Prior infection with SARS-CoV-2 will protect only about half of elderly people from repeat infection, according to the largest study to date on natural immunity.

Overall, however, reinfection is rare and previous infection will protect about 80% of adults younger than 65, the Danish population-based study shows.

“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,” the researchers wrote in the Lancet.

The University of Copenhagen-led observational study estimated the level of protection against reinfection by comparing PCR positivity rates among 525,000 people tested during both the first (March to May) and second (September to December) waves of the pandemic.