Effectiveness of repetitive influenza vaccination against SARS-CoV-2 infection among a cohort of health care workers in Portugal

Vaccination for influenza has been essential over the years to protect the most vulnerable populations. Moreover, it was recently suggested that influenza vaccination might confer some nonspecific immunity to other viruses and be associated with a lower risk for coronavirus disease 2019 (COVID-19) morbidity and mortality. Therefore, we aimed to assess the effectiveness of repetitive influenza vaccination against SARS-CoV-2 infection in a cohort of health care workers (HCWs). This study was conducted among HCWs at São João University Hospital Center (CHUSJ), Porto, Portugal, a tertiary reference hospital for diagnosis and therapy, one of the largest hospitals in the country with approximately 6000 HCWs. We analyzed databases for influenza vaccination conducted between 2012 and 2019 and COVID-19 laboratory testing retrieved from the first and last registered positive COVID test date before HCW's COVID-19 vaccination started. The study outcome was the incidence of the first SARS-CoV-2 infection, as determined by reverse transcription polymerase chain reaction (RT-PCR). Age and sex were considered potential confounders. We used multivariable Cox regression to estimate odds ratios. Neither the absolute number nor the proportion of influenza shots influenced the risk of getting infected by SARS-CoV-2 (adjusted odds ratio 1.02, 95% CI: 0.9–1.06 and 1.17 95% CI: 0.86–1.58, respectively). Similar findings were observed in most cases when the analysis was restricted by year. The findings from our retrospective observational analysis of a HCWs cohort failed to support any protective effect between repetitive influenza vaccination and SARS-CoV-2 infection.