Boosters and time since last anti-COVID-19 vaccine dose: lead public health choices by real-time epidemiological assessment

ABSTRACT

BACKGROUND: the levels of anti-SARS-CoV-2 antibodies after the second vaccine dose decline in the following months;an additional vaccine dose (booster) is able to swiftly restore the immune system, significantly reducing the risk of severe disease. In the winter of 2021, a new, particularly infectious variant made the need to increase booster coverage in the population even more urgent. OBJECTIVES: to present, using real data, an evaluation of the effectiveness of the booster dose in reducing severe disease caused by SARS-CoV-2 infection in terms of COVID-19 hospitalization and intensive care admission, and all-cause mortality. DESIGN: descriptive study of vaccination uptake;associative study of the factors linked with uptake of vaccination and COVID-19 symptoms;associative study of vaccine effectiveness against hospital admission and mortality. SETTING AND PARTICIPANTS: population residing in the Milan and Lodi provinces (Lombardy Region, Northern Italy), eligible for anti-SARS-COV-2 vaccination, with subjects aged >= 19 years alive as at 01.10.2021, not residing in nursing homes, followed-up until 31.12.2021. MAIN OUTCOME MEASURES: COVID-19 symptoms, hospitalization, intensive care hospitalization, and all-cause mortality in the period 01.10.2021-31.12.2021. RESULTS: the cohort included 2,936,193 patients as of 01.10.2021;at the end of the observation period (31.12.2021), 378,616 (12.9%) were unvaccinated, 60,102 (2.0%) had received only 1 dose and had not had the disease, 68,777 (2.3%) had received only 1 dose and had had the disease, 412,227 (14.0%) were fully vaccinated with 2 doses less than 4 months earlier, 198,459 (6.8%) had received 2 doses [4,5) months earlier, 439,363 (15.0%) had received 2 doses [5,6) months earlier, 87,984 (3.0%) had received 2 doses [6,7) months earlier, 74,152 (2.5%) had received 2 doses more than 7 months earlier, 62,614 (2.1%) had received 2 doses and had had the disease, and, finally, 1,153,899 (39.3%) had received a booster shot. In the study period (01.10.2021-31.12.2021), characterized by a very high prevalence of the omicron variant, 121,620 cases (positive antigen/molecular test), 3,661 hospitalizations for COVID-19, 162 intensive care admissions for COVID-19, and 7,508 deaths from all causes were identified. Compared to unvaccinated subjects, subjects who received a booster had half the risk of being symptomatic, and had half the risk of experiencing fatigue, muscle aches, and dyspnoea. In comparison with boosted subjects, unvaccinated subjects had a 10-fold risk of hospitalization, a 9-fold risk of intensive care, and a 3-fold risk of dying. CONCLUSIONS: this work highlights the effectiveness of vaccination in reducing serious adverse events in boosted subjects and the need to implement specific policies of engagement to bring subjects who received their second dose earliest to get a booster.