Over-65s more likely to be reinfected with Covid, finds study

Records from Denmark’s coronavirus testing programme show elderly population are at a greater risk of catching disease twice while for younger people reinfection is ‘rare’, writes Tom Batchelor

A healthcare worker takes a swab sample from a person in the Partybus, where people can listen to music while being tested, in Ishoej, Denmark (via REUTERS)

The vast majority of people who have had coronavirus will avoid catching it again for at least six months, with older people the most likely to report a second bout of the disease, a study found.

Researchers studying records from Denmark's Covid-19 testing programme found that the elderly were at a greater risk of catching Covid twice while for younger people reinfection was “rare”.

The findings mirror health outcomes across the population, with older people more likely to fall seriously ill and die from the illness.

Statistics collected in 2020 from the Danish national PCR testing database revealed that prior infection gave those under the age of 65 years around 80 per cent protection against reinfection, but for people aged 65 and older it only gave 47 per cent protection.

Dr Steen Ethelberg, from the Danish Statens Serum Institut, said the study, published in The Lancet, showed that “reinfection with Covid-19 is rare in younger, healthy people, but the elderly are at greater risk of catching it again”.

He said: “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.”

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Among those who had the virus during the first wave between March and May 2020, only 0.65 per cent (72 out of 11,068) tested positive again during the second wave from September to December 2020.

At 3.3 per cent, 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, 0.6 per cent (55 out of 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 3.6 per cent.

Researchers said older people were found to be at greater risk of reinfection, with 0.88 per cent of those aged 65 or older who were infected during the first wave testing positive again in the second wave.

Dr Daniela Michlmayr, also from the Staten Serum Institut, 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 limitations to their study, including that clinical information was only recorded if patients were admitted to hospital, meaning it was not possible to assess whether the severity of Covid symptoms affected patients' protection against reinfection.

Their research follows a separate study released by the La Jolla Institute for Immunology in January which found that Covid survivors have the immune cells necessary to fight reinfection for at least eight months.

The findings, based on analyses of blood samples from 188 Covid patients, added weight to the theory that people who have had coronavirus enjoy some form of protective immunity against serious disease.

”Of course, the immune response decreases over time to a certain extent, but that's normal,” said Professor Alessandro Sette, of the La Jolla Institute.

“That's what immune responses do. They have a first phase of ramping up, and after that fantastic expansion, eventually the immune response contracts somewhat and gets to a steady state.”

But Professor Shane Crotty, who was also involved in the study, added: “There are some people that are way down at the bottom of how much immune memory they have, and maybe those people are a lot more susceptible to reinfection.”