ATLANTA – Influenza vaccine effectiveness during the 2010-2011 through 2013-2014 flu seasons was moderate for up to 6 months post vaccination – about the duration of the average flu season, according to surveillance data.
Vaccine effectiveness in 1,720 non–active duty U.S. Department of Defense beneficiaries ranged from 40% to 69% across the flu seasons, and after adjusting for age group, calendar season, and flu season, significant and fairly consistent protection was provided for up to 180 days, Dr. Jennifer M. Radin and her colleagues at the Naval Health Research Center, San Diego, reported in a poster at the International Conference on Emerging Infectious Diseases.
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The adjusted vaccine effectiveness was 61% during the first 2 weeks after vaccination, 62% from days 15 through 90, and 60% during days 91 through 180. After that, the effectiveness dropped to –11%, the investigators said.
Vaccine effectiveness in this study was assessed using outpatient febrile respiratory illness surveillance among a convenience sample of individuals of all ages, 75% of whom were under age 25 years, who presented with fever, cough, or sore throat at outpatient facilities in California and Illinois. Case patients were those who tested polymerase chain reaction–positive for influenza; those who were PCR negative for influenza served as controls.
“Previous studies have found that protection from contracting influenza declines over time following influenza vaccination due to decreasing antibody levels. However, we found ... moderate, sustained protection up to 6 months post vaccination,” Dr. Radin said in a press statement, explaining that at this level of effectiveness, vaccination reduces the risk of a doctor’s visit by 50%-70%.
The findings suggest that vaccine administration close to the start of flu season is associated with slightly increased vaccine effectiveness, but the start of flu season varies each year, thus optimal timing is hard to predict.
“Consequently, early flu vaccination may still offer the best overall protection,” Dr. Radin and her colleagues wrote.
The finding of a dramatic drop in effectiveness after 6 months also underscores the importance of yearly vaccination, they noted.
The investigators reported having no disclosures.