Most people who have had coronavirus are protected from catching it again for at least six months, new research has suggested.
However, the news is less positive for those aged 65 and over who are more prone to reinfection.
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 COVID-19 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 new study analysed data collected as part of Denmark’s national testing strategy, through which more than two-thirds of the population (69%, four million people) were tested in 2020.
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 or older 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.
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.