Study finds COVID-19 reinfections rare, but more common in elderly patients

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Most people who have had novel coronavirus (COVID-19) are protected from catching it again for at least six months, but elderly patients above age 65 years old are more prone to reinfection, according to a new study published in The Lancet.

For the study, researchers analyzed data collected as part of Denmark's national COVID-19 testing strategy, through which more than two-thirds of the population 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. Ratios of positive and negative test results were calculated taking account of differences in age, sex, and time since infection, and these were used to produce estimates of protection against reinfection.

Assessment of reinfection rates confirmed that a small proportion of people or 0.65 percent 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 conferred 47 percent protection, indicating that they are more likely to catch COVID-19 again. The researchers said they detected no evidence that protection against reinfection declined within a six-month follow-up period.

The researchers said the findings highlight the importance of measures to protect elderly people during the pandemic, such as enhanced social distancing and prioritization 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, they said.

In the research, the authors noted that the timeframe of their study meant it was not possible to estimate protection against reinfection with COVID-19 variants, some of which are known to be more transmissible. Further studies are needed to assess how protection against repeat infection might vary with different COVID-19 strains.

"Our study confirms…reinfection with COVID-19 is rare in younger, healthy people, but the elderly are at greater risk of catching it again,” said Steen Ethelberg, PhD, senior researcher at the Statens Serum Institut in Denmark, in a statement. “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 emphasize 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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