Getting COVID-19 a second time was once considered a rarity. Now, with the more transmissible Omicron subvariant BA.2 spreading throughout the country, reinfections are becoming more common, immunology and infectious diseases experts say.
Exactly how common is not clear. Testing is limited across the country and detailed COVID-19 data that's publicly available is also limited.
"There is a dearth of testing that's happening and there's some information coming out to suggest that the rapid antigen tests are not as accurate as we hoped against Omicron," said Jennifer Gommerman, professor of immunology at the University of Toronto, and the Canada Research Chair in tissue specific immunity.
"Prevalence is a difficult question to answer, but we know that there's lots of virus going around."
PCR test-positivity rates, hospitalization figures, and wastewater data in many provinces are showing increased transmission of the virus.
And data from around the world do show reinfection risk is higher with Omicron, experts say.
"When we think about this idea of reinfection, we have to appreciate we are still facing a virus that has a unique ability to circumvent those early defences from neutralizing antibodies," said Jason Kindrachuk, a virologist and assistant professor in medical microbiology and infectious diseases at the University of Manitoba in Winnipeg.
Yet, reinfection cases largely do not cause worse symptoms of the disease, said Kindrachuk, who is also the Canada Research Chair in the molecular pathogenesis of emerging viruses.
There are many factors as to why someone may be reinfected with the virus, and researchers are trying to get a better understanding of reinfections and what it means during Omicron.
Reinfections rising in global data
Data released by Public Health Ontario last week shows some 11,730 Ontarians have been reinfected with COVID-19 since November 2020.
Reinfection, according to Ontario health officials, is when someone has had two lab-confirmed cases of SARS-CoV-2 with evidence that these were "separate infections caused by different viral lineages."
South African scientists had warned in December that reinfections among people who've already battled COVID-19 appeared to be more likely with Omicron than with earlier coronavirus mutants.
A recent rise in reinfections has been seen in parts of the world like the United Kingdom, which had a similar vaccine rollout to Canada.
Before Dec. 6, the proportion of daily cases in England thought to be reinfections had been below two per cent for nearly six months.
That rate is estimated to have risen to 9.9 per cent as of February, a Reuters analysis of UKHSA data found.
In Italy, a spokesman for the National Institute of Health said in February that reinfection cases stood at around three per cent of all infections, up from around 1.5 per cent before Omicron.
Why does reinfection happen?
There are a lot of factors as to why someone is reinfected with COVID-19, including vaccination status and when a person got a vaccine or infection, said Dr. Lynora Saxinger, an infectious disease specialist at University of Alberta in Edmonton.
A study done in Qatar published in the New England Journal of Medicine last month found recovering from a previous infection was approximately 90 per cent effective at preventing reinfection from the Alpha, Beta and Delta variants.
Protection against reinfection with the Omicron variant was lower at 60 per cent but "still considerable," said the authors.
Another important aspect of the risk of reinfection is vaccine-induced immunity.
The strength of immune response will partially depend on a person's age and overall health. The very old, very young, and immunocompromised will have weakened responses, for example.
That's why fourth doses are now being rolled out to the more vulnerable populations across the country.
And although vaccines have repeatedly been proven effective in preventing serious illness and hospitalization, Omicron is still "quite good at punching through the equivalent of two doses and in many cases, two doses plus infection or three doses," said Saxinger.
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It's known that antibodies do drop off over time after someone is vaccinated or infected, said Benoit Barbeau, a professor in the biological science department at Université du Québec à Montréal who studies virology.
"So if you've been infected in December or early January, you'd already have less levels of antibodies. It doesn't mean that you've totally lost your protection against infection, but definitely you'll have less protection than say 10 days after being infected," he said.
The good news is that the immune response generated through vaccination "is strong enough to keep us from getting really severe disease and ending up in hospital," said Gommerman.
That's why experts are recommending people get a booster shot for that added protection.
As for the Omicron subvariant BA.2, early research out of Denmark hints that reinfections with that strain seem rare.
What do we need to learn?
Barbeau said as parts of Canada and other countries deal with a sixth wave, more data will emerge on who is getting reinfected and why.
"There'll be other variants. The risk of reinfection will always be there," he said.
But he emphasized that the immunity in the population, whether from vaccines or prior infection, is building up and allowing us to reduce transmission and hospitalization.
All of this is happening as research continues into improving vaccines, whether it's adapting to new variants or research into new methods to administer vaccines like through the nasal cavity, he said.
"It's very important to continue improving the vaccines."
What we know so far about getting COVID-19 twice in the age of Omicron - CBC News
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