With so few people carrying antibodies, everyone in Canada remains at risk for infection. It is a constant refrain during the pandemic: Life won't get back to normal without a vaccine.
Efforts to create one depend on better understanding what happened in the immune systems of patients who have recovered from COVID-19, and whether they have any protection from the disease now that they have antibodies to the coronavirus that caused it.
In Canada, the national Immunity Task Force has so far examined antibody levels from the blood of 35,000 Canadians across the country who donated blood or who had their blood drawn for medical tests in British Columbia, Alberta and Ontario.
The task force found that the level of coronavirus antibodies in the population, or seroprevalence, barely registered in some cases.
"The message is clear," said Dr. David Naylor, co-chair of the task force. "Immunity in the Canadian population remains very low."
Hema-Quebec's data suggests 2.23 per cent of samples had antibodies. Samples from Montreal were highest at 3.05 per cent, Naylor said. Samples from B.C., Alberta and Ontario fell in the one per cent range.
With so few people carrying antibodies, everyone in Canada remains at risk for infection.
"Broad population susceptibility, in turn, means that caution is required this fall as schools and worksites reopen," Naylor said in an e-mail interview. "These results also underscore the continuing urgent need for safe and successful deployment of effective vaccines against COVID-19."
Low infection rate, low immunity
But while a low rate of potential immunity leaves many Canadians susceptible, a higher rate brings other issues.
Dr. Allison McGeer, a member of the task force's leadership team and an infectious disease physician in Toronto, points to hard-hit New York, where early testing found more than 13 per cent of people had developed antibodies.
"It's important to remember that the price of being at 13 per cent was a very large number of dead people and catastrophic health care for six or eight weeks," McGeer said.
It's not yet known how long coronavirus antibodies remain in the body, or whether their presence, and at what level, protects someone against reinfection. There have been documented cases of patients reinfected months after they recovered.
But Naylor called those reinfections "an outlier" among the millions of cases of COVID-19 worldwide.
Still, they add to the uncertainty about how both natural immunity and vaccines could work in the long term.
Kelvin Kai-Wang To of the University of Hong Kong's microbiology department co-authored a paper on the first documented reinfection, which used genome sequencing to show the first and second infections were caused by different viral strains.
He said the findings suggest that the best hope for stemming the pandemic remains a vaccine.
That's because SARS-CoV-2, the virus that causes COVID-19, may persist in people worldwide, as other human coronaviruses do — even if patients acquire natural immunity after infection.
"Our study only shows that immunity induced from natural infection cannot prevent reinfection for our patient," he said. "Vaccine-induced immunity can be much stronger and last much longer than infection-induced immunity."
More infected than known
For Dawn Bowdish, Canada Research Chair in aging and immunity and an associate professor at McMaster University in Hamilton, the Canadian seroprevalence results are good news.
"It means Canadians are doing the right things and not too many of us were infected," said Bowdish, who is independent from the task force. "It also does tell us that it's higher than the number of infections that we knew about."
Serology helps uncover the number of asymptomatic carriers of the virus, whereas most testing has been done on people with symptoms.
"It speaks to the importance of doing lots and lots of testing for even people with very minor symptoms." She also notes that the research to date suggests that children with COVID-19 are less likely to show symptoms than adults.
School could bring fundamental shift
Underscoring how screening for symptoms alone can miss COVID-19 cases in children, researchers in South Korea reported that of 91 children who were positive for the virus, 22 per cent were asymptomatic. Half were over the age of 10.
"We are expecting a fundamental shift in the dynamics of the pandemic … as we start opening up and go back to school," Bowdish said.
Bowdish believes "no COVID is good COVID," and pursuing near elimination should be the collective goal in Canada because of its long-term health consequences in people of all ages, including young adults, a demographic that's seeing more cases across Canada.
Dr. Caroline Quach, a pediatric infectious disease physician and medical microbiologist in Montreal, is encouraged by what she saw when she helped treat some of the 63 children and pregnant women with COVID-19 that have come through Montreal Children's Hospital so far.
"What's very interesting is, I think, that the immune system of children is much better equipped to face COVID than adults," said Quach, who is also a member of the immunity task force. "With the same number of viruses in their nose, and sometimes even higher, they don't develop any of that hyperinflammation that brings adults to the hospital. Somehow — and that's what research needs to tell us — they deal better with this virus."
Advantage of kids' blank slates
Bowdish said the difference between the immune systems of adults and those of children might offer some clues to designing vaccines.
In many of the adults who died due to COVID-19, the immune response was too strong and the wrong kinds of immune cells went into organs and caused damage. That reaction hasn't been as prevalent in children.
Children's immune systems also tend to respond better to many vaccines, Bowdish said, because they tend not to have "immune memory" — the immune system's recognition of an antigen the body has previously encountered.
Vaccines for adults are more complicated, Bowdish said.
She pointed to two main forms a coronavirus vaccine could take.
It could be like the tetanus vaccine, which requires a booster every 10 years because the immune response doesn't last. Or, "if we're really lucky," it could be like the measles vaccine, where even if antibody levels decrease over a year, they pop back up if reinfected.
"We need time to figure this out," Bowdish said.
Experiments are underway worldwide to sort out the relationship between disappearing antibodies and recall of immune cells with a memory of the virus. When safe and effective vaccines become available, that's something the task force hopes to measure.