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This undated electron microscope image made available by the U.S. National Institutes of Health shows the virus that causes COVID-19.HO/The Associated Press

As many as four million Ontarians may have been infected with the Omicron variant, according to the province’s science advisers, but their illnesses alone – particularly the milder ones – may not stop them from getting reinfected.

It’s too early to know for certain, but it doesn’t seem as if Omicron provides durable immunity against COVID-19, said epidemiologist and University of Ottawa associate professor Raywat Deonandan.

“The hope was that Omicron rips through the population, doesn’t render a lot of suffering, and leaves behind the gift of immunity,” he said.

“That is now in question. But it might be possible that if you get reinfected multiple times, maybe then you’ll get something resembling … durable immunity. It’s too early to know. Signs are not pointing in a positive direction.”

A recent preprint study – one that has not been peer-reviewed – suggested that mild Omicron infection doesn’t render enough immunity to prevent future infections, while infections from the Delta variant, which tended to be more severe, produced higher protection.

Fahad Razak, an internist and member of Ontario’s science advisory table, said the province is not seeing “significant” numbers right now of people with more than one Omicron infection.

But milder illnesses that give people congestion, cough, fever, and leave them feeling unwell but not so sick that they need to go to the hospital – like many Omicron infections – may leave people more susceptible to reinfection, he said.

“The parts of your immune system that protect against those milder infections, they tend to wane much faster than the parts of your immune system that protect against the more severe infection,” Dr. Razak said.

Omicron probably provides some short-term protection from reinfection, but there may not be long-term protection, said University of Toronto immunology professor Tania Watts. The combination of vaccination and infection is much better, she said.

“One thing our vaccines do is they give us what we call systemic immunity, because you inject it in the muscle, and the antibodies circulate around our body,” she said.

But the reason that immunity starts to wane is that people need those antibodies in the nose and throat, where the virus enters, she said.

“So when you get an infection, after two doses of vaccine, it can actually pull that immune response rapidly into the nose, and it leaves behind cells and those tissues that will respond again,” Prof. Watts said.

People who received two doses of a COVID-19 vaccine and then got Omicron will “almost certainly” see an immunity boost, she said, but that may not hold true for unvaccinated people who get Omicron.

Kieran Moore, Ontario’s chief medical officer of health, said getting vaccinated plus receiving an mRNA booster dose “significantly” adds to people’s level and length of protection against Omicron, but for unvaccinated people who get infected their immunity may not last as long.

“It appears with Omicron, if you get natural immunity – so you’re not vaccinated and you get infected by Omicron – you do have some immunity against further reinfection,” Dr. Moore said.

“But there is significantly a risk for reinfection from Omicron over time. So we’re not confident in how long that duration of immunity is from natural exposure yet. It’s still early days.”

Available figures from Public Health Ontario show there have been 3,208 confirmed cases of COVID-19 reinfections since Nov. 1, 2020, although the data don’t indicate what strain of the virus individuals were infected with.

Case numbers since late December – after Omicron became dominant – are likely an undercount, since the province has limited access to PCR testing, and therefore genomic sequencing, since then.

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