Flu Vaccine Effective for Up to 6 Months After Administration

Atlanta—When is the best time to administer flu vaccination?

A presentation at the 2015 International Conference on Emerging Infectious Diseases in Atlanta sought to answer that question, noting that the influenza vaccine provided protection for up to a half year in a group of patients, with effectiveness dropping off dramatically after the 6-months point.

Study authors from the Naval Health Research Center in San Diego also pointed out that 6 months is the typical duration of a flu season.

“Few studies have assessed how quickly protection against influenza declines within a year following vaccination, specifically among U.S. populations,” said lead presenter Jennifer Radin, PhD, adding that the effectiveness of flu vaccine generally is measured each season but not how long it lasts.

For the study, Radin and colleagues assessed influenza vaccine protection using febrile respiratory illness surveillance in more than 1,500 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 and Great Lakes, Illinois. Over four flu seasons, beginning in 2010 and lasting until 2014, an upper respiratory swab was obtained from a sample of the patients and tested for influenza. 

“Previous studies have found that protection from contracting influenza declines over time following influenza vaccination due to decreasing antibody levels,” Radin said in an American Society for Microbiology press release. “However, we found during this study that those who received the vaccine had moderate, sustained protection up to six 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%.”

Vaccine effectiveness (VE) in the study group—of which 55% were female and 75% under 25 years old—ranged from 40% to 69% over the flu seasons. The vaccine provided “significant and fairly consistent protection” up to 180 days post vaccination, according to the researchers: 61% during 0 to 14 days, 62% during 15 to 90 days, and 60% during 91 to 180 days post vaccination. 

Beyond 180 days post vaccination, the VE dropped to –11%, study results showed.

The results also indicate that administering influenza vaccines early in the fall, before flu begins circulating, prevents the greatest number of infections. 

“There was some fluctuation in VE across flu seasons which was adjusted for in our model,” according to the study authors. “Administering flu vaccines closer to the start of flu season may slightly increase effectiveness of the vaccine. However, the flu season starts at different times of each year, making optimal timing difficult to predict. Consequently, early flu vaccination may still offer the best overall protections.”