Influenza Vaccine's Effectiveness Following Repeated Vaccination

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The most current flu vaccine offers the greatest protection again influenza H1N1 and B.
The most current flu vaccine offers the greatest protection again influenza H1N1 and B.

Prior season influenza vaccination did not decrease the effectiveness of the current season vaccination, according to a recent study published in BMC Medicine.

In this systematic review and meta-analysis, researchers evaluated observational studies that reported vaccine effectiveness against laboratory-confirmed influenza and vaccination history. Participants were categorized as receiving vaccination during current season only, prior season only, both seasons, or neither season.

Vaccination in both seasons was correlated with better protection against influenza strains H1N1 (difference in vaccine effectiveness [∆VE] 26%; 95% CI, 15%-36%) and B (∆VE 24%; 95% CI, 7%- 42%) but not H3N2 (∆VE 10%; 95% CI, –6% to 25%) compared with prior-season vaccination only.

Furthermore, vaccination in both seasons did not appear to have an impact on the vaccine effectiveness compared with current-season-only vaccination for H1N1, H3N2, or B. Current-season-only vaccination was also more effective than no vaccination for H1N1, H3N2, and B.

Except for the 2014-2015 season, trends in individual seasons were consistent with the overall results. Vaccination was less effective in the 2014-2015 season for patients who were vaccinated in both seasons compared with the current season alone (n=3 studies; ∆VE –54%; 95% CI, –88% to –20%). According to the researchers, this negative interference may be a result of antigenic distance, which occurs when vaccine strains remain constant for 2 years in a row but the previous season's vaccine does not match the current circulating strain.

Co-investigator Bryna Warshawsky, MD, of Public Health Ontario, Canada, explained that the study results "provided reassurance that overall influenza vaccine effectiveness will not be substantially lower if the person has been vaccinated in the prior season." Based on these data, "vaccination in the current season is generally the best option regardless of whether or not the patient was vaccinated in the prior season."

Reference

Ramsay LC, Buchan SA, Stirling RG, et al. The impact of repeated vaccination on influenza vaccine effectiveness: a systematic review and meta-analysis. BMC Med. 2017;15(1):159. doi:10.1186/s12916-017-0919-0

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