Study of 10-million person-identifiable PCR-test in Denmark shows young people protected from SARS-CoV-2 reinfection

The following study was published by The Lancet on March 27, 2021. It was conducted by Christian Holm Hansen, Daniela Michlmayr, Sophie Madeleine Gubbels, Prof. Kåre Mølbak,. and Prof. Steen Ethelberg.

Utilizing individual level data collected from 10 million person-identifiable PCR tests conducted in Denmark in 2020, the researchers found that protection against repeated infection among the young (aged below 65 years) was at 80.5%, with those aged 0 to 34 years having the highest protection against reinfection. Meanwhile, protection from repeated infection among those aged 65 and older was lower, but still substantial at 47.1%. Observed protection against reinfection was also estimated to be at 80% after 6 months. The researchers said that no sign of waning protection was reported within the year 2020.

The data used in the study included the entire population of Denmark and covered all tests for SARS-CoV-2 from February 26 to December 21, 2020. The article also asserts that vaccination of infected individuals should be done because natural protection could not be relied on.

Editor’s Note: The completeness of this study from Denmark highlights to us some very important points. First, reinfection among the young is low. This is expected as the immune systems of younger populations are much more robust than that of the elderly.

Second, though protection from reinfection of SARS-CoV-2 is not perfect, it is nonetheless significantly high and must not be ignored. Notice, however, that though variants of SARS-CoV-2 were not yet covered by the the research, no particular variant of concern has recently been proven to have higher threat than the original strain.

Third, the research shows clearly why governments must implement focused protection. Not only are the elderly more prone to complications of COVID-19, they are also more prone to reinfection. Using one-size-fits-all approaches wastes resources which could have been utilized to protect those who are most at risk with the disease.

As we analyze this article, several questions come to mind. For one, the research raises too many unanswered questions regarding reinfections. Was there any marked difference between the clinical outcomes of original infection compared to reinfections? Were reinfections more severe than the original infection, or were they typically asymptomatic? Did any of the reinfections lead to death? We also need to know, what was the cycle threshold used for the PCR tests? Did Denmark use a fixed threshold from February to December 2020? It is important for us to know the answers to these questions because they will help determine of the reinfections were real infections, or whether they are false positives [recent studies are showing that a cycle threshold beyond 35 is too sensitive and will detect even inactive fragment of other coronaviruses which might be mistaken as SARS-CoV-2, see New York Times: More experts questioning RT-PCR testing]. Until we are able to verify what the cycle thresholds are, we cannot confirm whether the reinfections mentioned here are real infections or false positives.

In light of this last point, we cannot agree to vaccination for those who have already had COVID. New evidence is showing that those who were previously infected could experience more severe adverse reactions from the vaccines [article on severe disease for those who had covid]. Moreover, other scientific studies show that our immune response to SARS-CoV-2 is long-lasting [read New York Times: Immunity to coronavirus may last for years]. Also, even as the virus is mutating, our immune system is also mutating to respond more effectively to control the virus [see Our immune system evolves to fight coronavirus variants]. With all these facts, we can say our protection from COVID-19 is robust and reinfections, even if they occur, are rare and not usually severe.