People who have recovered from a COVID-19 infection are likely to be protected from reinfection for at least six months, but according to a study published Wednesday, that protection drops dramatically for people over the age of 65.

The study, which was published in The Lancet, found protection in the general population to be 80 percent or higher in those younger than 65, but approximately 47 percent in those aged 65 years and older, meaning they were more likely to be infected again. 

The study found that among people under age 65 who had COVID-19 during the first wave, between March and May 2020, only 72 out of more than 11,000 tested positive again during the second wave from September to December 2020. 

The infection rate during the second wave was five times higher for people who tested negative during the first wave.

For people over age 65, the numbers were slightly higher, with 17 out of 1,931 testing positive twice. The study found 2 percent of people over age 65 who tested negative in the first wave subsequently tested positive during the second wave.

The authors found no evidence that protection against repeat infection was waning after six months of follow-up, but because COVID-19 was only discovered in December 2019, they said longer-term studies are needed.

However, the study concluded that natural protection particularly among the elderly can’t be depended upon, and vaccination of all populations is the most reliable to ensure protection against COVID-19. 

The authors said those results highlight the importance of measures to protect elderly people during the pandemic, such as enhanced physical distancing and prioritizing them for vaccines, even for those who have recovered from the virus. 

The Centers for Disease Control and Prevention recommends vaccination for people who have previously been infected with COVID because there isn’t clear evidence about how long natural immunity lasts.

Several studies have found the COVID-19 vaccines have much higher levels of neutralizing antibodies than what are present with natural infection, giving vaccinated people a much more durable and higher-quality level of protection.

The Lancet study analyzed patient data collected in Denmark because the county has instituted a successful national testing program. The country has been expanding its free PCR testing program, and by Dec. 31, more than 10 million PCR tests were conducted on 4 million people, which is more than two-thirds of the country’s population.

Recent studies have suggested that reinfections are rare and that immunity can last at least six months, but the Lancet study was the first to examine the question on such a large scale

The study was conducted before the emergence of more contagious variants, and the authors said they also couldn’t determine whether the severity of symptoms played a role in reinfection protection.