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

Background: The severe acute respiratory syndrome coronavirus 2 (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 Centers for Disease Control and Prevention serosurveillance data. To estimate vaccine effectiveness, we created counterfactual scenarios with no vaccination either in the entire population or in select age groups.

Results: We found that total hospitalizations were reduced 74% (95% credible interval [CrI], 69%-78%) and 15% (95% CrI, 9%-19%) by primary vaccination and boosters, respectively. Vaccination effectiveness was highest during the Alpha wave, averting 90% (95% CrI, 88%-93%) of hospitalizations and in people aged 65+, averting 78% (95% CrI, 73%-81%). Relative to only vaccinating individuals aged 50+, vaccination of individuals aged 18-49 averted 52% (95% CrI, 44%-58%) of hospitalizations overall and 42% (95% CrI, 35%-48%) of hospitalizations among individuals 65+.

Conclusions: The SARS-CoV-2 vaccination program in Washington and Oregon averted the majority of hospitalizations. Vaccinating individuals aged 18-49 significantly reduced hospitalization among individuals aged 65+.