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

Choi A, Mâsse L.C., Bardwell S, Zhao Y, Xu YXZ, Markarian A, Coombs D, Watts A, Macdonald A, Dhillon N, Irvine M O’Reilly C, Lavoie PM, Goldfarb D. Symptomatic and asymptomatic transmission of SARS-CoV-2 in K-12 schools, British Columbia, April to June 2021Microbiology Spectrum. 6 July 2022. doi:

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

A CITF-funded study now published in Microbiology Spectrum estimated rates of symptomatic and asymptomatic transmission of SARS-CoV-2 among K-12 students and staff in British Columbia from April to June 2021. From the 69 primary cases detected, 392 close contacts were identified and tested even in the absence of symptoms. Of these, 229 (58%) were close school contacts, among whom three tested positive (1.3%). This is in contrast to the 117 household contacts, of whom 37% went on to test positive for COVID-19. This shows significantly higher transmission outside than within schools. The study was led by Drs. Pascal M. Lavoie and David M. Goldfarb from the University of British Columbia.

Key Findings:

  • Of the 392 contacts, 168 (43%) were asymptomatic when tested, and 13 (7.7%) tested positive. 5 of the 13 (38%) cases identified through asymptomatic testing developed symptoms later.
  • 224 participants (57%) declined asymptomatic testing – of these, 35 (16%) later tested positive.
  • Aggregating the asymptomatic and symptomatic testing results, the majority (43/48, 90%) were household contacts. Of the 229 school contacts with the closest interactions with primary cases, only three became secondary casesCOVID-19 infection transmitted from an already infected person that was initially recruited for the study..

Overall, the study found a very low proportion of secondary cases among close contacts of primary cases identified in the school setting. Moreover, they observed that the majority of transmission occurred outside of school and that household contacts had the highest secondary transmission rate after the initial infection. The study further highlights the need for asymptomatic testing for people in close contact with the primary cases.

Of the 69 primary cases identified, 23 (33%) were female and 46 (67%) were male, 65 (94%) were students with a median age of 12 years, and four (6.2%) were staff with a median age of 48 years. The majority of cases resided in Vancouver, British Columbia.

Of the 392 close contacts identified, 229 were from school, 117 were household contacts, while the rest were social contacts. Of the school contacts, 66% spent over one hour with the case, 84% interacted with the case indoors, and 56% wore a mask sometimes or not at all.