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This is a retrospective observational study including all COVID-19 patients admitted at our Institute throughout three successive pandemic waves, from January 2021 to June 2023. The main in-hospital outcomes (clinical progression [CP], defined as admission to Intensive Care Unit [ICU]/death, and death within 28 days) were compared among participants unvaccinated (NV), fully vaccinated (FV), with one (FV&B1) and two (FV&B2) booster doses. Vaccinated participants were stratified into recently and waned FV/FV&B1/FV&B2, depending on the time elapsed from last dose (≤ and >120 days, respectively). There were 4488 participants: 2224 NV, 674 FV, 1207 FV&B1, and 383 FV&B2. Within 28 days, there were 604 ICU admissions, 396 deaths, and 737 CP. After adjusting for the main confounders, the risk of both in-hospital outcomes was reduced in vaccinated individuals, especially in those who received the booster dose (approximately by 36% for FV and >50% for FV&B1 and FV&B2 compared to NV). Similarly, after restricting the analysis to vaccinated participants only, we observed a risk reduction of approximately 40% for FV&B1 and 50% for FV&B2, compared to FV, regardless of the distance since the last dose. Our data confirm the vaccine's effectiveness in preventing severe COVID-19 and support the efforts to increase the uptake of booster doses, mainly among older and frailer individuals, still at a greater risk of clinical progression.

Vaccination against SARS-CoV-2 is considered the most effective method of prevention to contain the pandemic. While highly effective SARS-CoV-2 vaccines are being applied on a large-scale, whether and to what extent the strength of the vaccine-induced immune response could be further potentiated is still an object of debate. Several reports studied the effect of different vaccines on the susceptibility and mortality of COVID-19, with conflicting results. We aimed to evaluate whether previous influenza and/or pneumococcal vaccination had an impact on the specific immune response to the SARS-CoV-2 BNT162b2 mRNA vaccine. The study population consists of 710 workers from our Institute who completed the BNT162b2 schedule and have been tested at least once after the second dose, from 27 December 2020 up to 15 April 2021. Of these, 152 (21.4%) had received an influenza and 215 (30.3%) a concomitant influenza and pneumococcal vaccination, a median of 102 days before the second dose of BNT162b2. Overall, 100% of workers were tested for anti-Spike receptor-binding domain (anti-S/RBD) antibodies, 224 workers for neutralization titer (Micro-neutralization assay, MNA), and 155 workers for a spike-specific T cell interferon-γ response (IFN-γ). The levels of anti-S/RBD, MNA and IFN-γ were evaluated and compared according to sex, age, involvement in direct care of COVID-19 patients, and previous influenza/pneumococcal vaccination. At the univariate analysis, no statistically significant association was observed with regard to a previous influenza and pneumococcal vaccination. A significant lower anti-S/RBD response was observed according to an older age and male sex, while MNA titers were significantly associated to sex but not to age. At the multivariable analysis, workers receiving a concomitant influenza and pneumococcal vaccination or only influenza showed a 58% ( p 0.01) and 42% ( p 0.07) increase in MNA titers, respectively, compared to those who did not receive an influenza/pneumococcal vaccination. Female workers showed an 81% MNA and a 44% anti-S/RBD increase compared to male workers ( p < 0.001). Compared to workers aged 21 to 49 years, those aged 50 or older were associated with a reduction in the anti-S/RBD (16%; p 0.005), MNA (31%; p 0.019), and IFN.g (32%) immune response. Maintaining the influenza and pneumococcal immunization program for the coming season, in which COVID-19 could still be spreading, remains strongly recommended to protect those who are more vulnerable and to limit the potential burden of these infections on the healthcare system.