Effectiveness of ChAdOx1 nCoV-19 (Vaxzevria) primary series vaccine against SARS-CoV-2 beta and delta variants: a nationwide study

Abstract

Background: This study assessed the real-world effectiveness of the ChAdOx1 nCoV-19 vaccine in adults against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, symptomatic infection, and severe coronavirus disease 2019 (COVID-19) during periods of Beta and Delta variant dominance in Qatar. Methods: A national, matched, test-negative case-control study was conducted using 186,130 PCR-positive tests (cases) and 667,289 PCR-negative tests (controls) collected between January 1 and December 18, 2021. Subgroup analyses were performed to evaluate vaccine effectiveness across key strata. Results: The median time between the first and second doses was 61 days (interquartile range, 56–64 days). Two-dose primary-series effectiveness was 66.0% (95% CI, 55.1–74.3%) against any SARS-CoV-2 infection and 73.0% (95% CI, 44.1–87.0%) against symptomatic infection. Effectiveness was estimated at 100% (95% CI, 64.0–100%) against any Beta variant infection and 65.3% (95% CI, 54.2–73.8%) against any Delta infection. Protection against any infection of any variant peaked at 78.4% (95% CI, 50.7–90.5%) within the first month after the second dose, gradually declining to 45.6% (95% CI, 5.5–68.7%) after 150 days. Effectiveness against severe, critical, or fatal COVID-19, irrespective of variant, was 100% (95% CI, 49.3–100%), with no vaccinated individuals progressing to severe, critical, or fatal disease after infection. Effectiveness of a single dose was 59.9% (95% CI, 51.0–67.3%) against any infection—65.0% (95% CI, 49.7–75.6%) against Beta and 55.9% (95% CI, 43.8–65.5%) against Delta—78.4% (95% CI, 60.9–88.0%) against symptomatic infection, and 100% (95% CI, 88.9–100%) against severe, critical, or fatal COVID-19. Conclusion: The ChAdOx1 nCoV-19 vaccine provided substantial protection against infection and strong protection against severe outcomes during periods dominated by the Beta and Delta variants, although protection against infection waned within the first few months following the primary series.