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

Nasreen S, Calzavara A, Buchan SA, Thampi N, Johnson C, Wilson SE, Kwong JC. Background incidence rates of adverse events of special interest related to COVID-19 vaccines in Ontario, Canada, 2015 to 2020, to inform COVID-19 vaccine safety surveillance. Vaccine. 2022 May 26. doi:  10.1016/j.vaccine.2022.04.065.

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

A study now published in Vaccine and led by CITF-funded researcher Dr. Jeff Kwong from IC/ES, tracked the incidence of nine different medical conditions in the five years preceding the pandemic (2015-2019) and in the first year of the pandemic (2020). These estimates, called background rates, will help establish the expected incidence rates for these conditions in the population. They can then be compared with the rates following the introduction of COVID-19 vaccines and facilitate the detection of vaccine safety signals.

Key findings: 

  • From 2015-2019, the overall mean incidence rate (displayed as one in 100,000 persons) of nine medical conditions, also called adverse events of special interest, for both sexes and across all ages were as follows:
    • In decreasing order: Idiopathic thrombocytopenia (43.9 per 100,000), Bell’s palsy (27.8 per 100,000), febrile convulsions (25.0 per 100,000), acute disseminated encephalomyelitis (22.8 per 100,000), myocarditis/pericarditis (11.3 per 100,000), pericarditis (8.6 per 100,000), myocarditis (2.9 per 100,000), Guillain-Barré syndrome (1.9 per 100,000), transverse myelitis (1.7 per 100,000), and Kawasaki disease (1.6 per 100,000).
  • When it came to children aged 0-11 from 2015-2019, the highest overall mean incidence rate of the nine conditions listed above across both sexes was febrile convulsion (at 453 per 100,000 for children aged 0-4 and 20.1 per 100,000 for children aged 5-11). The lowest mean incidence rate was for pericarditis (at 0.4 per 100,000 for children aged 0-4 and 5-11).
  • Incidence rates for the medical conditions listed above varied by year, age group, and biological sex. Likewise, the rates for most medical conditions assessed were generally lower in 2020 than in the five preceding years.

This work was conducted by the Canadian Immunization Research Network (CIRN) Provincial Collaborative Network (PCN), which is led by Dr. Kwong and funded in part by the CITF. Researchers assessed incidence rates by scanning medical records for hospitalizations and emergency room visits in Ontario.

A similar CIRN-PCN study estimating pre-pandemic rates of certain thromboembolic and coagulation disorders has recently been published in the British Medical Journal.

Many of Canada’s long-standing vaccine safety surveillance systems continue to monitor COVID-19 vaccine safety. Background estimates such as these are instrumental to ongoing surveillance efforts.