In a publication led by Dr. David Fisman from the University of Toronto in the Annals of Internal Medicine, CITF-funded researcher Dr. Sheila O’Brien and CITF Testing Working Party member Dr. Steven Drews, both from Canadian Blood Services, worked with colleagues from the University of Toronto and the University of Guelph and found that when accounting for testing rates, younger people in Ontario contracted SARS-CoV-2 more frequently than older people. Their results indicate that after adjustment for testing frequency, the infection rates were lowest in children (less than 10 years old) and in adults aged 70 years or older. They were however markedly higher in adolescent males (aged 10 to 19 years) and in young adult males (aged 20 to 49 years) compared to the overall population.


Key points:

  • This research was possible due to the existence of databases containing all COVID-19 PCR tests completed in Ontario, all COVID-19 cases in that province, as well as aggregated blood donor serologic data from Ontario collected by Canadian Blood Services.
  • The evaluation period included when schools were closed to in-person learning (March to August 2020) as well as when they reopened (September to December 2020). This involved associated changes in testing patterns.
  • Between March 1st and December 8th, 2020, over 6 million unique COVID-19 PCR tests were performed, leading to the diagnosis of 132,075 cases of COVID-19 in Ontario.

When the frequency of testing is accounted for and adjusted – an important adjustment, because older individuals are more likely to be symptomatic and get tested, compared to younger populations – pre-adolescent and adolescent males (aged 10 to 19 years) and young adult males (aged 20 to 49 years) had the highest infection rates: they should therefore be considered to an underrecognized group at high risk for SARS-CoV-2 infection. See detailed results in Table 1.


Table 1: Adjusted infection rates recorded

  All Female Male
0-9 y 0.59 0.59 0.59
10-19 y 1.47 1.52 1.61
20-29 y 1.46 1.01 2.50
30-39 y 1.16 1.00 1.40
40-49 y 1.09 1.01 1.33
50-59 y 1.02 0.93 1.10
60-69 y 0.80 0.74 0.93
70-79 y 0.67 0.61 0.73
>80 y 0.53 0.53 0.53


Fisman DN, Greer AL, Brankston G, Hillmer M, O’Brien SF, Drews SJ, Tuite AR. COVID-19 Case Age Distribution: Correction for Differential Testing by Age. Ann Intern Med. doi:10.7326/M20-7003