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

Barlow, R. S.; Larson, L.; Jian, K.

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). During July 2021, Multnomah County experienced an epidemic expansion coinciding with increased Delta variant prevalence. We assess COVID-19 VE during this time. 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 (ALERT IIS). Results: 500 case-control pairs were matched (n=1000). Overall effectiveness of any completed COVID-19 immunization was 73% (95% Confidence Interval [CI] 49-86%) and 74% (95% CI 65-85%) for mRNA immunizations and 72% (95% CI 47-85%) for individuals that had started but not completed 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.