Natural immunity to Covid-19 is likely to fade relatively quickly, meaning unvaccinated people who have recovered from the disease are at significant risk of reinfection from the coronavirus, according to research published in The Lancet Microbe.

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Because SARS-CoV-2, the coronavirus that causes Covid-19, began emerging less than two years ago, there's still little real-world evidence on the durability of natural immunity to reinfection.

So to better understand how long natural immunity is likely to last, researchers from Yale School of Public Health and the University of North Carolina at Charlotte, analyzed reinfection and immunological data from various types of coronaviruses including those that cause common colds, Middle East Respiratory Syndrome, and Covid-19.

The researchers then used evolutionary principles to model how likely natural immunity to Covid-19 would fade over time.

Ultimately, they determined that, under conditions where the virus that causes Covid-19 is endemic, reinfection likely would occur between three and 63 months after a person's peak antibody response, with a median timespan of 16 months.

"Reinfection can reasonably happen in three months or less," Jeffrey Townsend, a professor of biostatistics at Yale and co-leader of the study, said. "Therefore, those who have been naturally infected should get vaccinated. Previous infection alone can offer very little long-term protection against subsequent infections."

"Just like common colds, from one year to the next you may get reinfected with the same virus," Townsend added. "The difference is that, during its emergence in this pandemic, Covid-19 has proven to be much more deadly."

"We tend to think about immunity as being immune or not immune. Our study cautions that we instead should be more focused on the risk of reinfection through time," Alex Dornburg, assistant professor of bioinformatics and genomics at UNC Charlotte and co-leader of the study, said. "As new variants arise, previous immune responses become less effective at combating the virus. Those who were naturally infected early in the pandemic are increasingly likely to become reinfected in the near future."

Dornburg added that, as a result of the coronavirus's ability "to evolve and reinfect, it, too, is likely to transition from pandemic to an endemic disease." (Bean, Becker's Hospital Review, 10/6; UNC Charlotte press release, 10/1)