Single-dose mRNA Vaccine Effectiveness Against Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), Including Alpha and Gamma Variants: A Test-negative Design in Adults 70 Years and Older in British Columbia, Canada

Abstract

Randomized-controlled trials of mRNA vaccine protection against SARS-CoV-2 included few elderly participants. We assess single-dose mRNA vaccine effectiveness (VE) in adults ≥70-years-old in British Columbia (BC), Canada where second doses were deferred by up to 16 weeks and where a spring 2021 wave uniquely included co-dominant circulation of Alpha (B.1.1.7) and Gamma (P.1) variants of concern (VOC). Analyses included community-dwelling adults ≥70-years-old with specimen collection between April 4 (epidemiological week 14) and May 1 (week 17) 2021. Adjusted VE was estimated by test-negative design. Cases were RT-PCR test-positive for SARS-CoV-2 and controls were test-negative. Vaccine status was defined by receipt of a single-dose ≥21 days before specimen collection, but a range of intervals was assessed. Variant-specific VE was estimated against viruses genetically characterized as Alpha, Gamma or non-VOC lineages. VE analyses included 16,993 specimens: 1,226 (7.2%) test-positive cases and 15,767 test-negative controls. Of 1,131 (92%) genetically-characterized viruses, 509 (45%), 314 (28%) and 276 (24%) were Alpha, Gamma and non-VOC lineages, respectively. At 0-13 days post-vaccination, VE was negligible at 14% (95% CI 0-26) but increased from 43% (95% CI 30-53) at 14-20 days to 75% (95% CI 63-83) at 35-41 days post-vaccination. VE at ≥21 days post-vaccination was 65% (95% CI 58-71) overall: 72% (95% CI 58-81), 67% (95% CI 57-75) and 61% (95% CI 45-72) for non-VOC, Alpha and Gamma variants, respectively. A single dose of mRNA vaccine reduced the risk of SARS-CoV-2 by about two-thirds in adults ≥70-years-old, with protection only minimally reduced against Alpha and Gamma variants.