When researchers analyzed test results of 4 million people in Denmark, they found that less than 1% of those who tested positive experienced reinfection.
Initial infection was associated with about 80% protection overall against getting SARS-CoV-2 again. However, among those older than 65, the protection plummeted to 47%.
“Not everybody is protected against reinfection after a first infection. Older people are at higher risk of catching it again,” co–lead author Daniela Michlmayr, PhD, said in an interview. “Our findings emphasize the importance of policies to protect the elderly and of adhering to infection control measures and restrictions, even if previously infected with COVID-19.”
Verifying the need for vaccination
“The findings also highlight the need to vaccinate people who had COVID-19 before, as natural immunity to infection – especially among the elderly 65 and older – cannot be relied upon,” added Dr. Michlmayr, a researcher in the department of bacteria, parasites, and fungi at the Staten Serums Institut, Copenhagen.
The population-based observational study was published online March 17 in The Lancet.
“The findings make sense, as patients who are immunocompromised or of advanced age may not mount an immune response that is as long-lasting,” David Hirschwerk, MD, said in an interview. “It does underscore the importance of vaccination for people of more advanced age, even if they previously were infected with COVID.
“For those who were infected last spring and have not yet been vaccinated, this helps to support the value of still pursuing the vaccine,” added Dr. Hirschwerk, an infectious disease specialist at Northwell Health in Manhasset, N.Y.
Evidence on reinfection risk was limited prior to this study. “Little is known about protection against SARS-CoV-2 repeat infections, but two studies in the UK have found that immunity could last at least 5 to 6 months after infection,” the authors noted.
Along with co–lead author Christian Holm Hansen, PhD, Dr. Michlmayr and colleagues found that 2.11% of 525,339 individuals tested positive for SARS-CoV-2 during the first surge in Denmark from March to May 2020. Within this group, 0.65% tested positive during a second surge from September to December.
By the end of 2020, more than 10 million people had undergone free polymerase chain reaction testing by the Danish government or through the national TestDenmark program.
“My overall take is that it is great to have such a big dataset looking at this question,” E. John Wherry, PhD, said in an interview. The findings support “what we’ve seen in previous, smaller studies.”
Natural protection against reinfection of approximately 80% “is not as good as the vaccines, but not bad,” added Dr. Wherry, director of the Institute for Immunology at the University of Pennsylvania, Philadelphia.
Age alters immunity?
“Our finding that older people were more likely than younger people to test positive again if they had already tested positive could be explained by natural age-related changes in the immune system of older adults, also referred to as immune senescence,” the authors noted.