Development and evaluation of an Influenza virus subtype H7N2 vaccine candidate for pandemic preparedness

Abstract

Influenza virus of the H7N2 subtype has been introduced into noncommercial poultry in the United States, and this probably resulted in incidents of transmission of H7N2 virus to humans, documented in 2002 and 2003. This virus could be considered a potential threat to public health if it acquired person-to-person transmissibility. A favored approach for global pandemic preparedness includes development of prepandemic vaccines for any potential pandemic virus. To this end, we created a high-growth reassortant virus (H7N2-PR8) containing the genes for the hemagglutinin and the neuraminidase from a low-pathogenicity (H7N2) virus strain and the remaining six genes from a human vaccine strain (H1N1). The reassortant strain was evaluated to assess its antigenicity, safety, and protective efficacy using a mouse model. Antigenicity studies using ferret antibodies raised against H7N2-PR8 indicated that this virus confers broad cross-reactivity with divergent H7 viruses of different years and lineages. Mice and chickens inoculated with high doses of H7N2-PR8 supported virus replication but survived, indicating that this virus is comparable to other avian viruses of low pathogenicity. To assess the protective efficacy of H7N2-PR8, mice were immunized with two doses of formalin-inactivated H7N2-PR8, alone or with alum. Vaccinated mice subsequently challenged with highly pathogenic viruses from homologous and heterologous lineages A/Canada/444/04 (H7N3) and A/Netherlands/219/03 (H7N7) showed pronounced reduction of wild-type virus replication. These studies indicate that H7N2-PR8 is immunogenic, safe, and protective in animal models; these are the essential attributes to qualify for phase I human clinical trials as a prepandemic vaccine.

FIG. 1.

Daily body weight recordings after infection with escalating virus doses. In order to assess the pathogenicity of TK/VA (a), CAN/04 (b), and H7N2-PR8 (c), groups of mice were infected i.n. with six different dilutions of either virus and monitored daily for signs of mortality and morbidity during a period of 14 days. The data shown are from mice inoculated with the lowest, highest, and intermediate virus concentrations only and are expressed in log EID₅₀.

FIG. 2.

Protective efficacy of the H7N2-PR8 virus in BALB/c mice. Groups of mice were immunized with H7N2-PR8 (▴), TK/VA (▪), or PBS (⋄), either lacking adjuvant or containing alum. Each group was challenged with 100 MID₅₀ of either (a) CAN/04 or (b) NL/219. Daily body weight was recorded for 14 days after challenge with homologous and heterologous viruses. The body weight variation is expressed as the percentage of averaged weight of each group of mice.