A/AnnArbor/6/60 virus (AAca). AH/AAca was attenuated in chickens, mice, and monkeys, and it induced robust neutralizing antibody responses as well as HA-specific CD4+ T cell immune responses in rhesus macaques immunized twice intranasally. Importantly, the vaccinated macaques were fully protected from challenge with either the homologous AH/05 virus or a heterologous H5N1 virus, A/bar-headed goose/Qinghai/3/05 (BHG/05; clade 2.2). These results demonstrate for the first time that a cold-adapted H5N1 vaccine can elicit protective immunity against highly pathogenic H5N1 virus infection in a nonhuman primate model and provide a compelling argument for further testing of double immunization with live attenuated H5N1 vaccines in human trials.
Highlights
In 1996, a highly pathogenic H5N1 avian influenza virus was detected in geese in China [1]
The development of effective vaccines against these viruses is the highest priority for H5N1 pandemic preparedness
A high dosage or adjuvants improve the immunogenicity of H5N1 inactivated vaccines; limited production capacity for conventional inactivated influenza virus vaccines could severely hinder the ability to control the spread of H5N1 influenza through vaccination
Summary
In 1996, a highly pathogenic H5N1 avian influenza virus was detected in geese in China [1]. A year later, a reassortant H5N1 virus caused disease outbreaks in poultry in Hong Kong [2] and was transmitted to humans, infecting 18 people, six of whom died [3,4]. Beginning in late 2003, outbreaks of H5N1 influenza A virus infection appeared among poultry, and wild birds in numerous countries in Asia and subsequently were reported in Europe and Africa Despite substantial efforts to control the infection in poultry, H5N1 viruses have continued to evolve and spread, producing human infections in 14 countries, with 236 of the 372 confirmed cases proving fatal (World Health Organization [WHO]; http:// www.who.int). The emergence of H5N1 viruses resistant to adamantanes and oseltamivir [5,6,7] has raised serious concerns over the ability of current antiviral agents to prevent global influenza outbreaks. The development of an effective vaccine has assumed the highest priority in preparedness for an H5N1 influenza pandemic
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