Covid-19 reinfection rare, but more common in 65+: Large Danish study in Lancet

Most people who have had Covid-19 are protected from catching the virus again for at least six months, but elderly patients are more prone to reinfection, according to new research.

Most people who have had Covid-19 are protected from catching it again for at least six months, but elderly patients are more prone to reinfection, according to research published in The Lancet.

SARS-CoV-2, the cause of the Covid-19 epidemic, has resulted in over 117 million cases and over 2.6 million deaths worldwide as of March 7, 2021, as estimated by the World Health Organisation. However, the degree to which infection with SARS-CoV-2 confers protection towards subsequent reinfection is not well described.

In 2020, as part of Denmark’s extensive, free-of-charge PCR-testing strategy, approximately 4 million individuals (69 percent of the population) underwent tests. Using these national PCR-test data from 2020, researchers estimated protection towards repeat infection with SARS-CoV2.

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 percent protection against reinfection, for people aged 65 and older, it gave only 47 percent protection, indicating that they are more likely to catch Covid-19 again.

The authors of the first large-scale study of its kind detected no evidence that protection against reinfection declined within a six-month follow-up period.

Their findings highlight the importance of measures to protect elderly people during the pandemic, such as enhanced social distancing and prioritisation for vaccines, even for those who have recovered from Covid-19. The analysis also suggests that people who have had the virus should still be vaccinated, as natural protection – particularly among the elderly – cannot be relied upon.

As of January 2021, Covid-19 had resulted in more than 100 million cases and over 2 million deaths worldwide. Recent studies have suggested that reinfections are rare and that immunity can last at least six months, however, the degree to which catching Covid-19 confers protection against repeat infection remains poorly understood.

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.”

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