Older people are more at risk of catching Covid-19 again after recovering from a previous bout of the virus, new research shows.

While most people are unlikely to get the disease again for at least six months, the elderly are more prone to reinfection, according to the results published on March 17 in the Lancet. The study of test results in Denmark last year showed that those under age 65 who’d had Covid were about 80% protected from getting it again. Protection dropped to 47% for those 65 and older.

The data suggest that those who have had the virus should still be vaccinated, the authors said. Natural protection can’t be relied upon, particularly for the elderly, who are most at risk of severe disease. The Denmark analysis focused on the original coronavirus strain and made no assessment of new variants thought to be more transmissible.

“Our findings make clear how important it is to implement policies to protect the elderly during the pandemic,” even if they have already had Covid-19, Steen Ethelberg, a senior researcher from the Statens Serum Institut in Copenhagen, said in a statement. “Our insights could also inform policies focused on wider vaccination strategies and the easing of lockdown restrictions.”

The authors of the study analyzed data collected as part of Denmark’s national SARS-CoV-2 testing strategy. More than two-thirds of the population, or about 4 million people, were tested over a period spanning the country’s first and second waves. The analysis found that only 0.65% of people returned a positive PCR test during both waves. A higher proportion -- 3.3% -- got a positive result after a previous negative one.

In line with findings from other studies, there was no evidence that protection against reinfection waned within six months, the authors said. However, since the virus was only identified in December 2019, the complete period of protective immunity conferred by infection has still to be determined. It also wasn’t possible to assess whether the severity of symptoms affected a patient’s degree of immunity, they wrote.

“These data are all confirmation, if it were needed, that for SARS-CoV-2 the hope of protective immunity through natural infections might not be within our reach and a global vaccination program with high-efficacy vaccines is the enduring solution,” professors Rosemary Boyton and Daniel Altmann from Imperial College London wrote in a comment linked to the study.