"Attack Rate
We calculated detected attack rates in the province by dividing the number of reported cases
within an age group by the estimated population from that age group. Similarly, the detected
attack rate for Ontario was calculated as the total number of reported cases divided by the
estimated population of Ontario."
Information taken from this report-
"Enhanced Epidemiological Summary
COVID-19 Case Fatality, Case Identification, and Attack
Rates in Ontario"
They inflated the rates by 50% because muh NYT article.
Link to PDF of the report -
https://www.publichealthontario.ca/-/media/documents/ncov/epi/2020/06/covid19-epi-case-identification-age-only-template.pdf?la=en