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‘Natural immunity’ provides strong protection from COVID reinfection, but vaccination still safer


The study, and others like it, is offering insights into an enduring debate over the risk of SARS-CoV-2 reinfection among the unvaccinated, a debate as politicized as COVID itself

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New research shows that a COVID-19 infection can grant the unvaccinated strong and durable protection from reinfection, or mild or severe COVID — protection that even stacks up to that reported for mRNA vaccines.

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The major caveat: The study ended before Omicron took off, “and we can’t necessarily extrapolate our findings to the situation today,” said co-author Dr. Jessica Ridgway, an infectious diseases specialist at the University of Chicago.

But her team’s study, and others like it, is offering insights into an enduring debate over the risk of SARS-CoV-2 reinfection among the unvaccinated, a debate as politicized as COVID itself.

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Among those opposed to vaccination, a popular trope is that it’s more “natural” to gain immunity from infection, which is one reason why Ridgway is careful that her work is not misinterpreted to imply that people shouldn’t get vaccinated against COVID.

Some have argued that immunity is immunity — that immunity from natural infection contributes to population-wide immunity the same way vaccine-induced immunity does. Others say it’s impossible to predict, especially with emerging variants, who will actually be protected or not. How much immunity do they have, how can you measure it and is it enough? “Vaccination, although imperfect, provides a much more standard level of protection” without risking serious disease, said McMaster University immunologist Dawn Bowdish.

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For their study, Ridgway and colleagues looked at data from more than 100,000 people tested for SARS-CoV-2 in six western U.S. states between 2020 and November 2021. Only the unvaccinated were included. The researchers found a past infection was 85 per cent protective against reinfection, and 88 per cent protective against hospitalization. The researchers didn’t see any waning in the level of protection up to nine months out from the initial infection.

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“Obviously if you have to choose between getting a vaccine versus getting COVID, I would highly recommend going the vaccination route,” Ridgway stressed.

“But I do think it’s an interesting finding.” She was surprised to see similar protection from severe COVID, as well as mild, and by the degree of protection. “I expected to see some level of protection, but personally I was surprised that it was as high as it was.”

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It fits with what others have reported. A large study out of Qatar conducted between February 2020 and March 2022 found natural infection was associated with stronger, and more durable protection against infection, regardless of the variant, than two doses each of Pfizer and Moderna. Vaccine protection against infection waned with time after dose two, but natural immunity “demonstrated hardly any waning in protection for eight months of follow-up after the primary infection,” the researchers wrote.

The Omicron wave led to a massive increase in reinfections in both groups, yet even then, natural infection was associated with stronger protection against reinfection.

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Nonetheless, “vaccination remains the safest and optimal tool” against infection and death, the authors wrote, and other academics writing in the Journal of Medical Ethics argue that the notion that natural infection is somehow superior to vaccine-induced immunity is a “grave mistake and a form of the naturalistic fallacy.”

“It is ‘natural’ to become immune through contracting infections, but it is also natural to die from serious infections.”

While arguing that people with proof of a past infection shouldn’t have to be vaccinated to qualify for a vaccine passport, “it would be prudentially irrational to choose to be infected rather than to have the vaccine, for those who are vulnerable to COVID-19,” the authors wrote. COVID can damage the heart, lungs, liver, kidneys, and lead to long-term consequences like long COVID. One in four people with severe infections suffer impaired cognitive functioning. COVID is being linked with accelerated aging. The Qatar study also compared natural immunity to two doses of vaccines only. The differences with a booster thrown in may be smaller, and research suggests hybrid immunity — infection plus vaccination — offers the strongest protection.

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“When you have an Omicron infection you don’t generate really strong immune responses that protect you from subsequent Omicron in the absence of vaccination,” McMaster’s Bowdish said.

“Omicron needs at least three doses of vaccine, because you have to have sky high levels of antibodies to deal with it. It would be lovely if your third shot gave you zero chance of picking up the infection. But what your third shot does is provide you with a better infection — shorter, less severe, less likely to develop long-term health consequences.”

Most variants now are subvariants of Omicron, and it’s likely that everything coming in the near future is going to be Omicron-like as well, said University of Ottawa molecular virologist Dr. Marc-André Langlois.

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“The good news is that vaccines still protect very well against severe disease,” he said. But the super-transmissible subvariants require high levels of neutralizing antibodies to prevent reinfections, meaning “we will see large numbers of double and triple vaccinated people getting infected,” Langlois said. Hence the urgency to develop next-generation vaccines, including vaccines that target the dominant circulating strain, and nasal vaccines that can trigger high levels of neutralization where the virus enters the body – the upper respiratory tract — to block infection.

The other caveat to the most recent study? “It’s got, inevitably, what we call a survivor bias,” Bowdish said. “Meaning, anyone who died of their first infection is not in the study. It doesn’t account for all the people who didn’t live to be in this study because of their first infection.” Still, it’s important, well-designed and confirms recently published work, said Dr. Jorg Fritz, an immunology professor at McGill University. But Fritz doesn’t see any argument here “that would favour letting the virus run free, aim for ‘natural immunity’ through infection of unvaccinated people,” and the authors themselves certainly aren’t arguing for it.

“SARS-CoV-2 in unvaccinated people can cause severe pathologies and death, especially in the aging and immunocompromised population, often with long term consequences,” Fritz said. “The only way to lower the negative effects is vaccination and not natural immunity.”

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