RESUMO
BackgroundAs of December 30, 2021, Ontario long-term care (LTC) residents who received a third dose of COVID-19 vaccine [≥]84 days previously were offered a fourth dose to prevent a surge in COVID-19-related morbidity and mortality due to the Omicron variant. Seven months have passed since fourth doses were implemented, allowing for the examination of fourth dose protection over time. MethodsWe used a test-negative design and linked databases to estimate the marginal effectiveness (4 versus 3 doses) and vaccine effectiveness (VE; 2, 3, or 4 doses versus no doses) of mRNA vaccines among Ontario LTC residents aged [≥]60 years who were tested for SARS-CoV-2 between December 30, 2021 and August 3, 2022. Outcome measures included any Omicron infection, symptomatic infection, and severe outcomes (hospitalization or death). ResultsWe included 21,275 Omicron cases and 273,466 test-negative controls. The marginal effectiveness of a fourth dose <84 days ago compared to a third dose received [≥]84 days ago was 23% (95% Confidence Interval [CI] 17-29%), 36% (95%CI 26-44%), and 37% (95%CI 24-48%) against SARS-CoV-2 infection, symptomatic infection, and severe outcomes, respectively. Additional protection provided by a fourth dose compared to a third dose was negligible against all outcomes [≥]168 days after vaccination. Compared to unvaccinated individuals, vaccine effectiveness (VE) of a fourth dose decreased from 49% (95%CI 44%-54%) to 18% (95%CI 5-28%) against infection, 69% (95%CI 62-75%) to 44% (95%CI 24-59%) against symptomatic infection, and 82% (95%CI 77-86%) to 74% (95%CI 62-82%) against severe outcomes <84 days versus [≥]168 days after vaccination. ConclusionsOur findings suggest that fourth doses of mRNA COVID-19 vaccines provide additional protection against Omicron-related outcomes in LTC residents, but the protection wanes over time, with more waning seen against infection than severe outcomes.
RESUMO
BackgroundAs of December 30, 2021, Ontario long-term care (LTC) residents who received a third dose of COVID-19 vaccine [≥]84 days previously were offered a fourth dose to prevent a surge in COVID-19-related morbidity and mortality due to the Omicron variant. MethodsWe used a test-negative design and linked databases to estimate the marginal effectiveness (4 versus 3 doses) and vaccine effectiveness (VE; 2, 3, or 4 doses versus no doses) of mRNA vaccines among Ontario LTC residents aged [≥]60 years who were tested for SARS-CoV-2 between December 30, 2021 and April 27, 2022. Outcome measures included any Omicron infection, symptomatic infection, and severe outcomes (hospitalization or death). ResultsWe included 13,654 Omicron cases and 205,862 test-negative controls. The marginal effectiveness of a fourth dose (with 95% of fourth dose vaccine recipients receiving mRNA-1273) [≥]7 days after vaccination versus a third dose received [≥]84 days prior was 19% (95% Confidence Interval [CI], 12-26%) against infection, 31% (95%CI, 20-41%) against symptomatic infection, and 40% (95%CI, 24-52%) against severe outcomes. VE (compared to an unvaccinated group) increased with each additional dose, and for a fourth dose was 49% (95%CI, 43-54%), 69% (95%CI, 61-76%), and 86% (95%CI, 81-90%), against infection, symptomatic infection, and severe outcomes, respectively. ConclusionsOur findings suggest that compared to a third dose received [≥]84 days ago, a fourth dose improved protection against infection, symptomatic infection, and severe outcomes caused by Omicron among long-term care residents. Compared to unvaccinated individuals, fourth doses provide strong protection against severe outcomes, but the duration of protection remains unknown.
