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. 2025 Feb 8;25(1):194.
doi: 10.1186/s12879-024-10331-1.

Early COVID-19 and protection from Omicron in a highly vaccinated population in Ontario, Canada: a matched prospective cohort study

Altynay Shigayeva  1 , Christopher Kandel  2   3 , Lubna Farooqi  1 , Zoe Zhong  1 , Anne-Claude Gingras  4   5 , Brenda L Coleman  1   6 , Lois Gilbert  1 , Wayne L Gold  3   7 , Maria Major  8 , Tony Mazzulli  1   9 , Samira Mubareka  9   10 , Jelena Vojicic  8 , Jingyan Yang  11 , Pingping Zhang  11 , Catherine Martin  11 , Moe H Kyaw  11 , John M McLaughlin  11 , Allison McGeer  12   13   14   15   16
Affiliations

Early COVID-19 and protection from Omicron in a highly vaccinated population in Ontario, Canada: a matched prospective cohort study

Altynay Shigayeva et al. BMC Infect Dis. .

Abstract

Objectives: Predictions regarding the on-going burden of SARS-CoV-2, and vaccine recommendations, require an understanding of infection-associated immune protection. We assessed whether early COVID-19 provided protection against Omicron infection.

Methods: We enrolled a cohort of adults in Ontario, Canada, with COVID-19 prior to October 2020 (early infection, EI), and a matched cohort with COVID-19 testing and a negative PCR (non-EI). Participants completed baseline surveys then surveys every two weeks until January 2023. Multivariable Cox regression was used to assess factors associated with COVID-19 infection during the first 14 months of Omicron.

Results: Overall, 624 EI (70%) and 175 (77%) non-EI participants met criteria for analysis; 590 (95%) EI and 164 (94%) non-EI had received at least 2 COVID-19 vaccine doses prior to Omicron. Of 624 EI, 175 (28%) had one SARS-CoV-2 re-infection and 8 (1.3%) had two, compared to 84 (48%) non-EI participants with one, 5 (2.9%) with two and 1 (0.6%) with 3 infections (P < 0.0001). In multivariable analysis of risk factors for Omicron infection, the overall hazard ratio (HR, 95%CI) associated with EI was 0.56 (0.43-0.74); HRs for BA.1/2, BA.4/5 and mixed BA.5/BQ.1/XBB periods were 0.66 (0.45-0.97), 0.44 (0.28-0.68) and 0.71 (0.32-1.56). EI and BA.1/2 infection combined reduced later Omicron infection (HR 0.07 (0.03-0.21) compared to no prior infection. Older age, non-White ethnicity, no children in household, and lower neighbourhood income were associated with reduced risk of infection.

Conclusions: In our highly vaccinated population, early SARS-CoV-2 infection was associated with a 44% reduction in symptomatic COVID-19 during the first 14 months of Omicron, providing significant protection against re-infection for more than 2 years.

Keywords: COVID-19; Re-infection; SARS-CoV-2 infection; Vaccination.

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Conflict of interest statement

Declarations. Ethics approval and consent to participate: Informed consent was obtained from all study participants. The study was approved by the research ethics boards of participating hospitals (North York General #2024-0233-1025; Sunnybrook Health Sciences Centre #149–1994; Michael Garron Hospital #084-0209-Lab-01, University Health Network #14-8339-AE, The Scarborough Health Network #MED-02-011, William Osler Health System #95 − 0001, and Mount Sinai Hospital #21-0069-E). Consent for publication: Not applicable. Competing interests: MHK, CM, MM, PZ, JV, JY and JMM are employees and shareholders of Pfizer Inc. AM declares grants and personal fees from Pfizer, grants from Sanofi, personal fees from AstraZeneca, GlaxoSmithKline, Moderna and Novavax, outside the submitted work. ACG has received research funds from Providence Therapeutics Holdings, Inc., outside the submitted work. All other authors, no conflicts of interest.

Figures

Fig. 1
Fig. 1
Flow chart of study procedures, variant evolution and vaccination in Toronto from January 2020 to January 2023
Fig. 2
Fig. 2
Flow chart of study consent, enrollment and follow-up. Note that 2 participants enrolled as non-EI were reclassified as EI because they reported in their baseline questionnaire having had a SARS-CoV-2 infection after their test-negative episode defining eligibility but before 30 September 2020
Fig. 3
Fig. 3
Incidence rate of PCR- or RAT-confirmed SARS-COV-2 infections comparing participants in the early infection (EI) cohort to those in the non-early infection (non-EI) cohort. Panel A displays the distribution of month of diagnosis in the early infection cohort in 2020, while Panel B displays the infection rate by cohort from October 2020 to January 2023
Fig. 4
Fig. 4
Cumulative probability of SARS-CoV-2 infection during Omicron, comparing participants in the early infection (EI) cohort to those in the non-early infection (non-EI) cohort. Panel A displays the unadjusted and Figure B the analysis adjusted for age, severity of 2020 illness, ethnicity, education, neighbourhood income quintile, presence of children in the household and COVID-19 vaccination
See this image and copyright information in PMC

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