Research, published in the journal Lancet, has revealed that prior infection with the coronavirus reduced the chances of a second bout by about 80% in people under 65.
In a large study from Denmark, researchers have also revealed that people older than 65 don’t get the same protection.
The new study’s design and size benefited from Denmark’s free and abundant testing for the coronavirus. Nearly 70% of the country’s population was tested for the virus in 2020.
The number of infected older people in the study was small. The researchers did not have any information beyond the test results, so it’s possible that only people who were mildly ill the first time became infected again and that the second infections were largely symptom-free.
Reinfections are likely to be asymptomatic or mild because the immune system will suppress the virus before it can do much damage. The researchers also did not assess the possibility of reinfection with newer variants of the virus.
“You can certainly not rely on a past infection as protecting you from being ill again, and possibly quite ill if you are in the elderly segment,” said Steen Ethelberg, an epidemiologist at Statens Serum Institut, Denmark’s public health agency.
The researchers looked at the results from 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 those people, or 0.65%, again tested positive, compared with 3.27% of people who became infected for the first time.
That translates to a 80% protection from the virus in those who had been infected before. The protection fell to 47% for those over 65. The team also analyzed test results from nearly 2.5 million people throughout the epidemic, some longer than seven months after the first infection, and found similar results.
“It was really nice to see that there was no difference in protection from reinfection over time,” said Marion Pepper, an immunologist at the University of Washington in Seattle.
The findings indicate that people who have recovered from COVID-19 should get at least one dose of a coronavirus vaccine to boost the level of protection.
The immune system grows progressively weaker with age, and people over 80 typically mount weak responses to infection with a virus. The lower protection in older people seen in the study is consistent with those observations, says the research.