Impact of COVID-19 Vaccination on Hospitalizations in Washington and Oregon
Overview
Mathematical modeling of COVID-19 vaccination in Washington and Oregon from 2020 to 2022 estimated that primary vaccination reduced hospitalizations by 74%, with boosters contributing an additional 15% reduction. Vaccination was most effective during the Alpha variant wave and among older adults aged 65+, while vaccinating younger adults (18–49 years) also significantly decreased hospitalizations in older populations.
Background
The COVID-19 pandemic caused significant morbidity and mortality in the United States, with Washington reporting the first US death in March 2020. mRNA vaccines demonstrated high efficacy against the ancestral SARS-CoV-2 variant, but waning immunity, emerging variants, and infection-induced immunity complicated population-level effectiveness assessments. Mathematical models using local hospitalization data and vaccine efficacy meta-analyses can quantify the real-world impact of vaccination programs. This study used a compartmental model incorporating vaccine- and infection-induced immunity to estimate hospitalizations averted by vaccination in Washington and Oregon through 2022.
Data Highlights
Measure
Estimated Reduction in Hospitalizations (%)
95% Credible Interval
Primary Vaccination
74
69–78
Booster Doses
15
9–19
Alpha Variant Wave (Primary Vaccination)
90
88–93
Age 65+ (Primary Vaccination)
78
73–81
Vaccination of 18–49 vs. only 50+ (Hospitalizations Averted Overall)
52
44–58
Vaccination of 18–49 vs. only 50+ (Hospitalizations Averted in 65+)
42
35–48
Key Findings
Primary COVID-19 vaccination reduced hospitalizations by approximately 74% in Washington and Oregon populations.
Booster doses contributed an additional 15% reduction in hospitalizations beyond primary vaccination.
Vaccination was most effective during the Alpha variant wave, averting 90% of hospitalizations.
Older adults aged 65 and older benefited substantially, with 78% of hospitalizations averted by vaccination.
Vaccinating younger adults aged 18–49 significantly reduced hospitalizations not only in their age group but also among older adults, averting 42% of hospitalizations in those aged 65+ compared to vaccinating only those aged 50+.
Clinical Implications
These findings underscore the critical role of widespread primary vaccination and booster doses in preventing severe COVID-19 outcomes. Vaccination strategies that include younger adults can indirectly protect older populations by reducing overall transmission and hospitalizations. Clinicians and public health officials should continue to promote vaccination across all eligible age groups to maximize community-level protection.
Conclusion
The COVID-19 vaccination program in Washington and Oregon substantially reduced hospitalizations, with primary vaccination and boosters providing significant protection. Including younger adults in vaccination efforts further enhanced protection for older, more vulnerable populations.
References
Assessing Community Immunity and the Effects of COVID-19 Vaccination in Washington and Oregon, 2023
by Mia Moore, Larissa Anderson, Chloe Bracis, David A Swan, Ian Painter, Erik Everson, Holly Janes, Joshua T Schiffer, Laura Matrajt, Dobromir Dimitrov
