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

Richard L, Nisenbaum R, Colwill K, Mishra S, Dayam RM, Liu M, Pedersen C, Gingras A, Hwang SW. Enhancing detection of SARS-CoV-2 re-infections using longitudinal sero-monitoring: demonstration of a methodology in a cohort of people experiencing homelessness in Toronto, Canada. BMC Infect Dis. 2 February 2024; doi:

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

A CITF-funded study, published in BMC Infectious Diseases, found that longitudinal sero-monitoring for antibodies, combined with PCR or RAT saliva testing for acute infection, increased the detection of re-infections four-fold compared to PCR or RAT tests alone. The study focused on people experiencing homelessness in Toronto. This study was led by Lucie Richard (Unity Health Toronto), Dr. Anne-Claude Gingras (Lunenfeld-Tanenbaum Research Institute), and Dr. Stephen Hwang (University of Toronto).

From June to September 2021, the researchers recruited 736 unhoused participants at random from over 60 shelters, physical distancing hotels and encampments across Toronto. From these, 381 at-risk individuals who had experienced an incident infection by their nine-month study visit were included in the final analysis. Participants completed a baseline data collection interview and were then re-contacted for follow-up at three, six, nine, and 12 months. Data collected included self-reported information on positive PCR or rapid antigen tests (RAT) and vaccination information. Participants also provided saliva and blood (plasma or dried blood spot samples) to determine SARS-CoV-2 re-infection.

Studies relying only on PCR testing as a measure of SARS-CoV-2 re-infection often find that re-infection rates are very low despite the emergence of numerous new variants. Previously, these researchers had found that only 28% of incident infections among unhoused individuals were identified through PCR testing. These authors suggest that their comprehensive method, combining PCR/RAT testing and longitudinal serology, can address this under detection bias.

Key findings:

  • 37 re-infections were detected through PCR/RAT, while an additional 98 re-infections were detected through the longitudinal serology.
  • The comprehensive method (combining PCR/RAT testing and longitudinal serology) resulted in a significantly increased detection rate of 37.4 re-infection events per 100 person-years, more than four-fold higher than the rate detected by PCR/RAT alone.
  • 85% of PCR/RAT confirmed re-infections were also detectable via the longitudinal serology.

The authors suggest that current SARS-CoV-2 re-infection rates based on PCR testing are very likely much more common than currently reported in the literature. This has potentially important consequences for research aimed at understanding infection burden and downstream health outcomes. While the authors encourage more research to fully validate their method, they are confident that this study highlights the value of combining acute infection and serology data sources for effective public health surveillance.