Large Danish study: One in four had long-term symptoms after COVID-19
In 2020, Denmark was hit by the first wave of the COVID-19 pandemic, and many Danes were infected with the virus. Many of them noticed that the symptoms persisted for a long time, even if they had not been hospitalised.
A Danish study shows that one in four study participants who tested positive with a PCR test had at least one symptom due to COVID-19 of either a physical or mental nature more than 12 weeks after their positive test.
As part of the large national surveillance project 'We test Denmark', questionnaires were sent out to a large number of Danes to gather information about COVID-19. The participants were also asked if they experienced long-term symptoms after being infected.
The risk of long-term symptoms depends on the severity of the symptoms during infection.
"It depended very much on how you felt when you were ill. For those who had no symptoms or almost no symptoms, about 6.8 percent had long-term symptoms. The figure is 20 percent for those who had some symptoms, but did not feel so bad that they were confined to bed. For those with severe symptoms, but not so bad that they were hospitalized, the figure was 30 percent," says assistant professor Maarten van Wijhe from the Department of Science and Environment at Roskilde University.
He is first author on a paper with the results, which has been published in the scientific journal Open Forum Infectious Diseases. The results support other studies of the long-term symptoms of COVID-19.
As chief physician and professor at the Department of Infectious Diseases at Rigshospitalet, Susanne Dam Poulsen, one of the main authors of the article, has primarily researched COVID-19 in patients.
"This study provides a unique insight into the long-term effects of those who have not been hospitalised. It is worrying that such a high proportion have long-term symptoms, and we still don’t have a full picture of how long people can have such side effects after COVID," she says.
Particularly vulnerable groups
The most frequently occurring long-term symptoms were fatigue, shortness of breath, concentration problems and loss of taste and smell senses.
The study also shows that there are some factors that may have contributed to getting long-term symptoms. According to the study, it was women and people who had severe COVID-19 symptoms that were particularly at risk of long-term symptoms. A person who had a bad case of COVID-19 was about seven times more likely than those without COVID-19 to have poorer overall health than before the pandemic.
The control group consists of people who do not have antibodies and have not had a positive PCR test. Some people in the control group reported symptoms or worsening health during the pandemic, although there is no evidence that they were infected.
Better prepared for future pandemics
The study examines the first wave of COVID-19 in 2020. The long-term symptoms were strongest in the first wave and cannot therefore be directly compared to later variants of the virus, such as the delta and omicron variants.
However, although new variants have emerged since the first wave, the knowledge gained from the study is still relevant, as it provides both a better understanding of what happened during the first wave, and knowledge that could be used in a future pandemic.
"This study was only possible because so many Danes participated and because we have tested so much for coronavirus in Denmark. The study helps us to understand the issue of long-term symptoms, and it provides us with an insight into what might have happened if the infection had not been suppressed so effectively before the vaccines arrived. At the same time, it establishes that many people suffered from long-term effects and their health was effected considerably at the beginning of the pandemic. It is clear that long-term symptoms must be taken into account when we eventually assess the overall impact of the pandemic on public health," says Maarten van Wijhe.
The study includes 742 Danes who tested positive for COVID-19 with a PCR test during the first wave of the COVID-19 pandemic. They were divided into groups according to their symptoms: No symptoms, mild symptoms, and severe symptoms, which typically means they have been confined to bed.
These cases were compared to a control group of 7,420 people who did not test positive.
The study was a collaboration between researchers from Roskilde University, the Danish Serum Institute, Rigshospitalet, University of Copenhagen, Aalborg University Hospital, Aarhus University Hospital, Randers Regional Hospital, Odense University Hospital, University of Southern Denmark, North Zealand Hospital and Zealand University Hospital.
We test Denmark
'We test Denmark' was a national monitoring project aimed at gaining knowledge about how COVID-19 affected the Danish population.
A total of 474,000 citizens have completed a questionnaire and sent the answers to the Danish Serum Institute. A large number of them have also been sent an antibody test to see if they have antibodies in their blood.
One million randomly selected Danes over the age of 15 were asked to participate in the project by answering a questionnaire.
'We test Denmark' has been run by the Danish Serum Institute in collaboration with experts from the regions and universities.
The project has been supported by, among others, the Danish Health Authority, the Danish Patient Safety Authority, Local Government Denmark and the Danish Regions.