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

Walmsley S, Ravindran R, Clarke R, Wouters B, Silva A, Gingras A-C, Szadkowski L. COVID-19 breakthrough infections in vaccinated participants of the Safety and Efficacy of Preventative COVID Vaccines sub-study. Can J Infect Dis Med Microbiol. 2022 Dec. doi:

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

In the Journal of the Association of Medical Microbiology and Infectious Disease Canada, researchers with the CITF-funded Safety and Efficacy of Preventative COVID Vaccines (STOPCoV) study reported a low rate of symptomatic or asymptomatic breakthrough infections (3.4%) of COVID-19 between January 17 and March 29, 2022. The study also assessed the accuracy of rapid antigen tests. The research was led by Dr. Sharon Walmsley (University Health Network, Toronto).

In this paper, a subgroup of the STOPCoV study consented to perform a routine rapid antigen test (RAT)Participants were provided with Rapid Response COVID-19 antigen rapid test devices and instructed to complete nasopharyngeal sampling twice a week and to record results as positive or negative. twice weekly for up to 6 weeks in early 2022.  Of the 727 participants who took a part in this study, only 25 tested positive for an active infection and 702 tested negative for an active infection.

All participants had two doses of vaccine, while some participants had a third dose.

Key findings:

Breakthrough infection part of study:

  • Among the 25 participants with a positive self-administered RAT, 80% had had a third vaccine dose prior to testing positive.
  • Of the positive cases, all were mild, with no instances of hospitalization.
  • Older participants (aged >70 years) who had all had three doses of vaccine) had fewer infections than younger participants (aged 30-50 years), who had either had two or three doses).

Accuracy of rapid antigen tests (RATs) part of study:

  • Of the 25 who tested positive on the RAT, 22 also had infection-acquired antibodies (anti-nucleocapsid), detected via a dried blood spot test.
  • Of the 22, 64% (n=14) still had infection-acquired antibodies after a mean of 62 days, while 36% (n=8) no longer had infection-acquired antibodies after a mean of 40 days.
  • Of the 702 participants who tested negative on a RAT, 105 participants reported one symptom, while 96 reported two or more possible COVID-19 symptoms.
  • The false negative RAT was low (4-6.6%) compared with subsequent positive nucleocapsid antibody tests.

The STOPCoV study is an ongoing prospective study designed to longitudinally evaluate the IgG antibody response to COVID-19 vaccination and booster doses in an older (70 years and older) and a younger cohort (30-50 years old). Recruitment took place from January 17 to March 9, 2022, and RATS continued to be administered until March 29, 2022.