age, pregnant women, adults over 50 years of age, and persons with a history of hyper sensitivity to eggs or LAIV vaccine components (CDC, 2010b). It is also not recommended for persons with asthma and children between 2 and 4 years of age with a history of asthma or wheezing episodes in the 12 months prior to vaccination (CDC, 2010b). For these individuals and individuals with chronic conditions such as hematologic, hepatic, metabolic, neurologic or neuromuscular, pulmonary, or renal disorders; the immunosuppressed; and those between the ages of 6 months and 18 years receiving aspirin or other salicylates; ACIP recommends use of the age-appropriate TIV vaccine (Table 6-1) (CDC, 2010b).
In the 2008–2009 season, influenza vaccination was received by 29.1 percent of all persons aged 6 months to 18 years (CDC, 2010a). Thirty-three percent of individuals aged 19 to 49 years, who were considered high-risk for this age group, were vaccinated in comparison to 19.7 percent of individuals who were not considered high-risk for influenza (CDC, 2010a). The vaccine was administered to 51.5 percent of high-risk adults aged 50 to 64 years and 34.2 percent of non-high-risk adults in this age group (CDC, 2010a).
The committee reviewed four studies to evaluate the risk of encephalitis or encephalopathy after the administration of influenza vaccine. One study (Nakayama and Onoda, 2007) was not considered in the weight of epidemiologic evidence because it provided data from a passive surveillance system and lacked an unvaccinated comparison population. Three controlled studies (France et al., 2004; Goodman et al., 2006; Hambidge et al., 2006) had very serious methodological limitations that precluded their inclusion in this assessment. The studies by France et al. (2004), Goodman et al. (2006), and Hambidge et al. (2006) were unable to find any cases of encephalopathy or encephalitis following influenza vaccination using a case-crossover or case-control design, so no conclusions could be drawn from these analyses.
Weight of Epidemiologic Evidence
The epidemiologic evidence is insufficient or absent to assess an association between influenza vaccine and encephalitis or encephalopathy.