Review: vaccination reduces the incidence of serologically confirmed influenza in healthy adults.

MAIN RESULTS 47 trials (25 randomised controlled trials [RCTs], n=59 566) met the selection criteria. (1) Influenza vaccine (inactivated parenteral vaccine [IP], live aerosol vaccine [LA], or inactivated aerosol vaccine [IA]) v placebo. Rates of CDI and SCI were lower in the IP group than in the placebo group (table). Rates of specified CDI and SCI were also lower in the LA group than in the placebo group (table). The rate of unspecified CDI was lower in the IA group than in the placebo group (table). (2) >1 vaccine recommended for that year v placebo or other vaccines. The rate of CDI was lower in the vaccine group (IP, LA, and IA groups combined) than in the placebo groups (27 trials; relative risk reduction [RRR] 22%, 95% CI 14 to 30). The rate of SCI was lower in the combined IP and LA group than in the control group (placebo or non-influenza vaccine) (21 trials; RRR 68%, CI 57 to 76). (3) Vaccines matching circulating strains v placebo or other vaccines. The rate of CDI was lower in the vaccine group (IP, LA, and IA groups combined) than in the placebo group (16 trials; RRR 33%, CI 20 to 44). Overall the rate of SCI was lower in the combined vaccine group than in the control group (13 trials; RRR 75%, CI 62 to 84). CONCLUSIONS In healthy people 14–60 years of age, vaccines are effective for reducing the incidence of serologically confirmed influenza. However, they are not as effective for reducing the incidence of clinically defined influenza.