This is a summary, written by members of the CITF Secretariat, of:

Skowronski DM, Setayeshgar S, Zou M, Prystajecky N, Tyson JR, Galanis E, Naus M, Patrick DM, Sbihi H, El Adam S, Henry B, Hoang LMN, Sadarangani M, Jassem AN, Krajden M. Single-dose mRNA vaccine effectiveness against SARS-CoV-2, including P. 1 and B. 1.1. 7 variants: a test-negative design in adults 70 years and older in British Columbia, Canada. medRxiv. 2021 Jun 9. doi: 10.1101/2021.06.07.21258332.

The results and/or conclusions contained in the research do not necessarily reflect the views of all CITF members.

Dr. Mel Krajden, COVID-19 Immunity Task Force (CITF) Leadership Group member and Vaccine Surveillance Reference Group (VSRG) member, and Dr. Danuta Skowronski, VSRG Effectiveness Working Party member, was part of a team evaluating the effectiveness of a single dose of a mRNA vaccine in people over 70. The study indicated that 21 days after the first dose, the vaccine was 65% effective in preventing disease. Overall, one dose was observed to be 67% effective in preventing active infection with the Alpha (B 1.1.7) variant, be it symptomatic or asymptomatic, and 61% effective in preventing active infection with the Gamma variant (P.1).

This study aimed to determine the effectiveness of a single dose of a mRNA vaccine in preventing SARS-CoV-2 infection by using a test-negative design. To this end, researchers gathered information on people who had received one dose of the vaccine and linked those records to information on RT-PCR testing (nasal or throat swab test to determine an active infection) post-vaccination. Information collected was then analyzed to determine the effectiveness of the vaccine in preventing infection as determined by a positive or negative test. Further analyses were carried out to investigate variant lineage, restricted to individuals who tested positive for the Alpha or the Gamma variants.

A total of 16,993 SARS-CoV-2 PCR tests, collected between April 4th and May 1st, 2021, contributed to the vaccine effectiveness analysis, including 1,226 (7.2%) positive test cases and 15,767 negative test controls. Viral lineage was available for 1,131 cases of which 45% were Alpha, 28% were Gamma, and 24% were not variants of concern. The remaining samples could not be characterized or belonged to other lineages.

During the first 13 days after the first dose of vaccine, overall vaccine effectiveness was reported to be 14%, which increased with each passing week. Vaccine effectiveness increased to 43% by 14 to 20 days, and increased to 75% by 35 to 41 days after the first dose. After 21 days, vaccine effectiveness for non-variants of concern was 72%, and was 67% and 61% for Alpha and Gamma variants, respectively.

This study highlights the level of protection provided by a single dose of mRNA vaccine, with two-thirds of adults over 70 (not including those in long-term care or retirement homes) experiencing a reduction in the risk of SARS-CoV-2 infection.