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

Chung H, Austin PC, Brown KA, Buchan SA, Fell DB, Fong C, Gubbay JB, Nasreen S, Schwartz KL, Sundaram ME, Tadrous M, Wilson K, Wilson SE, Kwong JC. Effectiveness of COVID-19 Vaccines Over Time Prior to Omicron Emergence in Ontario, Canada: Test-Negative Design Study. Open Forum Infect Dis. 2022 Sep 7;9(9):ofac449. 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 Open Forum Infectious Diseases and led by Dr. Jeffrey Kwong (University of Toronto) on behalf of the Canadian Immunization Research Network (CIRN), highlighted that – prior to Omicron – the effectiveness of two doses of COVID-19 vaccine decreased over time against infection but remained high against severe outcomes over 11 months (January to November 2021). This applied to all dosing regimens whether homogeneous or mix-and-match (heterologous) and taking into account varying intervals between doses.

Key Findings:

  • VE of two mRNA vaccine doses against any infection decreased from 90% 7–59 days after the second dose to 75% after ≥240 days.
  • VE against symptomatic infection decreased from 94% to 87% over 11 months, but remained stable at 98% over the course of that time against severe outcomes.
  • Similar estimates were seen between individuals with and without comorbidities.
  • Similar trends in VE were seen with heterologous AstraZeneca and mRNA vaccine schedules. VE against symptomatic infection at 120–179 days post dose 2 was 94% for AstraZeneca/Pfizer, 93% for AstraZeneca/Moderna, 90% for 2 Pfizer doses, 91% for 2 Moderna doses, and 93% for mixed mRNA schedules. This was distinctly higher than for 2 doses of AstraZeneca – VE against symptomatic infection at 120–179 days was 80%.
  • Looking at the effects of dosing intervals on VE, VE from two doses of mRNA vaccine against symptomatic infection at 120–179 days was:
    1. 86% for dosing intervals <35 days,
    2. 92% for dosing interval 35–55 days, and
    3. 91% for dosing interval ≥56 days.
  • When stratified with age and subperiod, no differences between dosing intervals were observed.

More than 260,000 test-positive cases (of any SARS-CoV-2 lineage) and nearly 2.8 million individuals as test-negative controls were included in the study.

A test-negative study design was used to estimate VE against any SARS-CoV-2 infection, any symptomatic infection, and severe outcomes (COVID-19-related hospitalizations or death) by time since the second dose of any combination of Pfizer (BNT162b2), Moderna (mRNA-1273), and AstraZeneca (ChAdOx1) between January 11, and November 21, 2021, in Ontario.