Reactogenicity of trivalent inactivated influenza vaccine in young children: Pronounced reactions by previous successive vaccinations.

In order to assess factors associated with reactogenicity of trivalent inactivated influenza vaccine (IIV3) among young children, data on 1538 vaccinees aged 0-5 years in a previous vaccine effectiveness study were analyzed. The most frequent reaction was redness (19%), followed by induration, swelling, itching, and pain (6-12%); there were no serious adverse events. For some local reactions, multivariate analyses indicated associations of younger age, preschool attendance, presence of siblings, and allergy with lower risk, and use of thinner needles with higher risk. Most notably, administration of one or more IIV3 vaccines during the previous 3 seasons was positively associated with each local reaction (adjusted odds ratios: 3.6-5.4). For subjects aged ≥3 years, prior successive annual vaccinations were associated with substantially increased local reactions, with clear dose-response relationships (P for trend: <0.001 for each); for example, an 9.8-fold greater risk of swelling following three successive annual vaccinations before the study season.
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