A new study from Héma-Québec, the province's blood service, reveals that approximately 600,000 Quebec adults had been infected with the coronavirus by the end of the second wave in March 2021, a number twice as high as the official toll of reported cases.
This is the second serologic study of blood donors conducted by Héma-Québec in collaboration with Quebec's public health institute (INSPQ) and in partnership with the COVID-19 Immunity Task Force (CITF).
The study involved 7,924 participants aged 18 and over who donated blood between Jan. 25 and March 11 of this year, Héma-Québec said in a news release Thursday.
It found that 15 per cent of the adult population had developed antibodies to the COVID-19 virus, but that number includes the five per cent of people who had received at least one dose of the vaccine.
"This means that at the turn of the second wave, about 10 per cent of the adult population had been exposed to the virus," said Dr. Marc Germain, vice-president of medical affairs and principal researcher at Héma-Québec.
In its news release, Héma-Québec says a study conducted after the first wave of the pandemic in spring 2020 showed that only about two per cent of tested blood donors had antibodies.
After the second wave, at least three times more people were infected, the release says.
More than double confirmed cases
Based on these results, it is possible to extrapolate that almost 600,000 people between the ages of 20 and 69 have developed antibodies to COVID-19 following an infection since the start of the pandemic, the release says.
It should also be noted that the first study showed at least half of the infections were asymptomatic, Héma-Québec says.
According to the INSPQ, as of March 11 in Quebec, 286,912 people of all ages had contracted COVID-19.
This means that the actual number of cases is about twice the number of reported cases.
This statistic does not surprise Germain, who said the first serology study had already shown a high proportion of people infected with COVID-19 had no symptoms or symptoms so mild that they did not get tested.
That's why, he explained, these types of studies reveal that there were so many more cases than confirmed via conventional testing of those who show symptoms.
Young people and visible minorities hardest hit
The second Héma-Québec study also found that antibodies were more likely to be found in young people and gradually decrease with age.
It rises to 19 per cent among those aged 18 to 24 and drops to 9.5 per cent among those aged 70 and up.
The study found 16.9 per cent of women had antibodies compared to 12.5 per cent of men, while visible minorities are twice as likely to have antibodies than white people.
Germain said factors explaining this significant difference between visible minorities and white people have yet to be identified, though the hypothesis under consideration is that certain community groups were less likely or less able to adhere to the public health guidelines.
Héma-Québec, together with the public health authorities, is finalizing the protocols for the third study on the serology of blood donors, which will be carried out in the coming months.
It will mainly aim to assess the progression of immunity, in particular that conferred by vaccines, and to determine the impact of the vaccination campaign on the progression of infections.