Effectiveness and Duration of Protection of a Fourth Dose of Coronavirus Disease 2019 Messenger RNA Vaccine Among Long-term Care Residents in Ontario, Canada

We estimated the effectiveness of a fourth dose of messenger RNA coronavirus disease 2019 vaccine against Omicron infections and severe outcomes over time among long-term care residents in Ontario, Canada. Fourth doses provide additional protection against Omicron-related outcomes, but the protection wanes over time, with more waning seen against infection than severe outcomes.