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

Background: Randomized-controlled trials of messenger RNA (mRNA) vaccine protection against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) included relatively few elderly participants. We assess single-dose mRNA vaccine effectiveness (VE) in adults ≥ 70 years old in British Columbia, Canada, where second doses were deferred by up to 16 weeks and where a spring 2021 wave uniquely included codominant circulation of Alpha (B.1.1.7) and Gamma (P.1) variants of concern (VOC).

Methods: Analyses included community-dwelling adults ≥ 70 years old with specimen collection between 4 April (epidemiological week 14) and 1 May (week 17) 2021. Adjusted VE was estimated by test-negative design. Cases were reverse-transcription polymerase chain reaction (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.

Results: VE analyses included 16 993 specimens: 1226 (7%) test-positive cases and 15 767 test-negative controls. Of 1131 (92%) genetically characterized viruses, 509 (45%), 314 (28%), and 276 (24%) were Alpha, Gamma, and non-VOC lineages, respectively. At 0-13 days postvaccination, VE was negligible at 14% (95% confidence interval [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 postvaccination. VE at ≥ 21 days postvaccination 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.

Conclusions: 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.

Keywords: SARS-CoV-2; case-control; test-negative design; vaccine effectiveness; variants of concern.

Figure 1.

Percentage vaccinated among SARS-CoV-2 test-positive cases and test-negative controls and case tallies by epidemiological week, participating adults ≥ 70 years of age, British Columbia, Canada, weeks 14–17. Abbreviation: SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

Figure 2.

Adjusted VE estimates by interval in days since vaccination and restricted by subgroup, adults ≥ 70 years of age, British Columbia, Canada, weeks 14–17. All vaccine effectiveness estimates are adjusted for age group (70–79, 80–89, 90+ years); sex (men, women); epidemiological week (14, 15, 16, or 17); and health authority (HA) (Fraser HA, Interior HA, Northern HA, Vancouver Coastal HA, Vancouver Island HA). See Supplementary Tables 2–8 for details. VE estimates that are based upon a ≥ 21-day interval between vaccination and specimen collection combine specimens collected 21 or more days since vaccination. Similarly, VE estimates based on a ≥ 42-day interval combine specimens collected 42 or more days since vaccination. Abbreviations: CI, confidence interval; mRNA, messenger RNA; VE, vaccine effectiveness.