First SARS-CoV-2 vaccine booster and influenza vaccination: risk assessment of COVID-19 hospitalisation and death.

Background: the influenza and SARS-CoV-2 viruses share a common respiratory symptomatology and transmission mode. COVID-19 and influenza R0 overlapped in the first epidemic wave. In autumn 2021-winter 2022, the influenza epidemic had a delayed onset compared to pre-COVID-19 years and lower incidence rates than in the pre-pandemic period. The SARS-CoV-2 and influenza vaccination campaign overlapped in 2021-2022.

Objectives: to evaluate in the SARS-CoV-2 vaccinated cohort the effect of different timing of influenza vaccination on hospitalisations for COVID-19 and overall mortality.

Design: prospective cohort study.

Setting And Participants: subjects aged 65 years or older who were administered the first booster dose of SARS-COV-2 vaccine between 01.10.2021 and 01.03.2022. Based on the date of influenza vaccination, subjects were divided into the following 4 different mutually exclusive groups: 1. two vaccinations in the same vaccination session; 2. influenza vaccination following SARS-CoV-2 vaccination; 3. influenza vaccination preceding SARS-CoV-2 vaccination; 4. no influenza vaccination. Using Cox regression models, hazard ratio (HR) and corresponding 95% confidence intervals (95% CI) of hospitalisation and death were estimated for the influenza-vaccinated subjects compared to influenza-unvaccinated subjects.

Main Outcome Measures: ordinary hospital admissions for COVID-19 and general mortality.

Results: the cohort included 618,964 subjects: 16.3% received two vaccinations in the same vaccination session, 8.5% received the influenza vaccination after SARS-CoV-2 vaccination, 33.9% received it before and 41.1% did not receive an influenza vaccination. Those vaccinated against both SARS-CoV-2 and influenza had a combined HR of 0.73 (0.62-0.86) of hospitalisation for COVID-19 and 0.55 (0.49-0.62) of overall mortality compared to those vaccinated against SARS-CoV-2 only.

Conclusions: influenza vaccination combined with SARS-CoV-2 vaccination increases the protective effect against hospitalisations and overall mortality compared to SARS-CoV-2 vaccination alone. Both organisational and communication actions aimed to promote and encourage vaccination are required.