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

Chanchlani R, Shah BJ, Bangdiwala SI, de Souza R, Luo J, Bolotin S, Bowdish DME, Desai D, Lear AS, Loeb M, Punthakee Z, Sherifali D, Wahi G, Anand SS. COVID-19 vaccine effectiveness among South Asians in Ontario: A test-negative design population-based case-control study. medRxiv. 2023 December 9. 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A CITF-funded study, published as a preprint and not yet peer-reviewed, aimed to evaluate the effectiveness of COVID-19 vaccines among South Asians in Ontario compared to non-South Asians. The researchers also assessed the odds of symptomatic COVID-19 infection and related hospitalizations and deaths among non-vaccinated South Asians and non-South Asians. They found that two doses of a COVID-19 vaccine (prior to the advent of booster doses) were effective in protecting South Asians. Unvaccinated South Asians were twice as likely to experience negative outcomes, compared to unvaccinated non-South Asians. The research was led by Dr. Rahul Chanchlani and Dr. Sonia S. Anand (both from McMaster University).

This test-negative design study included 888,155 Ontarians, who had been tested by PCR for SARS-CoV-2 between December 14, 2020 and November 15, 2021. Individuals who had symptoms and tested positive at least once were considered “cases” and individuals who were symptomatic but tested negative were the “controls.” The study cohort was divided into four sub-cohorts: South Asian vaccinated, South Asian non-vaccinated, Non-South Asian vaccinated, and Non-South Asian non-vaccinated.

The primary outcome measured was vaccine effectiveness (SARS-CoV-2 infections, hospitalizations, death) and the secondary outcome measured was the odds of COVID-19 infections, hospitalizations, and death comparing non-vaccinated South Asians to non-vaccinated non-South Asians.

Key findings:

  • Among South Asians, two doses of a COVID-19 vaccine prevented 93.8% of infections and 97.5% of hospitalizations and deaths (the study was done before the advent of booster doses).
  • Among non-South Asians, vaccines prevented 86.6% infections and 93.1% of hospitalizations and deaths.
  • Non-vaccinated South Asians were twice as likely to experience a symptomatic SARS-CoV-2 infection compared to non-vaccinated non-South Asians.
  • Non-vaccinated South Asians experienced double the risk of COVID-19 hospitalizations and deaths compared to non-vaccinated non-South Asians.
  • More research is needed to determine whether complex sociocultural factors and specific biological pathways among South Asians may account for the higher risk of COVID-19 infection and negative outcomes.

The study had a large sample size (close to 900,000 individuals in Canada) and mitigated potential bias arising from differences in access to healthcare as it only included individuals who presented for SARS-CoV-2 testing. As such, these results are only generalizable to areas with universal healthcare coverage. Additionally, the researchers’ last name-based algorithm did not allow them to identify multiple ethnicities with specificity, so the non-South Asians group was quite heterogenous.