Copenhagen: Most people who had Covid-19 are protected from catching it again for at least six months, but elderly patients are more prone to reinfection, according to a research published in The Lancet.
The large-scale assessment of reinfection rates in Denmark in 2020 confirms that only a small proportion of people (0.65 per cent) returned a positive PCR test twice.
However, while prior infection gave those under the age of 65 years around 80 per cent protection against reinfection, for people aged 65 and above it conferred only 47 per cent protection, indicating that they are more likely to catch Covid-19 again.
"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," said researcher Steen Ethelberg from the Statens Serum Institut, Denmark.
For the study, the researchers analysed data collected as part of Denmark's national Covid-19 testing strategy, through which more than two-thirds of the population (69 per cent, 4 million people) were tested in 2020.
Researchers used this data, spanning the country's first and second waves, to estimate protection against repeat infection with the original Covid-19 strain.
Among people who had Covid-19 during the first wave between March and May 2020, only 0.65 per cent (72/11,068) tested positive again during the second wave from September to December 2020. At 3.3 per cent (16,819/514,271), 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 per cent (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 per cent (14,953/420,909).
Older people were found to be at greater risk of reinfection, with 0.88 per cent (17/1,931) 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 Covid-19, 2.0 per cent (1,866/93,362) tested positive during the second wave.