Effectiveness of COVID-19 Vaccines Against SARS-CoV-2 Infection During a Delta Variant Epidemic Surge in Multnomah County, Oregon, July 2021

Background Coronavirus disease 2019 (COVID-19) vaccines have been shown to be highly effective in preventing SARS-CoV-2 infection within controlled trials and real-world vaccine effectiveness (VE) studies. Recent reports have estimated reduced VE with the emergence and dissemination of the B.1.617.2 variant (“Delta variant”). We assess VE in Multnomah County, Oregon during a delta variant related epidemic expansion. Methods A test-negative design (TND) matched case-control analysis was performed to estimate the effectiveness of vaccination against SARS-CoV-2 infection during July 2021. Cases included a random sample of individuals that tested positive for SARS-CoV-2 and were reported by electronic laboratory report, were >15 years of age, and had no prior known SARS-CoV-2 infections. Controls were age and postal code matched individuals that tested negative for SARS-CoV-2 during July 2021. Immunization status was assessed using the Oregon ALERT Immunization Information system. Results 500 case-control pairs were matched (n=1000). 40.4% of cases were up-to-date on COVID-19 immunizations compared to 64.6% of controls. Effectiveness of any completed COVID-19 immunization was 73% (95% Confidence Interval [CI] 49-86%), 74% (95% CI 65-85%) for mRNA immunizations (BNT162b2, mRNA-1273), and 72% (95% CI 47-85%) for individuals partially immunized with mRNA immunizations. Conclusions Our findings estimate high, yet reduced, VE during Delta variant dissemination. These results highlight the importance of COVID-19 immunizations for reducing SARS-CoV-2 infection while juxtaposing the need for additional non-pharmaceutical interventions. Importantly, the reduced VE identified here may predict future reductions in vaccine performance in the context of ongoing viral genetic drift.