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.

"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."

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.

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.