The vast majority of people who recover from Covid-19 remain protected from the virus for at least six months, researchers reported on Wednesday in a large study in Denmark.
Previous coronavirus infection reduced the chances of a second attack by about 80% in people under 65, but only by about half in people over 65. But these results, published in the Lancet magazine, have been moderated by many caveats.
The number of elderly people infected in the study was small. The researchers had no information other than the results of the test, so it is possible that only people who were slightly ill the first time became infected again and that the second infection had no symptoms.
Scientists said reinfections are likely to be asymptomatic or mild because the immune system suppresses the virus before it can do much damage. The researchers also did not evaluate the possibility of reinfection with new variants of the virus.
Still, the study suggests that immunity to a natural infection is unpredictable and uneven, and underscores the importance of vaccinating everyone – especially the elderly, experts said.
“You certainly cannot rely on a past infection to protect you from getting sick again, and possibly very sick if you are in the elderly segment,” said Steen Ethelberg, an epidemiologist at the Statens Serum Institut, a public health agency in Denmark.
Because people over 65 are at the greatest risk of serious illness and death, he said, “they are the ones we are most eager to protect.”
Strict estimates of second infections are often rare because many people around the world did not initially have access to the tests, and laboratories require genetic sequences from both rounds of testing to confirm a reinfection.
But the findings are consistent with those of experiments in a wide variety of environments: sailors on a fishing trawler in Seattle, Marine Corps recruits in South Carolina, health professionals in Britain and patients in clinics in the United States .
The design and size of the new study benefited from Denmark’s abundant and free testing for coronavirus. Nearly 70 percent of the country’s population was tested for the virus in 2020.
The researchers analyzed the results of 11,068 people who tested positive for coronavirus during the first wave in Denmark between March and May 2020. During the second wave, from September to December, 72 of these people, or 0.65 percent, again tested positive, compared to 3.27% of people who were infected for the first time.
This translates to 80% protection against the virus for those who have already been infected. Protection dropped to 47 percent for those over 65. The team also analyzed the test results of nearly 2.5 million people during the epidemic, some more than seven months after the first infection, and found similar results.
“It was great to see that there was no difference in protection against reinfection over time,” said Marion Pepper, an immunologist at the University of Washington in Seattle.
She and other experts noted that while 80% does not look excellent, protection against symptomatic diseases is likely to be greater. The analysis included anyone who was tested, regardless of symptoms.
“Many of these infections will be asymptomatic, and many of them are likely to be people with a virus patch,” noted Florian Krammer, an immunologist at the Icahn School of Medicine at Mount Sinai in New York. “The eighty percent reduction in the risk against asymptomatic infections is great.”
The findings indicate that people who have recovered from Covid-19 should receive at least one dose of a coronavirus vaccine to increase their level of protection, added Dr. Krammer. Most people produce a robust immune response to a natural infection, “but there is a lot of variability,” he said. After vaccination, “we don’t see variability – we see very high responses in basically everyone, with very few exceptions”.
Experts were less convinced by the results in people over 65, saying the findings would have been more robust if the analysis had included more people in that age group.
“I wish it had really been broken down into specific decades over the age of 65,” said Pepper. “It would be nice to know if the majority of people who have been reinfected are over 80 years old.”
The immune system becomes progressively weaker with age, and people over 80 usually have weak responses to infection with a virus. The lower protection in the elderly seen in the study is consistent with these observations, said Akiko Iwasaki, an immunologist at Yale University.
“I think we tend to forget how amazing vaccines have been in providing protection in this age group, because you can see that natural infection does not provide the same type of protection,” she said. “It really emphasizes the need to cover the elderly with the vaccine, even if they took Covid first.”