Estimating population immunity and impact of COVID-19 vaccination in Washington State and Oregon

Background The SARS-CoV-2 mRNA vaccine showed high clinical efficacy against the ancestral variant, but immunological waning, emergence of new variants, and the durability of infection-induced immunity complicate the estimation of population-level effectiveness. We used mathematical modeling to calculate the proportion of hospitalizations averted by vaccination in Washington and Oregon. Methods We used an age- and region-structured compartmental model of vaccine-induced and infection-induced immunity from January 2020 until December 2022. We parameterized the strength and durability of immunity via a meta-regression of vaccine efficacy. We calibrated a time-varying contact matrix to weekly hospitalizations reported by the Washington Department of Health and Oregon Health Authority. We validated our model with CDC serosurveillance data. To estimate vaccine effectiveness, we created counterfactual scenarios with no vaccination either in the entire population or select age groups. Results We found that total hospitalizations were reduced 74% (95% Credible Interval 69% - 78%) and 15% (95% C.I. 9% - 19%) by primary vaccination and boosters, respectively. Vaccination effectiveness was highest during the Alpha wave, averting 90% (95% C.I. 88% - 93%) of hospitalizations and in people aged 65+, averting 78% (95% C.I. 73% - 81%). Relative to only vaccinating individuals ages 50+, vaccination of individuals ages 18-49 averted 52% (95% C.I. 44% - 58%) of hospitalizations overall and 42% (95% C.I. 35% - 48%) of hospitalizations among individuals 65+. Conclusion The SARS-CoV-2 vaccination program in Washington and Oregon averted the majority of hospitalizations. Vaccinating individuals 18-49 significantly reduced hospitalization among individuals 65+.