Researchers found that people who got sick with any pre-omicron version of the coronavirus had substantially reduced protection.
For at least 10 months after a COVID-19 infection, your immune system can provide good protection against symptomatic illness the next time around, a new study found, and the risk of severe illness is even lower.
The researchers, from the Institute for Health Metrics and Evaluation COVID-19 Forecasting Team, said their study is the largest review yet of available data on the subject. The study, published Thursday in The Lancet, is a meta-analysis that looks at 65 studies from 19 countries.
Ten-month immunity after COVID-19 infection seems strong against all variants of the coronavirus, the study found.
"There's quite a long sustained protection against severe disease and death, almost 90% at 10 months. It is much better than I had expected, and that's a good thing for the world, right? Given that most of the world has had omicron," said Dr. Chris Murray, director of the Institute for Health Metrics and Evaluation at the University of Washington. "It means there's an awful lot of immunity out there."
The US is still seeing an average of more than 40,000 new COVID-19 cases per day, according to the US Centers for Disease Control and Prevention.
Unlike with other viral infections like measles or chickenpox, protection from COVID-19 does not last forever, so even people who have this so-called natural immunity will probably still need an annual COVID-19 booster, Murray said. But with natural immunity lasting as long as it does, people don't probably need to get a shot any more often than once a year, he said.
The researchers found that people who got sick with any pre-omicron version of the coronavirus, though, had substantially reduced protection against infection from the BA.1 subvariant of omicron: only 36% at 10 months after infection.
Generally, protection from infection seems to be at least equal to two doses of mRNA vaccine, at least for the variants through BA.1. The study didn't include an analysis of protection against XBB and the variants that followed. XBB.1.5 is causing about 75% of current cases, according to the CDC.
More research will be needed to find out about those later variants, but Murray believes that protection should be about the same.
However, he's careful to note that the findings don't mean people should skip a vaccine or booster if it's recommended.
"The safest way to get immunity is vaccination," Murray said.
As of February 8, only about 69% of the total US population has gotten their primary series of vaccines. Only about 16% has gotten a dose of the updated booster, the CDC says.
Natural immunity is protective, but no doctor is going to recommend having a COVID party to catch it on purpose. Without vaccination, a first infection carries a substantial risk of severe illness or death, particularly for people with underlying health conditions.
There's also the risk of having symptoms that last a long time, called long COVID. Research has found that a more severe illness means a higher chance of long COVID.
"We never know what we're setting people up for by going out and getting a disease," said Dr. Claudia Hoyen, an infectious disease specialist and director of pediatric infection control at UH Rainbow Babies and Children's Hospital in Cleveland, who was not involved in the new study. "It's always safer to get vaccinated than get sick."
The higher a level of overall immunity in a population, she said, the less likely it is that an even more dangerous variant could develop.
Hoyen noted that COVID-19 isn't going away and said the new study makes a good argument for continuing to monitor for future variants.
The study had some limitations, including that there wasn't much research on natural immunity in Africa to include in the analysis. Some of the information about infection status and hospital admissions was also incomplete. And there were only a small number of studies that looked at BA.1 and its sublineages, so there is a need to continue monitoring and analyzing variants.
"What we all worry about is a new variant that has immune escape like omicron had on the prior variants," Murray said. "People like to hope the evolution of the virus will lead to less severe forms, but that's not a guarantee. So we just need to stay vigilant for the possibility of a different variant."
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