Every year, influenza infects 5 to 15 percent of the world’s population, resulting in up to 500,000 deaths. Influenza A H1N1 and H3N2 and influenza B currently cause the majority of human disease. Periodically, an antigenic shift in the circulating strain of influenza A occurs, resulting in an influenza pandemic. When this happens, the morbidity and mortality from influenza increases dramatically. Estimates are that in a severe pandemic, 200 million people in the U.S. alone would become infected, and 2.5 million would die.
New influenza viruses that can infect humans usually originate in birds. All 16 HA (hemagglutinin) and 9 NA (neuraminidase) subtypes of influenza viruses infect wild waterfowl, providing an extensive reservoir of influenza viruses. While avian influenza strains typically do not infect humans, several avian strains have caused sporadic human infections in recent years: H5N1, H7N2, H7N3, H7N7, and H9N2. These avian viruses serve as the potential progenitors of the next pandemic influenza strain. The highly pathogenic H5N1 virus is of greatest concern for human health, as it causes the most severe disease in humans.
As of January 2009, over 395 cases and 245 deaths from H5N1 infection have been reported in countries from Indonesia to Nigeria. Infection by the H5N1 virus often follows an unusually aggressive clinical course, with rapid deterioration and high fatality. Human infection with the other avian influenza viruses usually results in milder disease. Close contact with dead or sick birds is the principal source of human infection with the avian influenza viruses. Reassortment or adaptive mutation of a currently circulating H5N1 strain could result in a virus capable of sustained human to human transmission, which increases the likelihood of an H5N1 pandemic.
Vaccination is currently the best way to prevent morbidity and mortality from the yearly interpandemic influenza epidemics. Vaccines are being developed against avian influenza strains in an attempt to be prepared for and to perhaps preempt or abort the next pandemic. Vaccines against avian influenza strains currently being evaluated in clinical trials include:
- Live attenuated modified reassortant and recombinant vaccine viruses
- Inactivated whole virus vaccines
- Inactivated split virus and subunit vaccines
- Recombinant HA and recombinant M2 protein vaccines
- DNA vaccine encoding H5 HA
Avian Influenza (Bird Flu)
Epidemic and Pandemic Alert and Response
Here is a link to information about participating in pandemic/avian influenza vaccine studies at the CIR.