An infection with the coronavirus offers most, but not all, people protection against reinfection in the months that follow. This is the result of a large-scale study in Denmark, which has now been presented in the scientific journal "The Lancet". In older people over 65 years a repeated infection occurs afterwards more frequently than with younger ones.

The study authors analyzed extensive data collected in Denmark as part of a national corona testing strategy spanning the first and second corona waves. More than four million people have been tested with free PCR tests in 2020 in Germany's northernmost neighbor; the Danes are also using the results for research purposes.

According to the scientists, PCR tests were positive twice in 0.65 percent of patients during the first and second Corona waves. By comparison, the test of those who tested negative during the first wave in March to May 2020 was positive in 3.27 percent of patients during the second in the following September to December. The researchers reported protection against repeated infection at 80 percent for younger people - but only 47 percent for people over 65.

"Our study confirms what a number of others have already pointed to," said one of the study's authors, epidemiologist Steen Ethelberg of the Danish Health Institute (SSI). "Re-infection with Covid-19 is rare in young, healthy people, but there is a higher risk of re-infection for older people." No evidence was found of protection decreasing within a six-month period after a person has been infected once, explained author Daniela Michlmayr.

According to the researchers, these findings show that measures to protect older people, such as spacing and vaccination, are also essential for those who have already had Covid-19. Their analysis further suggests that people who have already contracted coronavirus should also be vaccinated, they said. Natural protection cannot be relied upon, especially among the elderly, they say.

The researchers point out that given the time frame of the study, it was not possible to estimate protection against repeated infection with the virus variants. Variant B.1.1.7, which first appeared in England, now accounts for more than 90 percent of all new Corona infections analyzed in Denmark.

The new data are worrying, The Lancet quotes researchers Rosemary J. Boyton and Daniel M. Altmann of Imperial College in London, who were not involved in the study. "All these data are confirmation that in the case of Sars-CoV-2, hope for protective immunity from natural infection may not be within reach and that a global vaccination program with highly effective vaccines is the permanent solution."

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