The protective effect of annual flu vaccines does not erode when older adults receive flu shots every year, a new study published in Vaccine shows. 

Researchers evaluated the confirmed flu cases in people 60 and older in Ningbo, China, over four influenza seasons from 2018 to 2022. The team compared 1,976 positive cases to the same number of negative controls. In China, the government recommends yearly flu vaccines in those 6 months and older, but the vaccination rate among the older population remains significantly below the World Health Organization’s target of 75%. Since 2020, the government in Ningbo has provided free flu shots to people over 65.

Participants were considered vaccinated if they received one dose of a flu vaccine at least 14 days before the onset of symptoms during the corresponding influenza season. The participants were put into four groups: not vaccinated in either the current or prior season; vaccinated in the previous season only; vaccinated in the current season only; and vaccinated in both seasons.

In total, 11.6% of people were vaccinated in two consecutive seasons, 16.8% were vaccinated only in the current season, 6.5% were vaccinated only in the previous season, and 65.1% were not vaccinated at all.

Males who were older and had hypertension were more likely to receive back-to-back vaccinations. 

There wasn’t a significantly increased risk of influenza in consecutively vaccinated participants compared to those receiving the vaccine only in the current season, the data showed. The risk of getting the flu was higher in those who only received the vaccine in the previous season. The highest risk for contracting the flu was in people who hadn’t gotten the flu shot in either of the consecutive two seasons.

Older adults who only received a vaccine in the previous vaccine and those who didn’t get it for two seasons had a higher risk for getting the flu.

“While there was a trend towards a reduction in protective efficacy, this difference did not attain statistical significance,” the authors wrote. “Additionally, we have also observed a high degree of stability in this phenomenon across different influenza seasons. On the other hand, elderly individuals who received the vaccine only in the previous season or those who did not receive any vaccine for two seasons are facing a significantly increased risk of influenza infection.”