The efficacy of the flu vaccine may vary from year to year, depending on factors such as the age and health of the recipient and the similarity of the virus strains included in the flu vaccine and the viruses that actually circulate during the flu season.

There are currently two types of flu vaccine that are licensed for the prevention of seasonal influenza – one contains inactivated viruses and the other contains live attenuated, or weakened, viruses. According to a study published in the September 24, 2009 issue of The New England Journal of Medicine, in the 2007-2008 flu season, the inactivated flu vaccine was more efficacious than the live attenuated vaccine in preventing symptomatic influenza A in healthy adults.

Researchers from the University of Michigan School of Public Health in Ann Arbor, Mich. and the Program for Appropriate Technology in Health (PATH) in Seattle, Wash. performed a randomized, double-blind, placebo-controlled trial of inactivated and live attenuated flu vaccine among 1,952 health adults. During the 2007-2008 flu season, about 90% of the circulating flu virus was influenza A (predominantly, H3N2). The researchers found that, in terms of relative efficacy, there was a 50% reduction in laboratory-confirmed cases of influenza among adults who received the inactivated flu vaccine compared to those who were given the live attenuated vaccine.

The findings of this study confirm suggestive evidence from previous study of the flu vaccine in the 2004-2005 flu season that the inactivated flu vaccine demonstrates superior performance over the live attenuated vaccine. With the emergence of the swine flu, it is becoming clear that in this era of influenza control, different types of flu vaccine may be appropriate for different age groups.

Source:

NEJM. 2009;361:1260-1267.