"Background: … In 2020, as part of Denmark's extensive, free-of-charge PCR-testing strategy, approximately 4 million individuals (69% of the population) underwent 10·6 million tests. Using these national PCR-test data from 2020, we estimated protection towards repeat infection with SARS-CoV-2….
Discussion: We used a large national surveillance dataset of individually referable PCR test results to estimate the degree to which previous infection with SARS-CoV-2 results in protection against repeat infection. We found protection in the population to be 80% or higher in those younger than 65 years, but to be approximately 47% in those aged 65 years and older. We did not see signs of waning protection against repeat infection within the year 2020.“
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