Younger individuals previously infected by SARS-CoV-2 had a low rate of reinfection during the second surge of the pandemic, while participants 65 years or older had a higher rate of reinfection, according to results of a study published in The Lancet.

Study authors analyzed Denmark’s national microbiology database of individually referable polymerase chain reaction (PCR) test results to estimate the degree to which previous infection with SARS-CoV-2 resulted in protection against repeat infection.

During the first surge, 533,381 people tested for SARS-CoV-2 and only 2.2% (n=11,727) were PCR positive. Of the people tested during the first surge, 525,339 were eligible for follow-up during the second surge, and 2.11% of the group (n=11,068) tested positive.

Study authors conducted an alternative cohort analysis, where they compared infection rates between participants with and without a previous confirmed infection at least 3 months prior. In this cohort, they also assessed for differences in outcome based on age group, sex, and time since infection.

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Among patients previously infected with SARS-CoV-2, data indicates that protection against reinfection was 80.5% (95% CI,75.4-84.5) in participants under 65 years of age, and 47.1% (95% CI, 24.7-62.8% in participants over 65 years of age. Results show no differences in reinfection rates based on sex (men, 78.4% [95% CI, 72.1-83.2]; women, 79.1% [73.9-83.3]; P =.84) and no evidence that protection in wanes over time. After a 3 to 6 month period of follow-up, the estimated protection was 79.3% (95% CI, 74.4-83.3), while after at least 7 months of follow-up, the estimated protection was 77.7% (95% CI, 70.9-82.9; P =.67). The alternative cohort analysis provided similar results.

“In our study, we did not identify anything to indicate that protection against reinfection declined within 6 months of having COVID-19,” states Dr Michlmayr.

Limitations of the study includes inability to correlate symptoms with protection against repeat infection due to lack of clinical parameters.

“Because the older age group is more prone to a serious clinical course of illness, this finding highlights the need to implement protective measures for the older population in the form of effective vaccines, physical distancing, and infection control, even in those known to be previously infected,” study authors concluded.


Hansen CH, Michlmayr D, Gubbels SM, Molbak K, Ethelberg S. Assessment of protection against reinfection with SARS-CoV-2 among 4 million PCR-tested individuals in Denmark in 2020: a population-level observational study. Lancet. Published online March 17, 2021. doi:10.1016/S0140-6736(21)00575-4