Abstract
Background: With possible resurgence of the SARS-CoV-2 and low seasonal influenza virus circulation next winter, reviewing evidence on a possible interaction between influenza vaccination and COVID-19 risk is important. Objective: To review studies on the effect of influenza vaccines on non-influenza respiratory disease (NIRD). Methods: Using different search strategies, 18 relevant studies were identified and their strength, limitations and significance were assessed. Results: Analysis of 4 RCT datasets did not suggest increased NIRD risk in recipients of live-attenuated vaccines (LAIV) and results of a cohort study suggested short-term protection consistent with the hypothesis of trained immunity. One RCT, four cohort studies and one test-negative case-control suggested increased NIRD risk in recipients of inactivated influenza vaccines (IIV), whereas five test-negative case-control studies did not show an increased risk associated with a specific viral pathogen. Cross-protection against COVID-19 was suggested in one cross-sectional study on IIV but major biases could not be excluded. Results of four recent ecological studies on COVID-19 were challenging to interpret. Conclusions: Available data on LAIV are reassuring but not all those on IIV. A drastic reorientation of 2020-2021 influenza campaigns is probably not warranted but studies aiming to test COVID-19 risk modification among recipients of seasonal influenza vaccines should be planned and funded.
Competing Interest Statement
Philippe De Wals received research grants from GSK, Pfizer and Sanofi for studies unrelated to the present work. Maziar Divangahi has no potential conflict of interests to report.
Funding Statement
No funding
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