If You’re Unvaccinated, COVID Reinfection Is Likely

One of the greatest unknowns about COVID-19 is how long natural immunity can last—and how susceptible people might be to reinfection. But researchers are working to clear up some of the mystery. New data show that, if you're unvaccinated, protection against the virus after infection may not last very long.

Based on Yale University researchers' analysis, there was an estimated 5% risk of reinfection by three months after peak antibody response for people who are unvaccinated. That means you could get reinfected with the virus as early as three months after your last COVID case.

“These estimates are based on endemic conditions with no interventions," Hayley B. Hassler, MS, research associate in the department of biostatistics at Yale University and co-author of the study, tells Verywell. "Our 50% risk of reinfection is by approximately 17 months."

So, if you're unvaccinated, you can expect to get COVID-19 more than once. By five years, there was a 95% risk of reinfection. The October study was published in The Lancet Microbe.

Analyzing Reinfection

According to Jeffrey Townsend, PhD, professor of ecology and evolutionary biology at Yale School of Public Health and the study’s coauthor, looking at COVID-19's family tree gave them insight into reinfection estimates. 

To estimate reinfection, Townsend and Hassler compared SARS-CoV-2’s—the virus that causes COVID—closest relatives: SARS-CoV-1 MERS-CoV, and three other human-infecting coronaviruses.

“Once we know the [evolutionary] tree, we can understand how the traits evolve on that same tree,” Townsend tells Verywell. 

“Whenever we’re trying to compare organisms to each other, we look at their sequence data," Hassler adds. "Within that sequence data, we want to be able to see each and every single one of them [organisms] evolves."

Townsend and Hassler also utilized the data from related COVID-19 species to estimate antibody levels post-infection. They found that antibody levels decreased over time.

Vaccine Protection Is Stronger Than Natural Infection 

Previous research suggests that immunity achieved through vaccination can offer stronger and more long-lasting protection compared to natural infection.

However, both natural immunity and vaccines don’t protect people against viruses over the long term, according to Townsend. While waning is a small factor, it has more to deal with the fact that viruses evolve constantly. 

“It’s not just because your immunity wanes, it’s really because the organism that you’re vaccinated against evolves and becomes a different organism,” Townsend says. 

For COVID-19, the virus evolves quickly at the end of the spike protein. “That protein has a very particular structure that makes it so a few changes can apparently change its structure a lot,” Townsend explains. “For that reason, it can evolve to a stage where our previous immunity doesn’t suffice to target it.” 

For example, because the flu evolves quickly, there is a need for yearly shots. “We get that [flu] vaccine every single year," Townsend says. "The bottom line is that viruses that infect us need to be able to continue to infect us to persist in the human population."

Because of COVID-19’s constant evolution, it makes it more difficult to garner 100% immunity, underscoring the best defense against getting severe illness: vaccines and booster shots.

"The problem is just like the flu, this is a virus that evolves with relative speediness, at least with regard to its interaction with our immune system," Townsend says. "As long as it is still causing significant sickness and illness in individuals, we’re going to need continued boosting."

Further research is still needed to understand antibody generation and the severity of sickness, according to Townsend. He says that larger datasets are needed to understand the degree to which this is happening. 

“Historically, evolutionary biology was considered a historical enterprise where you try to understand what happened a long time ago. But there are a lot of ways where evolutionary biology can be useful to us,” Townsend says. “This paper is an example of it. Evolutionary biology provided us the tools and expertise to get an answer that otherwise we would have to wait years to obtain.” 

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