Estimating Population Immunity and Impact of COVID-19 Vaccination in Washington State and Oregon

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The mRNA COVID-19 vaccine significantly reduced hospitalizations in Washington and Oregon, averting 74% of total hospitalizations from January 2020 to December 2022.
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Vaccination effectiveness peaked during the Alpha variant wave, preventing 90% of hospitalizations, particularly among individuals aged 65 and older.
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Vaccinating younger individuals aged 18-49 contributed to a 52% reduction in hospitalizations overall and 42% among those aged 65 and older.
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The study utilized a compartmental model to assess the impact of vaccination and infection-induced immunity on COVID-19 hospitalizations.
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Counterfactual scenarios demonstrated the critical role of vaccination in reducing severe COVID-19 outcomes amidst evolving variants and waning immunity.

Open Forum Infectious Diseases

by Mia Moore, Larissa Anderson, Chloe Bracis, David A Swan, Ian Painter, Erik Everson, Holly Janes, Joshua T Schiffer, Laura Matrajt, Dobromir Dimitrov
August 30, 2025

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