exacerbation of chronic pulmonary and cardiopulmonary diseases such as asthma and congestive heart failure (Bridges et al., 2008).

The influenza viruses were first isolated in the early 1900s by Smith and his associates (influenza A, 1933), Francis (influenza B, 1939), and Taylor (influenza C, 1950) (Francis, 1940; Smith et al., 1933; Taylor, 1951). In 1936, Burnet discovered that the virus could be grown in embryonated hen eggs, and in the 1950s animal cell culture systems were developed (Burnet, 1936; Mogabgab et al., 1954; Treanor, 2010). In 1943, the first commercial influenza vaccines were approved for use in the United States, and consisted of inactivated virus grown in chicken eggs (Treanor, 2010). With a few adaptations, propagation of influenza viruses in chicken eggs remains the primary means for growing virus for vaccine production and biomedical research (Treanor, 2010).

Currently, two types of vaccines are available in the United States—the trivalent, inactivated influenza virus (TIV) vaccine, and the live, attenuated, cold-adapted influenza virus (LAIV) vaccine (also trivalent). TIV vaccines, which were first licensed for use in the United States in 1943, are inactivated (killed) virus vaccines that provide immunity against the viruses without causing any signs or symptoms of the infection (Treanor, 2010). The LAIV vaccine is a live but attenuated virus vaccine that is capable of causing mild signs and symptoms of vaccine virus infection (Treanor, 2010). Approved in 2003, LAIV is a live virus vaccine that is cold-adapted (attenuated) so that it does not replicate in the warmer body temperature of the lower airways (CDC, 2003; Treanor, 2010). It is capable of causing mild signs and symptoms of wild-type influenza infection (Treanor, 2010). TIV is administered through an intramuscular injection, while LAIV is administered intranasally via an aerosol sprayer. Both vaccines contain two influenza A and one influenza B subtypes which are recommended by the World Health Organization (WHO) Global Influenza Programme—for example A/California/7/2009 (H1N1)-like,1 A/Perth/16/2009 (H3N2)-like, and B/Brisbane/60-2008-like for the 2010–2011 season (WHO, 2010).

The Advisory Committee on Immunization Practices (ACIP) recommends that all persons 6 months or older receive an annual influenza virus vaccine (CDC, 2010b). For healthy, nonpregnant persons aged 2 to 49 years either TIV or LAIV vaccine is recommended without preference (CDC, 2010b). LAIV is not recommended for children under 2 years of

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1 A/California/7/2009 (H1N1)-like is derived from the 2009 pandemic influenza A (H1N1) virus. This strain was included in the trivalent vaccine in 2010. The monovalent vaccine developed for the pandemic is not covered under the National Vaccine Injury Compensation Program (VICP); it is covered under the Countermeasures Injury Compensation Program and is therefore beyond the scope of this report.



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