Past COVID-19 Infection Does Not Fully Protect Young People
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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 underwent 10·6 million tests. Using these national PCR-test data from 2020, we estimated protection towards repeat infection with SARS-CoV-2.

In this population-level observational study, the team collected individual-level data on patients who had been tested in Denmark in 2020 from the Danish Microbiology Database and analysed infection rates during the second surge of the COVID-19 epidemic, from Sept 1 to Dec 31, 2020, by comparison of infection rates between individuals with positive and negative PCR tests during the first surge.

For the main analysis, they excluded people who tested positive for the first time between the two surges and those who died before the second surge. They did an alternative cohort analysis, in which they compared infection rates throughout the year between those with and without a previously confirmed infection at least 3 months earlier, irrespective of date.


During the first surge, 533381 people were tested, of whom 11,727 were PCR positive, and 525,339 were eligible for follow-up in the second surge, of whom 11068 had tested positive during the first surge.

Among eligible PCR-positive individuals from the first surge of the epidemic, 72 tested positive again during the second surge compared with 16819 of 514271 who tested negative during the first surge. Protection against repeat infection was 80·5%. The alternative cohort analysis gave similar estimates, estimated protection 78·8%.

In the alternative cohort analysis, among those aged 65 years and older, the observed protection against repeat infection was 47·1%. We found no difference in estimated protection against repeat infection by sex vs female 79·1% or evidence of waning protection over time vs more than or equal to 7 months of follow-up 77·7%.


Their findings could inform decisions on which groups should be vaccinated and advocate for vaccination of previously infected individuals because natural protection, especially among older people, cannot be relied on.