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Two types of seasonal influenza vaccines are currently licensed: an inactivated formulation, injected intramuscularly, and a live-attenuated formulation, administered by intranasal spray. Now, results from a double-blind trial conducted in Michigan during the 2007–2008 influenza season (and supported in part by the inactivated vaccine's maker) shed light on the comparative efficacy of these preparations in healthy adults. During that season, influenza A (H3N2) viruses predominated, with a slight antigenic drift from the vaccine strain.
A total of 1952 healthy adults aged 18 to 49 were randomly assigned to receive inactivated vaccine or matching intramuscular placebo, or live vaccine or matching nasal-spray placebo, in a 5:1 vaccine:placebo r…