A new study published in Vaccine reveals that repeated flu vaccination does not appear to significantly weaken the protective effect of annual vaccines in older recipients.
The study looked at confirmed flu cases among those 60 and older in Ningbo, China, over four influenza seasons from 2018 to 2022. The researchers compared 1,976 cases of influenza-positive and 1,976 cases of influenza-negative controls.
In China, annual flu vaccines are recommended for those 6 months and older, but the vaccination rate among the older population remains significantly below the World Health Organization's target of 75%.
To increase vaccination uptake, the local government in Ningbo has funded free influenza vaccinations to residents aged 65 and older since 2020.
The authors of the study wanted to examine the relationship between repeated flu vaccinations and vaccine protectiveness. To do so, each flu-positive patient was matched with a negative control. Participants were considered vaccinated if they received one dose of flu vaccine at least 14 days before the onset of symptoms during the corresponding influenza season.
Participants were grouped into four categories: 1) not vaccinated in either the current or prior season; (2) vaccinated in the previous season only; (3) vaccinated in the current season only; (4) and vaccinated in both seasons.
Highest risk of flu in unvaccinated adults
Overall, 11.6% 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.
Being older, male, and having hypertension were all factors associated with consecutive vaccination. The authors found no significant increased risk of influenza in consecutively vaccinated participants compared to those receiving the vaccine only in the current season (adjusted odds ratio [aOR],1.22; 95% confidence interval [CI], 0.94 to 1.58).
The risk of influenza was found to be elevated in individuals who received the vaccine only in the previous season (aOR, 1.56; 95% CI, 1.15 to 2.10). The highest risk was in those who had not received the vaccine in either of the consecutive two seasons (aOR, 3.39; 95% CI, 2.80 to 4.09).
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.
"While there was a trend towards a reduction in protective efficacy, this difference did not attain statistical significance. 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," the authors concluded.
