Does it make a difference whether the COVID-19 infection was treated in hospital? Whether a woman was infected or a man? Scientific institutes in Austria and Belgium have analysed studies on long COVID.
Some people complain of symptoms such as exhaustion, fatigue, headaches, high blood pressure, smell and taste disorders or respiratory problems even months after they have survived a corona infection. In order to clarify the prevalence of long COVID after a confirmed or suspected SARS-CoV-2 infection, which symptoms occur how frequently and to give an overview of possible risk factors, the Austrian Institute for Health Technology Assessment (AIHTA), in cooperation with the Belgian Health Care Knowledge Center (KCE), analysed the current data on long COVID. As of May 2021, 28 suitable studies could be identified to summarise the existing evidence of the condition.
"Severe COVID-19 courses are more often associated with long COVID according to the studies," says study leader Sarah Wolf from AIHTA, "but looking at several studies also showed that the range of individual symptoms is very large." According to the study, 39 to 72% of the COVID-19 patients admitted to hospital experienced long COVID symptoms within one to three months after acute SARS-CoV-2 infection; in the group of outpatients, the figure was 5 to 36%. Even after more than six months, up to 60% of formerly hospitalised patients still reported fatigue, exhaustion, cognitive impairment and/or respiratory problems; in the cohort of SARS-CoV-2 outpatients, this symptomatology applied to 13 to 25%.
The most common symptoms among long COVID patients up to three months after the onset of acute SARS-CoV-2 infection were "fatigue/exhaustion" with 16 to 98%, followed by "shortness of breath (10 to 93%) and headache with 9 to 91%. Within the first three months after the acute infection, 11 to 34% of the long COVID patients complained of "cough", "chest pain" affected between 10 and 86%, and "cognitive difficulties" between 4 and 89% of the test persons, depending on the study.
After three to six months, "fatigue/exhaustion" (16 to 78%) and "cognitive impairment" (13 to 55%) were among the most common long COVID symptoms. In addition, 16 to 21% had to deal with "respiratory problems".
Twelve of the 28 studies also looked at possible risk factors. The results of six studies suggest that the "female sex" may favour the development of long COVID. "However, the difference in the incidence of the disease between men and women could also have other reasons than biological sex and the immune response associated with it. For example, it is known that there are gender-specific differences in health behaviour, according to which women, for example, more often report poorer health than men in surveys," emphasises study leader Sarah Wolf.
Another potential risk factor for long COVID, which also could not be confirmed yet, is the high number of symptoms during the acute infection phase. Even an older age of the patient does not per se increase the probability of contracting long COVID.
"The exact causes and risk factors that lead to the development of long COVID symptoms are currently unknown. Due to the great variety of different symptoms, it can be assumed that several causes are interwoven," says the KCE and AIHTA report. For example, patients with severe courses of COVID-19, who had to be artificially ventilated, have an increased risk of developing long COVID symptoms. In these patients, possible organ damage caused by intensive care treatment, for example, could be the cause of long COVID. Long COVID symptoms that are not due to organ damage are to be distinguished from this. "However, the studies do not distinguish between long COVID symptoms due to organ damage and other causes," explains Sarah Wolf. The study authors therefore emphasise that "for future studies, a more precise characterisation and classification of long COVID symptoms and their causes is needed in order to efficiently design treatment strategies for different long COVID patient groups."
In addition, a consistent definition of long COVID is needed to distinguish symptomatology from other conditions (e.g. "post-intensive care syndrome") or causes (e.g. mental health problems due to the long lockdowns/psychological problems as a consequence of pandemic response measures).
Wolf, S. and Erdös, J. for the Belgian Health Care Knowledge Center (KCE). Epidemiology of long COVID: a preliminary report. German summary of the KCE report of the same name. AIHTA Project Report No. 135a; 2021. Vienna: Austrian Institute for Health Technology Assessment GmbH.