"Single-Shot" Vaccines May Protect Against H5N1 Influenza Virus

Two newly developed "single-shot" H5N1 influenza vaccines protected ferrets against lethal infection with the H5N1 influenza virus and may allow for mass vaccination in humans in the event of a pandemic outbreak. The researchers from Australia report their findings in the August 2009 issue of the Journal of Virology.

As the highly infectious H5N1 influenza A virus continues to persist in bird populations and infect humans through poultry, concerns of a pandemic outbreak remain high. Although human-to-human transmission has remained limited, the fatality rate among those reported human cases is greater than 60 percent. The threat that the virus will mutate and achieve efficient human-to-human spread emphasizes the need for effective preventive therapies.

Vaccination is considered the optimal method for controlling an influenza pandemic. Vaccines must be rapidly available to reach mass populations and they must include the minimal antigen dose (substance that promotes the generation of antibodies) to result in full immunity. The use of adjuvants (substances to improve the immune response) in vaccine development may lower the antigen dose required and ultimately ease the demand on vaccine supply during a pandemic.

Clinical trials on prepandemic vaccines containing adjuvant suggested that two injections were necessary to induce protective immunity. In this study researchers first inoculated ferrets twice with two H5N1 influenza virus adjuvant vaccines (Iscomatrix and AIPO4) and observed for protective efficacy following a lethal challenge with the H5N1 virus. Results showed that ferrets were completely protected against death and disease for at least 15 months. More significantly, a secondary study found that both adjuvant vaccines also protected ferrets from death following only a single inoculation. Specifically, ferrets receiving a single shot of the Iscomatrix adjuvant vaccine displayed fewer signs of infection and remained highly active.

"Our data provide the first indication that in the event of a future influenza pandemic, effective mass vaccination may be achievable with a low-dose 'single shot' vaccine and provide not only increased survival but also significant reduction in disease severity," say the researchers.