How effective is the flu vaccine?

The flu vaccine is very effective for older children and adults, aged 10 to 49 years, but may be less effective than believed for the population overall, according to two recent studies.

A study of 6,757 patients seeking medical care for respiratory illness evaluated the pandemic vaccine effectiveness in communities in Michigan, Wisconsin, Tennessee and New York. Fifteen percent of them tested positive for influenza.1

The study, funded by the Centers for Disease Control and Prevention, found that the inactivated vaccine was 89% effective among people ages 10 to 49, but that few children 9 and under received the recommended two doses, the authors stated. Including children in the analysis lowered the overall effectiveness of the vaccine to 56%.

"Our results suggest that a single dose of a US licensed non-adjuvanted pandemic vaccine was capable of preventing over half of medical care visits associated with pandemic virus infection, and that inactivated vaccines were very effective for those aged 10 to 49 years," the authors said.

Another study at the University of Michigan School of Public Health found that flu vaccine efficacy may be overestimated in some studies if they used cell culture rather than real-time PCR to identify influenza virus.2

In the study, all influenza A (H3N2) and B cases that were isolated in cell culture were also identified by rtPCR, but only 69% of the influenza A cases identified by rtPCR were also isolated in cell culture, the authors stated.

The overall vaccine efficacy, based on rtPCR testing, was about 70%, they said. "That may suggest that we should lower the usual description of vaccine efficacy from 70%–90% in healthy adults to closer to 70%; however, further confirmation by other studies is desirable," the authors said. "Given 70% efficacy in a population with 50% vaccine coverage, approximately one-quarter of influenza cases may occur among vaccinated persons, regardless of attack rate in a given year."

Interestingly, there was less viral shedding among people who were vaccinated but still developed influenza, the study found.


1. Griffin MR, Monto AS, Belongia EA, et al. Effectiveness of non-adjuvanted pandemic influenza A vaccine for preventing pandemic influenza acute respiratory illness visits in 4 US communities. PLoS ONE 2011. Available at

2. Petrie JG, Ohmit SE, Johnson E, et al. Efficacy studies of influenza vaccines: effect of end points used and characteristics of vaccine failures. J Infect Dis 2011; 203:1309-15.