Surveillance
According to a 2008 article in the journal "Vaccine", influenza strains are constantly evolving and mutating through two processes known as "antigentic shift" and "antigenic drift" genes. Therefore, 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. Genetic analysis is used 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 U.S. Centers for Disease Control and Prevention (CDC) in March 2009--as they arise. This process of surveillance is continuous.
Selection Process
In February of each year, experts at the U.S. Food and Drug Administration (FDA) and the WHO review surveillance data in order to make decisions about the which viruses the 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 United States vaccine. 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).
Vaccine Composition
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 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. Vaccines work when the body makes antibodies to markers on the surface of the influenza virus. Some influenza viruses make few such markers or the markers they make are so similar to markers on the body's own cells that vaccination could trigger an autoimmune disease. These strains are poor candidates for a vaccine, no matter how common or dangerous they are.
References
- Vaccine; The Biology of Influenza Viruses; N.M. Bouvier, P. Palese; September 12 2008
- World Health Organization
- Food and Drug Administration
