New study estimates that 80% of those who have been infected with COVID-19 are protected against becoming infected again

Researchers at Statens Serum Institut have analysed the distribution of the more than 10 million PCR tests that were performed in Denmark last year. The results indicate that approximately 80% of those testing positive for the covid-19 virus would be protected against becoming infected again. Among the elderly, however, this figure is down to only 50%.

Covid-19 is a new disease and there is still much we do not know about it. This includes the question of how large a proportion of the population becomes immune after a first infection with the covid-19 virus.

Last year, more than 10 million PCR tests were performed in Denmark. Four million Danes had at least one test done. Researchers at the Statens Serum Institut (SSI) have now analysed patterns in these test data with a focus on the individuals who tested positive more than once. The data were used to estimate the degree of protection against a new infection after a past infection.

The study has been published today in the English medical journal The Lancet.

Only few became infected twice

In the analysis, the researchers identified those who were tested by PCR in the first epidemic wave in the spring and looked at their test results, if any, during the second wave in the autumn and winter of 2020. The analysis showed that among those who tested positive during the first wave, 0.65% tested positive again during the second wave. By comparison, 3.3% of those who tested negative in the first wave had a positive test during the second wave. Those who had not previously tested positive were thus roughly five times more likely to test positive later on. This corresponded to an 80.5% degree of protection against a later infection.

There was no difference in protection between males and females. There was also no difference between the first part and the last part of the study period. That is, there was no sign that protection began to wane after six months. However, when dividing the population into age groups, a different pattern emerged for the senior age group. Among those aged 65 years and above, the protection was estimated to be just 47%.

Older persons may have reduced immunity to covid-19

"We estimated protection after natural infection to be 80% in the population overall. But we also saw that this figure, what we may call the observed immunity, was markedly lower in the older age segment, being just 50% or lower. ” said senior researcher Steen Ethelberg who together with colleagues from the SSI conducted the study. Steen Ethelberg continued:

“Our study suggests that most people will be protected from being infected again for at least half a year. But not everyone is protected and especially among the elderly, only about half appeared to be protected after a first infection. Even if you have already been infected, our findings suggest that it is advisable to keep following the general advice on how to protect yourself from infection. Furthermore, the findings highlight the need to vaccinate all, since natural protection – especially among the elderly - cannot be relied upon."


Steen Ethelberg


Steen Ethelberg , Infektionsepidemiologi og Forebyggelse / Fødevare- og vandbårne sygd. og zoonoser
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