What do we know about breakthrough COVID-19 cases? Experts break down the science

Cases among fully vaccinated individuals — dubbed breakthrough infections if they occur at least two weeks following a second dose — are rare and typically milder, experts say, even against the more transmissible delta variant.

Infections among fully vaccinated expected to increase, but usually as milder infections

People wait in line outside Canada Place for their COVID-19 vaccination in Vancouver on June 21. (Ben Nelms/CBC)

As COVID-19 cases rise through parts of the country, experts expect the number of infections among fully vaccinated people will increase with them. But that doesn't mean the vaccines have stopped working.

Cases among fully vaccinated individuals — dubbed breakthrough infections if they occur at least two weeks following a second dose — are rare, experts say, even against the more transmissible delta variant of the virus.

And the chance a fully vaccinated person would get seriously ill or die following a COVID-19 infection is even less likely, they add.

"To date, the vaccines are doing exactly what we would expect them to be doing," said Dr. Isaac Bogoch, an infectious disease physician with the University of Toronto.

"They reduce people's risk of getting the infection, they significantly reduce the risk of people getting very sick and landing in hospital, and there's also good growing data demonstrating that vaccines reduce the degree to which someone is contagious."

So what do we know about breakthrough infections? The Canadian Press asked Bogoch and other health experts to break down the science.

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How often are breakthrough cases happening?

In the U.S., new data published by the Centers for Disease Control and Prevention (CDC) showed around a quarter of coronavirus infections among Los Angeles County residents were among fully vaccinated individuals between May and late July — a period in which the highly contagious delta variant was circulating.

That kind of data hints at what Canada can expect, too.

Data from Public Health Ontario showed known breakthrough infections accounted for less than one per cent of all reported COVID-19 cases in the province from Dec.14, 2020 to Aug. 7, 2021. But as the proportion of vaccinated Canadians grows, so too will the number of vaccinated people exposed to delta. And experts say we'll likely see more breakthrough cases.

Those without shots are still significantly more vulnerable, though. Ontario public health data found unvaccinated individuals were about eight times more likely to contract COVID-19 in the past 30 days.

Recent cases in British Columbia showed a 10-times higher rate of infection among unvaccinated people and a 17-times higher hospitalization rate.

A health worker administers a dose of the Pfizer-BioNTech vaccine at a clinic. Several experts who spoke to CBC News maintain that vaccinated individuals are helping to curb transmission to those not yet protected. (AFP/Getty Images)

Dr. Sumon Chakrabarti, an infectious disease physician in Mississauga, Ont., said that while delta is a problem, the vaccines are still offering excellent protection.

"There's just so much more of delta right now, so we're seeing people who are getting COVID after they have the vaccine — but they're almost uniformly getting mild disease," he said.

"As time goes on we'll see more (breakthroughs) because we're looking for them ... but you have to compare it to the vast number of fully vaccinated people being exposed to COVID and not (catching it)."

A U.K. study from July suggested vaccine effectiveness dipped against delta compared to the alpha variant, offering between 67 and 88 per cent protection against infection. But effectiveness against death and severe disease has remained high.

Who is more likely to suffer serious breakthrough infections?

Though it doesn't happen often, some fully vaccinated people have either required hospitalization or intensive care or died following a breakthrough infection.

According to the CDC's latest study on infections in Los Angeles County, only 3.2 per cent of fully vaccinated individuals who were infected with the virus were hospitalized, just 0.5 per cent were admitted to an intensive care unit and 0.25 per cent were placed on a ventilator.

Bogoch said emerging worldwide data suggests those who suffer serious breakthrough outcomes are likely to have other risk factors for severe disease.

"This is usually older or frail adults or immunocompromised individuals," he said. "And these are people that won't mount the same degree of an immune response to vaccination compared to younger cohorts."

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Since those segments of the population were also among the first prioritized for vaccination when Canada's rollout began in December, some experts say waning immunity may be at play for certain groups.

A pre-print study from immunologists at McMaster University suggests those in long-term care could soon need a booster shot to amplify their protection.

"In the general population, it is not believed that we have reached that waning immunity," said Dawn Bowdish, a co-author of the pre-print study, which is currently under review.

"In long-term care and in the vulnerable, yes, they're reaching waning immunity, but they never have immune responses that last as long."

Chakrabarti said most studies on immune longevity look at antibody levels over time. But while antibodies decrease, T-cell responses linger much longer to continue to help fight off severe infection.

"Antibodies are like a brick wall. They're strong but with enough force you can knock them down," he said. "But the kind of long-term immunity you have with your T-cells, that's like a concrete wall. That's not something that easily drops."

Can breakthrough infections lead to further transmission?

Recent data from the United Kingdom showed that some fully vaccinated COVID-19 patients had similar viral loads to unvaccinated people who contracted the virus.

While that would seem to suggest vaccinated people are just as contagious, experts say that's not the case.

Bowdish said further studies have indicated viral load drops much quicker in fully vaccinated people compared to those unvaccinated: "So you might have one day of being infectious versus five," she explained.

People line up outside Canada Place for their COVID-19 vaccination in Vancouver in late June. Data shows 'breakthrough infections' after full vaccination aren't common, and experts say that means vaccines are doing their job. (Ben Nelms/CBC)

"Right now we're seeing delta is just as contagious as chicken pox in unvaccinated people," Bowdish said. "In vaccinated people, it's probably closer to influenza ... so a fully vaccinated person, there's still potential to transmit, but it will be a lot less."

Bowdish added that the amount of viral load someone carries around doesn't necessarily translate to how infectious the person is.

She pointed out that studies during the first wave, before delta emerged, suggested children carried high loads of virus but weren't as contagious as adults. She said the presence of symptoms and the behaviour of the host — are they sneezing and surrounded by people? — impact transmission more than viral load itself.

Chakrabarti added viral load doesn't always indicate the presence of live virus, and since vaccinated people likely won't have symptoms, they also likely won't become super-spreaders.

How do we stop breakthrough cases?

Some experts believe the COVID-19 virus will become endemic, with small, seasonal waves continuing to pass through predominantly unprotected populations.

That means it will be hard to stop breakthrough infections entirely.

Bogoch said the best way to halt spread among both vaccinated and unvaccinated groups is to continue with added layers of protection, including mask-wearing and limits on indoor gatherings, to "stop infections in the community."

"Right now, we have to vaccinate, plus have other mitigation efforts in place simultaneously," he said. "We'll get out of this pandemic. We're just not there yet."

With files from Reuters

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