Comparative single-dose mRNA and ChAdOx1 vaccine effectiveness against SARS-CoV-2, including variants of concern: test-negative design, British Columbia, Canada

In British Columbia, Canada, most adults 50-69 years old became SARS-CoV-2 vaccine-eligible in April 2021, with ChAdOx1 restricted to ≥55-year-olds and second doses deferred ≥6 weeks to optimize single-dose coverage.Among adults 50-69-years-old, single-dose mRNA and ChAdOx1 VE against SARS-CoV-2 infection and hospitalization, including variant-specific, was assessed by test-negative design between April 4-October 2, 2021.Single-dose VE included 11,861 cases and 99,544 controls. Median (interquartile range) of post-vaccination follow-up was 32 (15-52) days. Alpha, Gamma and Delta variants comprised 23%, 18% and 56% of genetically-characterized viruses. At 21-55 days post-vaccination, single-dose mRNA and ChAdOx1 VE was 74% (95%CI: 71-76) and 59% (95%CI:53-65) against any infection and 86% (95%CI:80-90) and 94% (95%CI:85-97) against hospitalization, respectively. VE was similar against Alpha and Gamma infections for mRNA (80%;95%CI:76-84 and 80%;95%CI:75-84) and ChAdOx1 (69%;95%CI:60-76 and 66%;95%CI:56-73). mRNA VE was lower at 63% (95%CI:56-69) against Delta but 85% (95%CI:71-92) against Delta-associated hospitalization [non-estimable, ChAdOx1].A single mRNA or ChAdOx1 vaccine dose gave important protection against SARS-CoV-2, including early variants-of-concern. ChAdOx1 VE was lower against infection but one dose of either vaccine reduced the hospitalization risk by >85% to at least 8 weeks post-vaccination. Findings inform program options, including longer dosing intervals.