Description:
<jats:title>Abstract</jats:title>
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<jats:title>Background</jats:title>
<jats:p>Long-term care residents were among the most vulnerable during the COVID-19 pandemic. We estimated vaccine effectiveness of mRNA COVID-19 vaccines in Medicare nursing home residents aged ≥65 years during pre-Delta and high Delta periods.</jats:p>
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<jats:title>Methods</jats:title>
<jats:p>We conducted a retrospective cohort study from 13 December 2020 to 20 November 2021 using Medicare claims data. Exposures included 2 and 3 doses of Pfizer-BioNTech and Moderna COVID-19 vaccines. We used inverse probability weighting and Cox proportional hazards models to estimate absolute and relative vaccine effectiveness.</jats:p>
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<jats:title>Results</jats:title>
<jats:p>Two-dose vaccine effectiveness against COVID-19–related death was 69.8% (95% CI, 65.9%‒73.3%) during the pre-Delta period and 55.7% (49.5%‒61.1%) during the high Delta period, without adjusting for time since vaccination. We observed substantial waning of effectiveness from 65.1% (54.2%‒73.5%) within 6 months from second-dose vaccination to 45.2% (30.6%‒56.7%) ≥6 months after second-dose vaccination in the high Delta period. Three doses provided 88.7% (73.5%‒95.2%) vaccine effectiveness against death, and the incremental benefit of 3 vs 2 doses was 74.6% (40.4%‒89.2%) during high Delta. Among beneficiaries with a prior COVID-19 infection, 3-dose vaccine effectiveness for preventing death was 78.6% (50.0%‒90.8%), and the additional protection of 3 vs 2 doses was 70.0% (30.1%‒87.1%) during high Delta. Vaccine effectiveness estimates against less severe outcomes (eg, infection) were lower.</jats:p>
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<jats:title>Conclusions</jats:title>
<jats:p>This nationwide real-world study demonstrated that mRNA COVID-19 vaccines provided substantial protection against COVID-19–related death. Two-dose protection waned after 6 months. Third doses during the high Delta period provided significant additional protection for individuals with or without a prior COVID-19 infection.</jats:p>
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