"Flu" is a generic term for a syndrome of respiratory infection produced by several distinct influenza viruses. According to the Centers for Disease Control and Prevention (CDC), more than 200,000 people are hospitalized and about 36,000 people die from the flu in the United States each year. Immunization, especially among high-risk groups, would prevent many influenza infections.
Surveillance
Flu vaccines target specific strains of virus. Each year, those strains are identified from ongoing surveillance by the World Health Organization (WHO). Participating laboratories around the world routinely collect samples of circulating influenza viruses and submit them to the WHO Collaborating Centers for Reference and Research on Influenza. In the U.S., the CDC is a member of that network.
Characterization
According to a 2008 article in the journal "Vaccine," the influenza virus is continuously evolving through two simultaneous processes known as "antigenic shift" and "antigenic drift." Scientists use genetic analysis to identify mutations among previously characterized strains and to locate new strains of influenza virus--such as the novel H1N1 virus first identified by the CDC in March 2009--as they arise.
Selection
In February of each year, experts at the U.S. Food and Drug Administration (FDA), the CDC and the WHO review surveillance data in order to make decisions about which viruses the annual flu vaccine should target. Typically, the WHO will make recommendations for a Northern Hemisphere vaccine. The FDA then makes decisions about whether to adopt the WHO recommendations for the U.S. For example, in 2009, the WHO issued its recommendations on February 12; on February 18, these recommendations were endorsed by the FDA's Vaccines and Related Biological Products Advisory Committee (VRBPAC).
Candidates
According to the FDA, the seasonal flu vaccine is a "trivalent" vaccine, which means it combats three different strains of influenza. In recent years, the formula has included one influenza A (H3N2) virus, one regular seasonal influenza A (H1N1) virus--not the same as the 2009 H1N1 virus--and one influenza B virus. In 2009, the following viruses were selected: influenza A/Brisbane/59/2007 (H1N1)-like virus; influenza A/Brisbane/10/2007 (H3N2)-like virus and influenza B/Brisbane/60/2008-like virus.
Criteria
According to the FDA, factors considered in the selection of individual strains include the prevalence of the virus, the virulence of the virus and the ease of manufacturing vaccine. Dangerous strains may trump common ones, although in some cases, as with the H1N1 2009 virus, an additional separate vaccine may be approved to combat particularly virulent strains. A third, often overlooked consideration is the ease of manufacturing a vaccine. Some strains are more amenable to vaccine development than others.
References
- World Health Organization: Vaccine Production
- "Vaccine;" The Biology of Influenza Viruses; N.M. Bouvier, P. Palese; Sept. 12, 2008
- U.S. Food and Drug Administration: Influenza Vaccine Information
