Contracting COVID-19 may provide some immunity. But still get vaccinated, scientists say

Some new studies that look at natural immunity — the protection against COVID-19 that a person develops after being infected — have triggered discussion within the scientific community about the strength of such immunity. However, scientists tell CBC News it is no substitute for vaccination.

Canadian federal government doesn't count previous infection as part of vaccination status

Hand holds a vaccine and needle.
The level of so-called natural immunity from COVID-19 that results from a prior infection has become a topic of some debate in the scientific community recently. (John Woods/The Canadian Press)

When it comes to acquired immunity against COVID-19, also known as natural immunity, scientists agree that people looking for protection against the coronavirus certainly shouldn't be running out to get intentionally infected.

Yet the emergence of more studies — some that suggest prior infection can provide significant protection and others that show vaccination is much more effective — has triggered discussion within the scientific community about the strength of natural immunity.

Many scientists say vaccination is still essential for those who have contracted COVID-19, and that the combination of previous infection and vaccination may actually offer the best level of protection.

Monica Gandhi, a professor of medicine and infectious diseases at the University of California, San Francisco's School of Medicine, is among the experts who believe one dose of vaccine after prior infection offers the best protection. She says the extent of immunity after infection is a very legitimate scientific debate.

"And the problem with this current debate," she said, "is that to ignore natural immunity and say it isn't a thing is leading to a lot of distrust of public health officials."

'Strongly disagree' natural immunity better than vaccination

What goes beyond the bounds of legitimate debate, say many scientists, is the idea being suggested by some other scientists that acquired immunity from infection should be considered as effective or better than vaccination.

"I strongly disagree with that assessment," said Theodora Hatziioannou, a virologist at the Rockerfeller University in New York City.

Acquired immunity is the immunity that one's body develops to a disease after being infected. In Canada and the U.S., a previous infection is not counted as part of an individual's vaccination status. A person who has had COVID-19 still requires two doses of an approved vaccine to be considered fully vaccinated.

But citizens in many European countries who have had the illness and received a single dose of vaccine are considered fully vaccinated. And in Israel, a person who has recovered from COVID-19 is considered fully vaccinated without having received a shot of vaccine.

In Canada and the U.S., a previous COVID-19 infection is not counted as part of an individual's vaccination status. But citizens in many European countries who have had the illness and received a single dose of vaccine are considered fully vaccinated. (Evan Mitsui/CBC)

From a purely medical perspective, if someone has had a prior infection, they of course should be able to mount an immune response that can protect them for a certain amount of time, said Matthew Miller, an associate professor in the Michael G. DeGroote Institute for Infectious Diseases at McMaster University in Hamilton.

However, most of the studies that have compared the immunity resulting from infection with that of vaccination have found that two doses of vaccine, especially mRNA vaccines, provide higher levels of antibodies than a prior infection, he said.

Dawn Bowdish, Canada Research Chair in Aging and Immunity and a professor at McMaster University, said she's been working with people who were hospitalized with COVID-19 and found they "tend to have pretty robust immune responses because they had quite a bit of time with the virus."

She said immunity from previous infection may be enough for "some of the people, some of the time," but it's "quite proportionate to how sick you got, and there's a lot of variability in people who had low-level infections."

'The durability of the response'

For example, Bowdish recently had someone give blood who had previously been infected with COVID-19 but only with very mild symptoms.

"We struggled to find any evidence that she had any immunity whatsoever," Bowdish said of the test results.

When asked about natural immunity on CNN last month, Dr. Anthony Fauci, the top infectious disease expert in the U.S, said he couldn't provide a firm answer on the subject. 

"That's something that we're going to have to discuss regarding the durability of the response," said Fauci, director of the National Institute of Allergy and Infectious Diseases (NIAID).

It's an issue many people opposed to vaccines have seized upon, particularly the comments of some scientists who have gone so far as to advocate natural immunity as equal to or perhaps better than double-dose vaccination.

The problem with that, says Hatziioannou, and what many scientists will point out, is that the level of natural immunity is quite varied between different people, and that protection varies depending on the severity of their prior illness.

As public officials continue to encourage Canadians to get vaccinated against COVID-19, plans are underway in many jurisdictions to expand the rollout of booster shots. (Jean-Claude Taliana/CBC/Radio-Canada)

Based on her own data, she estimates very few of those previously infected with COVID-19, around 10 per cent, mount a "really significantly high neutralizing antibody response." 

The rest, she said, develop a medium to low response, with the majority pretty low.

"It appears the more sick you are, the higher the levels of your antibodies, generally speaking. But overall, the majority of infections are either asymptomatic or very, very mild to moderate. So I would not expect the majority of these people will have really high neutralizing antibodies."

Impact on variants

As well, the particular coronavirus variant that infected the individual will inevitably have some degree of impact on how well it protects them from infection with a different variant, Miller said, which adds another layer of complexity to the issue.

"I think that the scientific issue of whether symptomatic infection can protect you from a future infection — I think that's clear," he said. "Is it clear exactly how well and to what extent and for what period of time? No, it's not."

Other scientists, meanwhile, suggest the case is pretty clear that natural immunity may provide more protection than previously thought.

Matthew Memoli, director of NIAID's Laboratory of Infectious Diseases Clinical Studies Unit, told the BMJ, a U.K.-based peer-reviewed medical journal, that there probably isn't much difference between natural immunity and vaccination in terms of resistance to the spike protein — a crucial feature on the surface of the coronavirus that allows it to gain access to our cells.

Vaccines, he said, are focused only on that tiny portion of immunity that can be induced by neutralizing the spike, while someone who has had COVID-19 was exposed to the whole virus, "which would likely offer a broader based immunity" that would be more protective against variants.

Experts make case for natural immunity

Jeffrey D. Klausner, a professor of population and public health sciences at the University of Southern California published a study that concluded there is "consistent epidemiological evidence" that prior infection provides "substantial immunity" to repeat infection and provides similar protection when compared to vaccination.

Marty Makary, a professor at the Johns Hopkins School of Medicine and Bloomberg School of Public Health in Baltimore, Md., has been very vocal in making his case that policy-makers need to consider natural immunity as equal to or better than vaccination.

In a Washington Post column last month, Makary, a surgical oncologist, wrote that for far too long, public health officials have dismissed natural immunity as unreliable protection against COVID-19 — "a contention that is being rapidly debunked by science."

Makary pointed to some recent studies, including one in Israel that found people who were double vaccinated were six times more likely to get infected with the delta variant compared to those who had been previously infected with COVID-19 but not vaccinated.

But Fauci, during his appearance on CNN last month, said the Israeli study did not address the durability of immunity from infection compared to that which results from vaccination. 

"So you may be protected, but you may not be protected for an indefinite period of time," he said.

Meanwhile, other studies are suggesting limits to natural immunity. A study from the U.S. Centers for Disease Control and Prevention published last month found unvaccinated people previously infected with COVID-19 were twice as likely to be reinfected than those who were fully vaccinated after initially contracting the virus.

When asked about natural immunity on CNN last month, Dr. Anthony Fauci, the top infectious disease expert in the U.S, said one of the relatively unknown factors is how long it lasts. (Jeenah Moon/Getty Images)

Still, the efficacy of natural immunity could have potential policy implications, particularly in countries where vaccines are in short supply. 

 And researchers are finding that the combination of prior COVID-19 infection and vaccination, so-called hybrid immunity, may offer the best protection. 

Bowdish said McMaster University is currently conducting a study in long-term care with 60 COVID-19 survivors. 

"And we definitely found that after they got their vaccine, they seem to be the ones that are having really robust, long immune responses," she said.

Hatziioannou agreed that the research suggests people who were previously infected, even if their initial immune responses were not great, that once they get vaccinated, even with just with a single dose, their immunity "became remarkable."

"It really is really great immunity," she said. "It makes no sense to say that immunity from infections is sufficient."


  • A previous version of this story stated Canada does not count acquired immunity through previous infection to COVID-19 as part of an individual's vaccination status. The story has been updated to clarify that while the federal government requires a person to have received two doses of an approved vaccine to be considered fully vaccinated, in Quebec, a single dose and previous infection is considered fully vaccinated in certain circumstances.
    Nov 09, 2021 10:43 AM ET


Mark Gollom

Senior Reporter

Mark Gollom is a Toronto-based reporter with CBC News. He covers Canadian and U.S. politics and current affairs.

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