Evidence Suggests Variable Effectiveness for Flu Vaccine

Randomized controlled trials indicate vaccine efficacy varies between seasons, age groups

WEDNESDAY, Oct. 26 (HealthDay News) -- Influenza vaccines provide variable effectiveness and efficacy in young children and adults, according to a study published online Oct. 26 in The Lancet Infectious Diseases.

Michael T. Osterholm, Ph.D., from the University of Minnesota in Minneapolis, and colleagues reviewed available literature published between January 1967 and February 2011 to assess the efficacy and effectiveness of licensed influenza vaccines in the United States. The efficacy for reducing the risk of all circulating influenza viruses during individual seasons was assessed in 17 randomized controlled trials (RCTs), and the effectiveness of vaccination was measured in 14 observational studies.

The investigators found that the efficacy of the trivalent inactivated vaccine was shown in 67 percent of 12 seasons, in 10 RCTs, in adults aged 18 to 65 years. There were no trials included for children ages 2 to 17, or adults older than 65 years. Live attenuated influenza vaccine efficacy was seen in 75 percent of the 12 seasons, analyzed in 10 RCTs, in children aged 6 months to 7 years. There were no trials included for children aged 8 to 17 years. Variable effectiveness of the vaccine was seen for seasonal influenza, with significant protection against medically attended influenza in 35 percent of 17 analyses, in nine studies, in the outpatient or inpatient setting. In five observational studies, the median effectiveness for monovalent pandemic H1N1 vaccine was 69 percent.

"Influenza vaccines can provide moderate protection against virologically confirmed influenza, but such protection is greatly reduced or absent in some seasons," the authors write.

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