Most people who have had coronavirus 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 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 older it only gave 47 per cent 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.

“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 Dr Steen Ethelberg, from the Statens Serum Institut, Denmark.

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“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.”

Free, national PCR testing – open to anyone, regardless of symptoms – is one of the central pillars of Denmark’s strategy to control COVID-19.

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 per cent, four million people) were tested in 2020. Researchers used this data to estimate protection against repeat infection with the original COVID-19 strain.

Graphic showing global COVID-19 cases and deaths as of 17 March © PA Graphics
© PA Graphics

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. The rate of infection among people who had tested negative during the first wave was five times higher, at 3.3 per cent.

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Of those under the age of 65 who had COVID-19 during the first wave, 0.60 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 say 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. Among people 65 or older who had previously not had coronavirus, 2.0 per cent (1,866 out of 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 per cent (8 out of 658) of those who had COVID-19 during the first wave became reinfected, compared with 6.2 per cent (934 out of 14,946) of those who were negative during the first wave.

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“In our study, we did not identify anything to indicate that protection against reinfection declines within six months of having COVID-19," said Dr Daniela Michlmayr, from the Statens Serum Institut, Denmark.

“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.

How do scientists develop vaccines for new viruses?

Vaccines work by fooling our bodies into thinking that we’ve been infected by a virus. Our body mounts an immune response, and builds a memory of that virus which will enable us to fight it in the future.

Viruses and the immune system interact in complex ways, so there are many different approaches to developing an effective vaccine. The two most common types are inactivated vaccines (which use harmless viruses that have been ‘killed’, but which still activate the immune system), and attenuated vaccines (which use live viruses that have been modified so that they trigger an immune response without causing us harm).

A more recent development is recombinant vaccines, which involve genetically engineering a less harmful virus so that it includes a small part of the target virus. Our body launches an immune response to the carrier virus, but also to the target virus.

Over the past few years, this approach has been used to develop a vaccine (called rVSV-ZEBOV) against the Ebola virus. It consists of a vesicular stomatitis animal virus (which causes flu-like symptoms in humans), engineered to have an outer protein of the Zaire strain of Ebola.

Vaccines go through a huge amount of testing to check that they are safe and effective, whether there are any side effects, and what dosage levels are suitable. It usually takes years before a vaccine is commercially available.

Sometimes this is too long, and the new Ebola vaccine is being administered under ‘compassionate use’ terms: it has yet to complete all its formal testing and paperwork, but has been shown to be safe and effective. Something similar may be possible if one of the many groups around the world working on a vaccine for the new strain of coronavirus (SARS-CoV-2) is successful.

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Sara RigbyOnline staff writer, BBC Science Focus

Sara is the online staff writer at BBC Science Focus. She has an MPhys in mathematical physics and loves all things space, dinosaurs and dogs.