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Observational Study
. 2022 Mar 15;40(12):1755-1760.
doi: 10.1016/j.vaccine.2022.02.013. Epub 2022 Feb 7.

Effect of the 2020/21 season influenza vaccine on SARS-CoV-2 infection in a cohort of Italian healthcare workers

Alexander Domnich  1 , Andrea Orsi  2 , Laura Sticchi  3 , Donatella Panatto  4 , Guglielmo Dini  5 , Allegra Ferrari  6 , Matilde Ogliastro  7 , Simona Boccotti  8 , Vanessa De Pace  9 , Valentina Ricucci  10 , Bianca Bruzzone  11 , Paolo Durando  12 , Giancarlo Icardi  13
Affiliations
Observational Study

Effect of the 2020/21 season influenza vaccine on SARS-CoV-2 infection in a cohort of Italian healthcare workers

Alexander Domnich et al. Vaccine. .

Abstract

Objectives: Healthcare workers (HCWs) are a priority group for seasonal influenza vaccination (SIV). The 2020/21 SIV campaign was conducted during the second wave of the COVID-19 pandemic. Vaccines, including SIV, may exert non-specific protective effects on other infectious diseases which may be ascribable to the concept of trained immunity. The aim of this study was to explore the association between 2020/21 SIV and SARS-CoV-2 positivity in a cohort of Italian HCWs.

Methods: In this observational study, a cohort of HCWs employed by a large (ca 5000 employees) referral tertiary acute-care university hospital was followed up retrospectively until the start of the COVID-19 vaccination campaign. The independent variable of interest was the 2020/21 SIV uptake. Both egg-based and cell culture-derived quadrivalent SIVs were available. The study outcome was the incidence of new SARS-CoV-2 infections, as determined by RT-PCR. Multivariable Cox regression was applied in order to discern the association of interest.

Results: The final cohort consisted of 2561 HCWs who underwent ≥1 RT-PCR test and accounted for a total of 94,445 person-days of observation. SIV uptake was 35.6%. During the study period, a total of 290 new SARS-CoV-2 infections occurred. The incidence of new SARS-CoV-2 was 1.62 (95% CI: 1.22-2.10) and 3.91 (95% CI: 3.43-4.45) per 1000 person-days in vaccinated and non-vaccinated HCWs, respectively, with an adjusted non-proportional hazard ratio of 0.37 (95% CI: 0.22-0.62). E-values suggested that unmeasured confounding was unlikely to explain the association.

Conclusions: A lower risk of SARS-CoV-2 infection was observed among SIV recipients.

Keywords: COVID-19; Healthcare workers; Influenza; Influenza vaccines; SARS-CoV-2; Vaccination.

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Conflict of interest statement

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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