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Why an Omicron infection alone might not offer the immune boost you’d expect

In the span of just weeks, millions of Canadians became infected with SARS-CoV-2. Globally, more cases were reported in the first 10 weeks after the Omicron variant was identified than in all of 2020.

It was a mass infection event quite unlike anything we’d seen in the pandemic to date, hitting both the unvaccinated and vaccinated — but not in the same way.

While vaccinated and boosted individuals largely avoid dire outcomes from COVID-19, data continues to show that unvaccinated individuals remain at a far higher risk of serious illness, hospitalization, and death.

Emerging evidence also suggests high infection rates won’t necessarily translate into widespread protection against re-infections down the line — unless you’re layering Omicron exposure onto the broader immunity provided by vaccines.

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“In people who are not vaccinated, they’re not making a good response to Omicron,” said immunologist and University of Toronto professor Jennifer Gommerman.

“It’s very different than for people who are fully vaccinated against SARS-CoV-2; those people seem to make a good response to Omicron, which is good news.”

‘Wimpy’ immunity signal from just Omicron in lab 

It’s clear leading vaccines, designed to combat the original SARS-CoV-2 strain, struggle more against this latest variant. Its constellation of mutations — dozens of them, including many in the virus’s spike protein — allow it to break through that immunity, though two vaccine doses still cut your chance of getting seriously ill.

Boosters, meanwhile, seem to ramp up the level of protection even more, both studies and government data show.

A study shared online in January, which isn’t yet published or peer-reviewed, offered an early look at where Omicron fits into the ever-evolving immunity puzzle that scientists have been striving to piece together since SARS-CoV-2 first burst into global consciousness in early 2020.

Was there, perhaps, an upside to such widespread Omicron infection at once?

“The question that everybody raised is … is that really going to lead to immunity — herd immunity, mass immunity — so that we, at least for a while, are safe from other variants?” said researcher Melanie Ott, director of the Gladstone Institute of Virology and a professor of medicine at the University of California San Francisco. 

In hopes of finding an answer, Ott and other researchers exposed lab mice to different variants and found being infected with the previously-dominant Delta variant induced broad immunity against both Delta and Omicron. Catching Omicron, the team found, didn’t have the same effect, only shielding mice from an Omicron reinfection.

A close-up look at the Omicron variant of SARS-CoV-2, with low magnification showing a cell (left) after infection and high magnification (right) of an infected cell featuring viral particles with corona-shaped spikes on their surface. (HKUMed and Electron Microscope Unit/HKU)

Gommerman, who was not affiliated with the study, said the use of mice offered the researchers a blank slate, assuring the subjects didn’t have previous exposure to this virus, but was also a key limitation — more research is needed to confirm the early findings in humans.

The team did also analyze human samples from Omicron and Delta breakthrough cases in vaccinated individuals, and in this case, both variants appeared to offer an immune boost to protect against getting reinfected with the other.

Together, the results indicate that having an Omicron infection “enhances pre-existing immunity elicited by vaccines,” but might not induce broad immunity in unvaccinated individuals, the researchers wrote.

In an interview with CBC News, Ott put it this way: “In the unvaccinated people, I think this response is very narrow and limited, and might not have the anticipated result of broad herd immunity for protection against future variants.”

Vaccines, meanwhile, offer a “fantastic background for immunity to build up if you have a breakthrough infection,” she added.

Gommerman agreed the results signal there’s likely a “wimpy immune response” post-Omicron in people who get infected without previously being vaccinated.

“People who are unvaccinated and got Omicron might think they’re good to go,” she said. “And that’s likely not the case based on this preprint.”

WATCH | COVID-19 deaths, hospitalizations hit record highs:

COVID-19 deaths, hospitalizations hit record highs in Canada

6 days ago

Duration 3:43

Across Canada, the number of COVID-19 deaths and hospitalizations have climbed to record highs. But it appears the Omicron wave is waning with fewer overall infections, giving some experts optimism for the coming months. 3:43

Immunity like building up an army

The apparent differences in how our bodies tackle this evolving virus, depending on whether the exposures happen post-vaccination or without that extra protection, aligns with the basic science behind how vaccines work.

When your body first encounters a virus or a vaccine, vaccine researcher Alyson Kelvin explains, your immune system builds up an army to attack different parts of that threat.

“If you see the same virus or vaccine again, then you go back to that same army — and you pick out the strongest members of that army,” she continued. “If you encounter a completely different virus or vaccine, then you will no longer go back and recall that old army. You’ll have to make a new one.”

As this virus keeps evolving, that could mean a need for more fresh armies, building up and broadening our immune system’s defence network one infection at a time.

Kelvin, who works at the University of Saskatchewan’s Vaccine and Infectious Disease Organization, is among those hopeful that through a combination of vaccination and infections, that population-level immunity will continue to rise, while instances of severe disease continue to drop. 

“That’s an optimistic point of view,” she admitted. “But we can always have a brand new, antigenically-different coronavirus that might change all that, and we’re not able to lean back on our pre-existing immunity.”

WATCH | Pfizer testing Omicron-based vaccine:

Pfizer testing Omicron-based vaccine amid concerns about twin variant

9 days ago

Duration 2:02

Pfizer has started testing an Omicron-based COVID-19 vaccine to see if it can prevent infection and not only severe illness. The testing comes as researchers investigate an emerging variant described as Omicron’s twin. 2:02

The weeks and months ahead will provide researchers with a better sense of how robust and long-lasting post-Omicron immunity may be, and how it differs between the vaccinated and unvaccinated.

There’s also ongoing research to adapt existing Pfizer-BioNTech and Moderna vaccines to this variant, while other global teams are exploring universal coronavirus vaccines that could tackle any future variants, or intranasal options which may prevent more mild, breakthrough infections from happening in the first place.

All those efforts offer hope that regardless of how our bodies handle infections, vaccines will continue providing a way out of this pandemic — but that requires people actually getting the shots.

“We got kind of lucky with Omicron in that it causes more mild illness, but there’s no guarantee that a subsequent variant will likewise be mild,” Gommerman said. 

“So for those Canadians who have yet to be vaccinated, if they’ve had Omicron, that shouldn’t stop them; they should still go out and get the vaccine.”

Have questions about this story? We’re answering as many as we can in the comments or email us at ask@cbc.ca.

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