Prevention of Antigenically Drifted Influenza by Inactivated and Live Attenuated Vaccines

The efficacy of influenza vaccines may decline during years when the circulating viruses have antigenically drifted from those included in the vaccine. Methods We carried out a randomized, double-blind, placebo-controlled trial of inactivated and live attenuated influenza vaccines in healthy adults during the 2004–2005 influenza season and estimated both absolute and relative efficacies. Results A total of 1247 persons were vaccinated between October and December 2004. In-fluenza activity in Michigan began in January 2005 with the circulation of an anti-genically drifted type A (H3N2) virus, the A/California/07/2004-like strain, and of type B viruses from two lineages. The absolute efficacy of the inactivated vaccine against both types of virus was 77 % (95 % confidence interval [CI], 37 to 92) as measured by isolating the virus in cell culture, 75 % (95 % CI, 42 to 90) as measured by either isolating the virus in cell culture or identifying it through real-time poly-merase chain reaction, and 67 % (95 % CI, 16 to 87) as measured by either isolatin