Findings published in The Lancet on Wednesday drew from over 525,000 people in Denmark available for follow-up after two infection surges; the first occurring in spring 2020 and the second surging from September to December 2020.
Researchers from the Statens Serum Institute in Copenhagen determined reinfection risk through an analysis of PCR test data from the Danish Microbiology Database, and excluded about 600 people who tested positive for the first time and over 7,400 deaths from all causes. The team found that 72 of 11,068 people who tested positive during the spring-time surge went on to test positive again, which translates to a 0.65% reinfection rate.
Further, adults over age 65 saw a 47% “observed protection against repeat infection” compared to nearly 80% protection among younger age groups. Researchers found no difference in reinfection risk between the sexes, nor waning protection after over seven months of follow-up.
“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,” study authors wrote in part.
Given the findings, researchers emphasized the need for vaccination and mitigation measures among older adults.
“Our findings could inform decisions on which groups should be vaccinated and advocate for vaccination of previously infected individuals because natural protection, especially among older people, cannot be relied on,” they wrote.
Researchers still face unknowns, like the duration of antibody-induced natural protection after contracting the virus, though some studies have suggested this could range for about five months or longer.