Published Monday, August 3, 2020 8:51AM EDT
Last Updated Monday, August 3, 2020 9:12AM EDT
The agency said that of more than 7,000 blood samples tested during the month of June, 79 showed the presence of COVID-19 antibodies, indicating a prevalence of 1.1 per cent when adjusted to the characteristics of Ontario's population.
The lab found a higher prevalence of antibodies among those aged 80 and over (2.6 per cent) and no presence of antibodies in children under the age of nine.
The lab also found a great disparity in the presence of antibodies between different regions of the province.
In Toronto and the eastern GTA, they estimate 1.5 per cent of people have antibodies for COVID-19.
Whereas in northern and eastern Ontario, they believe only 0.3 per cent of people have been infected.
Up until June 30, Ontario labs had confirmed 35,200 cases of novel coronavirus infection, using a PCR test which detects the actual presence of the virus in the body at a single point in time.
Blood tests search for antibodies created when a person successfully fights off infection.
The fact that more than four times more people could show proof of prior COVID-19 infection in their blood is due to a variety of factors, including lack of access to testing and the fact that many people experience infection with few or no noticeable symptoms.
A similar serological study conducted in New York found up to 20 per cent of people in the New York City area showed the presence of COVID-19 antibodies, even though less than three per cent of residents ever tested positive during PCR testing.
Public Health Ontario says since it takes a week or two to develop antibodies in the blood stream, it is possible some blood samples tested negative even though they were taken from people who were infected with COVID-19 at the time.
There also people who do not develop enough antibodies after fighting an infection to be detected in a blood test.
An earlier blood screen conducted by federal officials and Canadian Blood Services found antibodies for COVID-19 at a rate of less than one per cent, using 10,000 samples from across the country.
Public Health Ontario said the blood samples it tested for the study were all submitted for purposes other than COVID-19 infection and identifying information was removed from all samples before they were tested.
The provincial lab said it still could not say whether the presence of COVID-19 antibodies made someone immune from future infection.