National Post (National Edition)
Report finds 1.5% have antibody immunity
`Herd' objective still distant, experts suggest
Despite high infection rates in the second wave of the COVID-19 pandemic, Canadians are still a long way from achieving herd immunity, according to a national report.
The report, compiled by the Canadian Blood Services and Canada's COVID-19 Immunity Task Force (CITF), found that only 1.5 per cent of healthy Canadians contain detected antibodies in the second wave of the pandemic. The result is based on the analysis of 33,680 blood samples from blood donor centres across Canada — with the exception of Quebec and the Territories — collected during October and November last year.
“Community transmission has been more intense in this sustained second wave,” Dr. Catherine Hankins, CITF co-chair, said in a statement on Monday. “However, even using serology to add cases to our count that never came to light through formal diagnostic tests, it is clear that the vast majority of Canadians remain vulnerable to COVID-19."
The overall level of seroprevalence — the Canadian population with detected COVID-19 antibodies — is twice that recorded during the first wave of the pandemic, using data compiled between May and July. Then, only 0.7 per cent of Canadians were found to have antibodies after exposure to the virus.
But the number is still low, Dr. David Naylor, CITF co-chair acknowledged in an interview — lower than a hypothetically ideal 2.3 per cent, which would have been the result if antibodies had been detected in all Canadians with confirmed infections.
“What this tells you ... is that there is a problem with antibodies waning over time,” he said.
Reports of re-infections around the world have confirmed that immunity from antibodies has limits. Scientists still aren't sure how long antibodies remain in the body post-infection and how long they can immunize someone from the virus.
The uncertainty could partly explain why the spike in infections isn't reflected in the number of people with detected antibodies, Naylor explained.
“I treat this data as more of a qualitative snapshot of the current wave rather than as a total indicator of the level of background immunity,” he said.
The Prairie provinces and Western Canada showed “greatest increases” in levels of seroprevalence. “Since the first wave in May and June, seroprevalence in the Prairies (Manitoba and Saskatchewan) increased fivefold, from 1.69 per cent to 8.71 per cent," said Dr. Sheila O'Brien, associate director of epidemiology and surveillance at the Canadian Blood Services. "Seroprevalence in British Columbia and Alberta has nearly tripled.”
The higher seroprevalence results in these regions are also “an unsettling indication of how hard-hit some of those regions have been,” Naylor said.
Remarkably, Ontario, despite reporting the highest daily number of infections per province, noted the lowest provincial level of seroprevalence in November — 0.77 per cent, a drop from 0.88 per cent in May-July.
This doesn't necessarily mean that there are fewer Canadians with detected antibodies in Ontario than in Manitoba for instance; rather it could be indicative of different provincial sizes, sampling bias of blood donors, and “the differential nature” of how the epidemic impacts different provinces, Naylor explained, referencing Ontario as an example.
“To me, the Prairie seroprevalence numbers make sense, while the Ontario numbers seem low,” he said. "That could be because Ontario's first wave hit early and there's less carry over from then.
“Another, probably bigger factor, is the clustering of epidemic combined with sampling bias,” he added.
Several middle-class neighbourhoods in Ontario report relatively few cases of the virus, while a “terrible concentration of cases” have been observed in lower-income and racialized neighbourhoods, he added. “If you're in a neighbourhood hit that way and already at risk because you're an essential worker, then no matter how big-hearted you are, it's hard to imagine you'd be keen to take a risk and go give blood.”
“You can end up with these paradoxical results, where you would expect accumulation of immunity, but you don't see it as you're just measuring antibodies,” Naylor said.
Data on Quebec's level of seroprevalence in November has not been included in the study, as the province uses its own non-profit agency, Héma-Québec, which plans to conduct their survey “very soon,” according to Naylor.
Seroprevalence studies, despite their limitations, are important as they pick up undetected cases of the virus — asymptomatic carriers for instance, and those who may decide against getting tested as their symptoms are mild. But, alone, they're not necessarily the best indicator of the nation's overall level of herd immunity, nor can they provide a definite timeline for when Canada will be able to achieve immunity.