Background: Influenza causes substantial morbidity in adults and children with asthma, and vaccination can prevent influenza and its complications. However, there is concern that vaccination may cause exacerbations of asthma. Methods: To investigate the safety of the inactivated trivalent split-virus influenza vaccine in adults and children with asthma, we conducted a multicenter, randomized, double-blind, placebo-controlled, crossover trial in 2032 patients with asthma (age range, 3 to 64 years). The order of injection of vaccine and placebo was assigned randomly, with a mean of 22 days between the injections. Each day during the two weeks after each injection, the patients recorded peak expiratory flow rates, symptoms thought to be related to the injection, use of asthma medications, unscheduled health care visits for asthma, and asthma-related absences from school or work. The primary outcome measure was an exacerbation of asthma in the two weeks after the injections. Results: The frequency of exacerbations of asthma was similar in the two weeks after the influenza vaccination and after placebo injection (28.8 percent and 27.7 percent, respectively; absolute difference, 1.1 percent; 95 percent confidence interval, - 1.4 percent to 3.6 percent). The exacerbation rates were similar in subgroups defined according to age, severity of asthma, and other factors. Among symptoms thought to be associated with the injection, only body aches were more frequent after the vaccine injection than after placebo injection (25.1 percent vs. 20.8 percent, P<0.001). Conclusions: The inactivated influenza vaccine is safe to administer to adults and children with asthma, including those with severe asthma. Given the morbidity of influenza, all those with asthma should receive the vaccine annually.
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