Inactivated influenza vaccine contains the hemagglutinins of strains (i.e., typically two type A and one type B), representing the influenza viruses likely to circulate in the United States during the 2007-2008 influenza season. The vaccine viruses are made noninfectious (i.e., inactivated or killed).¹ Because the vaccine viruses are initially grown in embryonated hens' eggs, the vaccine might contain limited amounts of residual egg protein. Inactivated influenza vaccine distributed in the United States also contains thimerosal, a mercury-containing compound, as the preservative.

Efficacy and effectiveness of inactivated influenza vaccine

The effectiveness of inactivated influenza vaccine depends primarily on the age and immunocompetence of the vaccine recipient and the degree of similarity between the viruses in the vaccine and those in circulation. The majority of vaccinated children and young adults develop high postvaccination hemagglutination inhibition antibody titers. These antibody titers are protective against illness caused by strains similar to those in the vaccine.¹

Adults aged < 65 years. When the vaccine and circulating viruses are antigenically similar, influenza vaccine prevents influenza illness in approximately 70%-90% of healthy adults aged < 65 years. Vaccination of healthy adults also has resulted in decreased work absenteeism and decreased use of health care resources, including the use of antibiotics, when the vaccine and circulating viruses are well matched.¹

Children. Children aged as young as 6 months can develop protective levels of antibody after influenza vaccination although the antibody response among children at high risk of influenza-related complications might be lower than among healthy children. In a randomized study among children aged 1-15 years, inactivated influenza vaccine was 77%-91% effective against influenza respiratory illness and was 44%-49%, 74%-76%, and 70%-81% effective against influenza seroconversion among children aged 1-5, 6-10, and 11-15 years, respectively. One study reported a vaccine efficacy of 56% against influenza illness among healthy children aged 3-9 years, and another study determined vaccine efficacy of 22%-54% and 60%-78% among children with asthma aged 2-6 years and 7-14 years, respectively. A 2-year randomized study of children aged 6-24 months determined that ≥ 89% of children seroconverted to all three vaccine strains during both years. During year 1, among 411 children, vaccine efficacy was 66% (95% confidence interval [CI] = 34% and 82%) against culture-confirmed influenza (attack rates: 5.5% and 15.9% among vaccine and placebo groups, respectively). During year 2, among 375 children, vaccine efficacy was –7% (95% CI = -247% and 67%; attack rates: 3.6% and 3.3% among vaccine and placebo groups, respectively; the second year exhibited lower attack rates overall and was considered a mild season). However no overall reduction in otitis media was reported. Other studies report that trivalent inactivated influenza vaccine decreases the incidence of influenza- associated otitis media among young children by approximately 30%.¹

Adults aged ≥ 65 years of age. Older persons and persons with certain chronic diseases might develop lower post-vaccination antibody titers than healthy young adults and thus can remain susceptible to influenza-related upper respiratory tract infection. A randomized trial among noninstitutionalized persons aged ≥ 60 years reported a vaccine efficacy of 58% against influenza respiratory illness, but indicated that efficacy might be lower among those aged ≥ 70 years.¹ The vaccine can also be effective in preventing secondary complications and reducing the risk for influenza-related hospitalization and death among adults ≥ 65 years with and without high-risk medical conditions (e.g. heart disease and diabetes). Among elderly persons not living in nursing homes or similar chronic-care facilities, influenza vaccine is 30%-70% effective in preventing hospitalization for pneumonia and influenza. Among older persons who do reside in nursing homes, influenza vaccine is most effective in preventing severe illness, secondary complications, and deaths. Among this population the vaccine can be 50%-60% effective in preventing hospitalization or pneumonia and 80% effective in preventing death, although the effectiveness in preventing influenza illness often ranges from 30%-40%.¹

REFERENCES

1. Centers for Disease Control and Prevention. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR. 2004;53 (Early Release):[1-40].

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