Public health officials say variants of COVID-19 were found in 5.5 per cent of cases screened on a single day in January.
The majority of those were linked to an outbreak at a nursing home in Simcoe-Muskoka.
The results are part of a Public Health Ontario study on the spread of variants in the province.
Dr. Vanessa Allen, the agency’s top microbiologist, says the results show how quickly variants of COVID-19 can spread in an outbreak.
Public Health Ontario screened 1,880 positive samples from Jan. 20 and found variants in 103 cases.
It says 89 of those were cases in the Simcoe-Muskoka health unit, where a deadly long-term care outbreak was driven by that strain.
Ontario reported 1,563 new cases of COVID-19 and 88 more deaths linked to the virus on Thursday.
Public health officials have noted, however, that updates to a provincial database are causing data fluctuations in this week’s case counts.
Ontario reported nearly 64,500 tests had been completed since the last daily update.
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