Most people who have had Covid-19 are protected from catching it again for at least six months, but those aged 65 and over are more prone to reinfection, new research suggests.
Large-scale assessment of reinfection rates in Denmark in 2020 confirms that only a small proportion of people (0.65%) returned a positive PCR test twice.
However, while prior infection gave those under the age of 65 years around 80% protection against reinfection, for people aged 65 and older it only gave 47% protection, indicating that they are more likely to catch the coronavirus again.
According to the study published in The Lancet, the researchers detected no evidence that protection against reinfection declined within a six-month follow-up period.
Dr Steen Ethelberg, from the Statens Serum Institut, Denmark, said: "Our study confirms what a number of others appeared to suggest: reinfection with Covid-19 is rare in younger, healthy people, but the elderly are at greater risk of catching it again.
"Since older people are also more likely to experience severe disease symptoms, and sadly die, our findings make clear how important it is to implement policies to protect the elderly during the pandemic.
"Given what is at stake, the results emphasise how important it is that people adhere to measures implemented to keep themselves and others safe, even if they have already had Covid-19.
"Our insights could also inform policies focused on wider vaccination strategies and the easing of lockdown restrictions."
The authors of the study analysed data collected as part of Denmark's national Covid-19 testing strategy, through which more than two-thirds of the population (69%, four million people) were tested in 2020.
Free, national PCR testing - open to anyone, regardless of symptoms - is one of the central pillars of Denmark's strategy to control the virus.
Researchers used this data to estimate protection against repeat infection with the original Covid-19 strain.
Among those who had the virus during the first wave between March and May 2020, only 0.65% (72/11,068) tested positive again during the second wave from September to December 2020.
At 3.3% the rate of infection was five times higher among people who returned a positive test during the second wave having previously tested negative.
Of those under the age of 65 who had Covid-19 during the first wave, 0.60% (55/9,137) tested positive again during the second wave.
The rate of infection during the second wave among people in this age group who had previously tested negative was 3.60%.
Researchers say older people were found to be at greater risk of reinfection, with 0.88% of those aged 65 and over who were infected during the first wave testing positive again in the second wave.
Among people 65 or older who had previously not had coronavirus, 2.0% (1,866/93,362) tested positive during the second wave.
Due to their high risk of exposure to the virus, a sub-analysis of healthcare workers was also carried out.
It found that with 1.2% (8/658) of those who had Covid-19 during the first wave becoming reinfected, compared with 6.2% (934/14,946) of those who were negative during the first wave.
Dr Daniela Michlmayr, from the Staten Serum Institut, Denmark, said: "In our study, we did not identify anything to indicate that protection against reinfection declines within six months of having Covid-19.
"The closely related coronaviruses Sars and Mers have both been shown to confer immune protection against reinfection lasting up to three years, but ongoing analysis of Covid-19 is needed to understand its long-term effects on patients' chances of becoming infected again."
The authors acknowledge some limitations to their study, including that clinical information is recorded only if patients are admitted to hospital, so it was not possible to assess whether the severity of Covid-19 symptoms affects patients' protection against reinfection.