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Three years after the pandemic began, Canada has managed to avoid a severe COVID-19 wave this winter despite a total lack of public health restrictions, a busy indoor holiday season and a rapidly mutating virus that is still very much circulating in the population.
"We are now at a point in Canada where COVID-19 activity has reached a relatively stable state," Canada's chief public health officer, Dr. Theresa Tam, said at a briefing on March 10.
"While uncertainty remains about the seasonal patterns of COVID-19, the current trend suggests we may not see any major waves in the coming months."
And new research continues to back up why: Hybrid immunity from vaccination and prior infection is holding up against hospitalizations and deaths and will likely continue to help control the severity of COVID-19 in Canada and around the world for the foreseeable future.
"We're certainly in a much better position now than we have been at any time during the pandemic," World Health Organization director general Tedros Adhanom Ghebreyesus said during a news conference on Friday.
"It's very pleasing to see that for the first time, the weekly number of reported deaths in the past four weeks has been lower than when we first used the word pandemic three years ago."
More than 76 per cent of Canadian adults and close to 90 per cent of young adults (aged 17 to 24) are estimated to have previously had the disease as of mid-January, according to national blood donor data released by the federal government's COVID-19 Immunity Task Force.
High levels of infection — combined with the more than 80 per cent of Canadians who've received at least two doses of a COVID vaccine, better treatment access and less severe infections than previous strains — have led to stronger immune protection against a virus that continues to spread globally.
"The high levels of hybrid immunity are one of the major factors explaining the contained number of COVID-19 hospitalizations and deaths this winter," said Dr. Sara Carazo, an epidemiologist and researcher with the Quebec National Institute of Public Health.
"This is explained also by the intrinsic characteristics of new circulating variants, which were not causing a more severe disease than previous Omicron subvariants."
But infection is not without risk — and vaccination is still the preferred route of acquiring immunity, due to the strong protection it provides against severe illness and the ongoing risk of COVID complications in vulnerable groups.
Hybrid immunity offers strongest protection
A growing body of research has consistently shown that the hybrid protection from vaccination and infection is superior to immunity from prior infection alone — meaning those who have previously been infected should still get vaccinated.
"Vaccine-induced immunity is what got us to the point of even asking the question of whether hybrid immunity is what's getting us out of the pandemic," said John Wherry, director of the Institute for Immunology at the University of Pennsylvania in Philadelphia.
"It doesn't look like it's really helping with transmission, but it almost certainly is adding to the overall population immunity in a way that's making [new subvariants] a lot less concerning."
A Canadian study of health-care workers in Quebec published in The Lancet Infectious Diseases in January found that two doses of an mRNA vaccine and a previous Omicron infection offered substantial protection against future infection from Omicron subvariants.
"Importantly, this protection seems to have little waning over time during one year followup, which contrasts with the loss of effectiveness with time among persons vaccinated but not previously infected," said Carazo, the lead author of the study.
"We also observed that protection from hybrid immunity was maintained even for distant variants and subvariants compared with protection from infection alone."
Carazo's research also found that those with a previous infection had a 90 per cent risk reduction of BA.4/5 hospitalization when combined with vaccination, compared with only about 70 per cent if they were unvaccinated and had immunity from infection alone.
"It's safe to say that the relative lack of severity of the waves that we've seen here is because of immunity — it's hard to argue anything differently than that," said Deepta Bhattacharya, an immunologist and professor at the University of Arizona in Tucson.
"And obviously, given what fraction of the population has had a prior infection, you would have to think that hybrid immunity is a big part of that."
Vaccination 'the safest way to get immunity'
A new study of 613 patients published this week in Science Translational Medicine found that people who had received a COVID-19 vaccine after an infection showed much stronger immune responses than those who were either only vaccinated or only infected.
"The level of protection expected from hybrid immunity is significantly higher than that afforded by vaccination only or infection only," said Thierry DeFrance, a lead author of the study and an infectious diseases researcher at the University of Lyon in France.
And a recent systematic review of 65 studies from 19 countries in The Lancet found that a previous COVID-19 infection reduced the risk of hospitalization and death from a reinfection by up to 88 per cent for at least 10 months — equivalent to two doses of an mRNA vaccine.
"Clearly, the good news is sustained protection against severe disease," said Dr. Christopher Murray, the lead author of the review and director of the Institute for Health Metrics and Evaluation at the University of Washington in Seattle.
"The less good news is that protection against infection is not as good and wanes much more quickly, meaning that there will be continued waves of transmission even though we have a very high level of immunity from either vaccination or infection."
It's also important to note that not all immunity carries the same risk, and an infection with Omicron or one of its subvariants is much different than an infection with previous variants, such as Alpha, Beta, Delta or even the original strain, prior to vaccines.
"The safest way to get immunity is through vaccination," Murray said. "The risk you were taking was huge back in the days of Delta or even the ancestral strain, because the infection fatality rate was 10 times higher than Omicron."
South Africa, a country that saw massive amounts of hospitalization and death following severe waves of infection early in the pandemic prior to the rollout of vaccines, is also now in a much different situation with COVID-19 due to high levels of immunity in the population.
"It has been over a year that we have had any large wave of infection that translates into hospitalization," said Tulio de Oliveira, director of South Africa's Centre for Epidemic Response and Innovation.
"Is that long lasting? That's the million-dollar question," he said. "But what we know is that the current immunity wall is holding very well."
Better access to antiviral treatments has helped
How well hybrid immunity continues to hold up in the population will determine how often additional booster doses should be offered, and it underscores the need to further protect elderly and immunocompromised Canadians who are less likely to have a prior infection.
Dr. Gaston De Serres, an epidemiologist at the Quebec National Institute of Public Health who researches hybrid immunity and co-authored the research with Dr. Sara Carazo, said the immunity landscape is drastically different in younger adults than elderly Canadians.
"Why it matters is because hospitalizations are, for the most part, occurring in elderly people," he said.
"Having a large proportion of the younger population that has been infected helps. But the pool of individuals who are over 70 and who have not yet been infected is still quite substantial, and we may expect that future hospitalizations will, for the most part, occur in these individuals."
De Serres said better access to antiviral treatments that can be given to older Canadians at the onset of a COVID-19 infection has helped reduce the number of hospitalizations, as well as the fact that Omicron and its subvariants appear less severe than previous strains.
"It's not to say that Omicron or its subvariants are completely mild and not harmful — that's not true," he said, adding that 2022 "had more deaths than the two previous years of the pandemic."
"Having said that, there has not been an overwhelming wave that flooded the hospital system last fall or now, and in that sense things are more under control."
Wherry, at the University of Pennsylvania, said key goals for the future should be trying to improve on COVID-19 vaccine technology to recreate the protection that hybrid immunity provides in people who haven't yet been vaccinated, such as children and those who are more vulnerable to severe illness.
"That still remains a major challenge, and hybrid immunity and vaccines are still not giving us a really durable benefit there," he said.
"Immunity from severe disease may also wane. We haven't been out long enough to really know. So I wouldn't assume that we're going to have five-year or 10-year immunity keeping us out of the hospital."
The University of Arizona's Bhattacharya said there could be some seasonal variation in terms of the severity of COVID infections — similar to the flu, where different strains emerge that are distinct from what the population had last acquired immunity to — that could drive future waves.
"But I still strongly believe that we're not going to go back into the pre-vaccination era of early 2020," he said. "I don't think we're going to see those dark days again."
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