Influenza vaccines provide moderate protection throughout the entire flu season

Each flu season, researchers work to find out how effective the flu vaccine was in order to measure its value as a health intervention. Factors such as age and health of an individual, as well the level of similarity between the flu virus and the flu vaccine can play a role in how well an influenza vaccine works.

"Few studies have assessed how quickly protection against influenza declines within a year following vaccination, specifically among US populations," said Dr. Jennifer Radin, at the Naval Health Research Center in San Diego, CA.

For this study at the Naval Health Research Center, the researchers assessed influenza vaccine protection using febrile respiratory illness surveillance, which identified Department of Defense (DoD) non-active duty beneficiaries of all ages who presented with fever, cough or sore throat at three outpatient facilities in San Diego, CA and Great Lakes, Illinois. An upper respiratory swab was taken from a sample of these individuals and then was tested in the laboratory for influenza. Researchers measured influenza vaccine protection declines using data from 1,720 individuals throughout the course of four flu seasons, from the 2010-2011 season through the 2013-2014 season.

"Previous studies have found that protection from contracting influenza declines over time following influenza vaccination due to decreasing antibody levels," said Radin, "However, we found during this study that those who received the vaccine had moderate, sustained protection up to 6 months post-vaccination, the duration of most influenza seasons. This means flu vaccination reduced one's risk of a doctor's visit by approximately 50-70%."

The results showed that administering influenza vaccines early in the fall, before influenza begins circulating, may still prevent the greatest number of infections. The researchers also saw a marked decline in protection after 6 months, when the vaccine offered little to no protection, suggesting that yearly vaccination may be prudent.