COVID-19 reinfections rare, but older adults are more at risk, large study suggests

A study in Denmark has found that people who were infected during the first surge of COVID-19 were unlikely to be reinfected during the second surge, but the risk of reinfection was higher for people older than 65.

The study, published Wednesday in The Lancet, "reinforces the notion that everybody should get the vaccine," said lead researcher Steen Ethelberg, a senior researcher in the epidemiology of zoonotic infections at the University of Copenhagen. It also "suggests you should be a bit careful if you had the disease, especially if you are a senior citizen.”

Ethelberg and his colleagues found that out of nearly 17,000 people who were infected during the first wave of the coronavirus pandemic from March to May, only 72 people (about 0.7%) were reinfected during the second surge from September to December.

They estimated that natural infection offers roughly 80% protection against a second infection -- but for adults over 65, natural infection offered an estimated 47% protection against a second infection. Older people may have weaker protection because their immune systems don't mount as robust of a response when infected naturally, experts said.

That means vaccination likely offers better protection than natural infection, especially for older adults. The Pfizer vaccine, for example, recently showed 97% protection against symptomatic infection and 94% protection against asymptomatic infection across all age groups studied in a real-world analysis.

“A key finding of the work seems to be that this protective effect was weaker in older adults, where new infection rates were only cut in half by having had a previous infection. But we’ve seen that the vaccines provide much better protection than this, even in older adults,” said David Benkeser, an assistant professor of biostatistics and bioinformatics at Emory University's Rollins School of Public Health.

“It’s absolutely imperative that people age 65 and older get vaccinated, even if they have had previous COVID infection," said Dr. William Schaffner, professor of preventive medicine and infectious diseases at the Vanderbilt University Medical Center, who was not involved in the research.

Experts caution that reinfection isn't necessarily something to be alarmed about. A second infection will probably be way less dangerous than the first. One of the key limitations of this study is that it did not look into what happened to the people who were reinfected, such as whether they got sick or if they felt fine. Also, reinfection will depend on the amount of active virus is spreading at the time, so the ability to generalize the protection to all people who have recovered from COVID-19 is not possible based on this study.

“This study highlights that natural immunity doesn’t provide full protection against reinfection, and it highlights the importance of how everybody should be getting the vaccine regardless of prior infection history,” said Dr. John Brownstein, chief innovation officer at Boston Children's Hospital and an ABC News contributor.

It also reinforces the idea that mass vaccination is the best way to stop the virus in its tracks.

“The study supports the need to continue aggressive rollout of vaccines and hopefully lays to rest any thought that achieving herd immunity through natural infection as a viable public health strategy,” Benkeser said.

“Population immunity may be able to help in the short run, but it’s not going to be as robust or as durable as vaccine-induced immunity," said Dr. Amesh Adalja, senior scholar at the Johns Hopkins University Center for Health Security

Scientists still don't know exactly how long protection lasts after an initial infection, but it's likely a person has strong protection within the first few months that tapers off gradually over time. Researchers only looked out to six months; protection is likely durable after that, but we don't know for how long.

Samuel Rothman, MD, is a psychiatry resident at the BronxCare Health System in New York City and a contributor to ABC’s Medical Unit. Sony Salzman is the coordinating producer of the Medical Unit.