COVID-19 vaccines coverage and effectiveness against SARS-CoV-2 infection among residents in the largest Health Authority of Lazio region (Italy): a population-based cohort study

Background The waning of the protective effect of COVID-19 vaccines and timing of booster doses are debated. Methods Population-based cohort study in the largest Health-Authority of Lazio region, Italy, on 946,156 residents aged 12+ (study period: 1 January 2021-10 January 2022). Vaccine effectiveness (VE) against any SARS-CoV-2 infection (symptomatic or asymptomatic) was estimated through multivariable negative-binomial models using unvaccinated person-time as a reference. Results The primary vaccination cycle was completed by 81% of residents; of these, 45% received a booster dose. Vaccine coverages were lower for foreigners, and people living in deprived areas, families with children aged 0-11, and households size 1 or 6+. Overall, VE waned from 71% (95% Confidence Interval (CI) 70-73%) 1 month after the second dose to 43% (CI 41-45%) after 4 months and 24% (CI 21-27%) after 6 months, especially in the elderly aged 70+. We observed a prompt restore of VE 15-19 days after the booster dose (69%, CI 67-70%). Conclusions Our results support the recommendation of a booster dose 4 months after completion of the primary cycle, giving priority to elderly and fragile individuals. The lower vaccine coverage among social disadvantaged subgroups suggests the need of targeted communication and interventions.