The vast majority of people who recover from Covid-19 remain protected from the virus for at least six months, researchers reported Wednesday in a large study from Denmark.
Previous coronavirus infection reduced the chance of a second attack by about 80 percent in people younger than 65, but only by about half in those older than 65. But the results, published in the journal Lancet, were by many reservations tempered.
The number of infected older people in the study was small. The researchers have no more information than the test results, so it is possible that only people who were mildly ill the first time became infected again and that the second infections were largely symptom-free.
Scientists have said that reinfections are likely to be asymptomatic or mild because the immune system will suppress the virus before it can cause much damage. The researchers also did not assess the possibility of re-infection with newer variants of the virus.
However, the study suggests that immunity to a natural infection is unpredictable and unequal, and it emphasizes the importance of vaccinating everyone – especially older people, experts said.
“You can certainly not rely on an infection in the past to get sick again, and possibly quite ill if you are in the elderly segment,” said Steen Ethelberg, an epidemiologist at Statens Serum Institut, Denmark for public health, said.
Because people over the age of 65 are most at risk for serious illnesses and deaths, he said: “these are the ones we most want to protect.”
Solid estimates of second infections were generally rare because many people worldwide initially did not have access to testing, and laboratories require genetic sequences from both tests to confirm a re-infection.
But the findings are consistent with experiments in a wide variety of settings: sailors on a fishing trawler in Seattle, Marine Corps recruit in South Carolina, health care workers in Britain and patients at clinics in the United States.
The design and size of the new study benefited from Denmark’s free and abundant testing of the coronavirus. Nearly 70 percent of the country’s population was tested for the virus in 2020.
The researchers looked at the results of 11,068 people who tested positive for the coronavirus during the first wave in Denmark between March and May 2020. During the second wave, from September to December, 72 of the people, or 0.65 percent, tested positive again, compared to 3.27 percent of people who became infected for the first time.
This equates to 80 percent protection against the virus in those who have been infected before. Protection dropped to 47 percent for those older than 65. The team also analyzed the test results of nearly 2.5 million people during the entire epidemic, some longer than seven months after the first infection, and found similar results.
“It was really nice to see that over time, there was no difference in protection against reinfection,” said Marion Pepper, an immunologist at the University of Washington in Seattle.
She and other experts noted that the protection against symptomatic diseases is likely to be higher than 80 percent probably does not look great. The analysis included everyone who was tested, regardless of the symptoms.
“A lot of these will be asymptomatic infections, and a lot of them will probably be people who have a bit of a virus,” noted Florian Krammer, an immunologist at the Icahn School of Medicine in Mount Sinai, New York. “The eighty percent reduction against asymptomatic infection is tremendous.”
The findings indicate that people who have recovered from Covid-19 should receive at least one dose of coronavirus vaccine to increase the level of protection, said Dr. Staple added. Most people produce robust immune responses to a natural infection, “but there is a lot of volatility,” he said. After vaccination, “we do not see volatility – we see very high responses in everyone, with very few exceptions.”
Experts were less convinced of the results in people older than 65 and said the findings would have been more robust if the analysis had included more people from the age group.
“I wish it was divided into 65 decades,” said Dr. Pepper said. “It would be nice to know if the majority of the people who are re-infected are older than 80.”
The immune system gradually weakens with age, and people older than 80 usually show poor responses to infection with a virus. The lower protection in older people seen in the study is consistent with the observations, says Akiko Iwasaki, an immunologist at Yale University.
“I think we tend to forget how the vaccines were pretty amazing at providing protection in this age group, because you can see that natural infection does not offer the same protection,” she said. “It really underscores the need to cover older people with the vaccine, even if they first had Covid.”