At the beginning of the pandemic, some jurisdictions, including those in the United Kingdom and Sweden, pursued a policy of herd immunity, which assumed that allowing enough people to catch the virus would confer enough immunity on the population to end the pandemic.
While there have been some reports of people developing COVID-19 more than once, these numbers are too small to carry out an epidemiological study. This means that it is difficult to determine how long immunity conferred from a SARS-CoV-2 infection lasts.
In the present study, researchers have conducted an analysis of previously published data on viruses similar to SARS-CoV-2. They set out to determine how long immunity following COVID-19 might last.
The research, which appears in The Lancet Microbe, shows that unvaccinated people can expect immunity against reinfection to last 3–61 months after developing COVID-19 — if the virus is still circulating in the community.
This study could help encourage people to get vaccinated, said Dr. Ajay Sethi, associate professor of population health sciences at the University of Wisconsin-Madison, who was not involved in the research. He told Medical News Today,
“The study underscores the importance for the public to understand that immunity from natural infection is not as long lasting as some may perceive, and certainly not lifelong.”
He also explained that “research has shown that vaccination following natural infection produces an even more robust immune response as compared [with] vaccination without any prior history of COVID-19. Hopefully, more people who’ve had infection in the past will choose to get vaccinated.”
A team from Yale School of Public Health in New Haven, CT, and the University of North Carolina at Charlotte looked at the genes of 177 coronaviruses known to affect humans. The researchers then determined which were the closest viral relatives of SARS-CoV-2.
They identified five viruses that met this criterion. They included SARS-CoV, responsible for the SARS outbreak in 2003, and MERS-CoV, which was first detected in 2012. They also included viruses that cause the common cold.
The researchers then analyzed existing data on how antibody levels decline over time — from 128 days to 28 years after infection. They also looked at the risk of reinfection at different antibody levels for those viruses.
Using this information, they predicted that natural immunity conferred by contracting SARS-CoV-2 would likely last less than half as long as the immunity due to contracting related coronaviruses.
They found that reinfection with SARS-CoV-2 in people who had not received a vaccine could occur as soon as 3 months after initial infection, with a median risk of reinfection within 16 months, under endemic conditions.
Co-leader of the study Alex Dornburg, assistant professor of bioinformatics and genomics at the University of North Carolina, says:
“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.”
The study authors conclude that as novel variants could evade existing immunity, it is important to focus efforts on accelerating global vaccine programs to reduce the likelihood of variants emerging in the first place.
It is of note that the study had certain limitations. For instance, it did not look at how severity of initial SARS-CoV-2 infection or immune status of an individual might affect how long their natural immunity would last.
Speaking on this point, Dr. Alexander Edwards, associate professor in biomedical technology at the University of Reading in the U.K., who was not involved in the study, told MNT:
“Reinfection isn’t really the critical problem for COVID-19. Instead, the most important point is severity of infection. If previous infection protects from serious disease developing, it becomes less important to the individual if they get infected a second time.”
“However,” he continued, “for COVID-19, we still don’t know if previous infection will fully protect from severe disease and death for everyone.”