The vast majority of people who recover from Covid-19 will remain protected from the virus for at least six months, researchers reported Wednesday in a large study from Denmark.

Previous coronavirus infection reduced the likelihood of a second fight for people under 65 years of age by about 80 percent, but only about half for people over 65. However, these results, published in the journal Lancet, have been tempered by many reservations.

The number of infected elderly people in the study was low. The researchers had no information beyond the test results, so it’s possible that only people who were mildly ill the first time were re-infected and the second infections were largely symptom-free.

Scientists have 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 re-infection with newer variants of the virus.

Still, the study suggests that immunity to natural infection is unpredictable and uneven, and it underscores the importance of vaccinating everyone – especially the elderly, according to experts.

“You certainly cannot rely on a previous infection to protect you from disease again and possibly be quite ill if you are in the elderly area,” said Steen Ethelberg, epidemiologist at Statens Serum Institute, Denmark’s public health department.

Because people over 65 are at the highest risk of serious illness and death, he said, “They are the ones we are most likely to want to protect.”

Rigorous estimates of secondary infections have generally been rare because many people around the world initially did not have access to testing and laboratories need genetic sequences from both rounds of testing to confirm re-infection.

However, the results are consistent with those from experiments in a variety of settings: sailors on a fishing trawler in Seattle, Marine Corps recruits in South Carolina, healthcare workers in the UK, and patients in clinics in the US.

The design and size of the new study benefited from Denmark’s free and extensive tests for the coronavirus. Almost 70 percent of the country’s population was tested for the virus in 2020.

Updated

March 18, 2021, 12:25 p.m. ET

The researchers examined 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 these people, or 0.65 percent, tested positive again. compared to 3.27 percent of people who were infected for the first time.

This means 80 percent protection from the virus in those who were previously infected. Protection fell to 47 percent for those over 65. The team also analyzed the test results of nearly 2.5 million people during the epidemic, some longer than seven months after the initial infection, and found similar results.

“It was really nice to see that there was no difference in protection against re-infection over time,” said Marion Pepper, an immunologist at the University of Washington in Seattle.

She and other experts found that 80 percent may not seem great, but protection from symptomatic illness is likely to be higher. The analysis included everyone who was tested, regardless of symptoms.

“Many of these will be asymptomatic infections, and many of them will likely be people who have a virus stain,” noted Florian Krammer, an immunologist at the Icahn School of Medicine on Mount Sinai in New York. “An 80 percent reduction in the risk of asymptomatic infections is great.”

The results show that people who have recovered from Covid-19 should receive at least one dose of a coronavirus vaccine to increase levels of protection, added Dr. Krammer added. Most people produce a robust immune response to natural infection, “but there is great variability,” he said. After vaccination “we don’t see any variability – with very few exceptions we see very high reactions in practically everyone.”

Experts were less convinced of the results in people over 65, saying the results would have been more robust if more people in that age group had been included in the analysis.

“I wish it had actually been broken down into specific decades over 65,” said Dr. Pepper. “It would be nice to know if the majority of the people who were re-infected were over 80 years old.”

The immune system becomes progressively weaker as we age, and people over 80 tend to respond weakly to infection with a virus. The lower levels of protection seen in the elderly in the study are consistent with these observations, said Akiko Iwasaki, an immunologist at Yale University.

“I think we tend to forget that vaccines are amazingly protective in this age group because you can see that natural infections don’t offer the same protection,” she said. “This really highlights the need to provide the elderly with the vaccine, even if they had Covid first.”