Repeated Influenza Vaccination May Reduce Protection Against Certain Strains

Results of a systematic review and meta-analysis published in The Lancet Respiratory Medicine showed evidence that repeated influenza vaccination may attenuate some effectiveness of vaccine protection against certain influenza strains.

Investigators at the University of Melbourne and the Royal Melbourne Hospital in Australia searched publication databases through June 2022 for studies that assessed influenza vaccine effectiveness (VE). The investigators aimed to evaluate whether influenza VE is decreased by repeated vaccination. They assessed influenza VE among 4 vaccination groups, including individuals who were vaccinated in the current season only, in the previous season only, in both the current and previous seasons, or in neither season. The investigators calculated the overall pooled and absolute differences in VE between the 4 groups, as well as by season and age group.

A total of 83 articles were included in the systematic review and 41 were included in the meta-analysis; the earliest assessed estimates on VE were from the 2007 to 2008 season. The majority (51%) of studies were conducted in outpatient settings across Europe and North America. In addition, 60% of included studies reported on the efficacy of trivalent inactivated influenza vaccines, though few of these studies indicated the specific type of vaccine administered.

For the current influenza season, VE ranged from 37% for protection against influenza A (H3N2) to 61% for protection against both influenza B Yamagata and Victoria. For the previous season, influenza VE was between 9% for protection against influenza A (H3N2) and 38% for protection against influenza B Yamagata. Vaccination in both the current and previous seasons was associated with a VE of 20% for protection against influenza A (H3B2) and 53% for protection against influenza A (H1N1)pdm09.

Compared with vaccination in the current season only, vaccination in both the current and previous seasons was not favored for any influenza strain (VE range, -18% to -2%). Compared with vaccination in the previous season only, vaccination in both the current and previous seasons was favored for all influenza strains (VE range, 7%-26%).

Compared with no vaccination, vaccination in the current season (VE, 58%; 95% CI, 48%-66%; I², 38.4%) and in both the previous and current seasons (VE, 53%; 95% CI, 44%-60%; I², 61.6%) was favored for influenza A (H1N1). Similar findings were observed for influenza A (H3N2), as vaccination in the current season (VE, 37%; 95% CI, 29%-45%; I², 45%) and in both the current and previous seasons (VE, -18%; 95% CI, -26% to -11%; I², 7%) conferred better protection when compared with no vaccination. These findings also were observed for influenza B, with vaccination in the current season (VE, 54%; 95% CI, 49%-59%; I², 0%) and in both the current and previous seasons (VE, 47%; 95% CI, 41%-53%; I², 48.3%) favored over no vaccination.

This study may have been limited by insufficient data and significant heterogeneity among the influenza seasons assessed.

“Vaccine effectiveness against influenza A (H3N2) is worse overall and there is a greater loss of effectiveness with repeated vaccination,” the investigators noted. “[C]urrent evidence does not suggest there is a consistent and severe enough attenuation to recommend any changes to annual vaccination recommendations,” they concluded.

Disclosure: Multiple authors declared affiliations with industry. Please see the original reference for a full list of disclosures.