question: Background: We used electronic health registries to estimate the mRNA vaccine effectiveness (VE) against COVID-19 hospitalizations and deaths in older adults. Methods: We established a cohort of individuals aged 65 and more years, resident in Portugal mainland through data linkage of eight national health registries. For each outcome, VE was computed as one minus the confounder-adjusted hazard ratio, estimated by time-dependent Cox regression. Results: VE against COVID-19 hospitalization [≥]14 days after the second dose was 94% (95%CI 88 to 97) for age-group 65-79 years old (yo) and 82% (95%CI 72 to 89) for [≥]80 yo. VE against COVID-19 related deaths [≥] 14 days after second dose was 96% (95%CI 92 to 98) for age-group 65-79 yo and 81% (95%CI 74 to 87), for [≥]80 yo individuals. No evidence of VE waning was observed after 98 days of second dose uptake. Conclusions: mRNA vaccine effectiveness was high for the prevention of hospitalizations and deaths in age-group 65-79 yo and [≥]80 yo with a complete vaccination scheme, even after 98 days of second dose uptake.