The risks of developing long Covid appear to have been exaggerated by flawed research, academics have suggested.
Most people who contract coronavirus feel better within days or weeks, but for some people symptoms can last longer than 12 weeks.
Those who have long Covid, also known as post-acute sequelae of Covid-19 (PASC), report fatigue, brain fog, chest pain and shortness of breath.
However, analysis from researchers in the UK, Denmark and the US suggests “major flaws” in the body of literature on the condition have probably exaggerated the true threat of developing it.
The analysis, published today, says “many scientific publications” overestimate the prevalence of long Covid due to “overly broad definitions, lack of control groups, inappropriate control groups, and other methodological flaws”.
Researchers said weaknesses in such studies may have fuelled public anxiety as well as causing misdiagnoses and diversion of funds from other long-term conditions.
The paper, published in BMJ Evidence-Based Medicine, looked at the definition of long Covid by a number of global health organisations, including the National Institute for Health and Care Excellence (Nice) and the World Health Organisation (WHO).
Researchers said “none of them requires a causal link” with coronavirus infection meaning any new symptoms after a confirmed or suspected infection can end up being assumed to be long Covid.
“In general, in the scientific literature, imprecise definitions have resulted in more than 200 symptoms being associated with the condition termed long Covid,” they said.
The team also points out that studies in the early stages of the pandemic are likely to have included fewer patients with mild or no symptoms.
They highlighted a recent review which found that 54 per cent of of studies of long Covid interventions included lab-confirmed infections.
This could lead to sampling bias, they said, which occurs when certain groups are more likely to be included in a study than others, limiting the findings.
Many symptoms assumed to be those of long Covid are shared with other upper respiratory viruses.
They added: “Our analysis indicates that, in addition to including appropriately matched controls, there is a need for better case definitions and more stringent (long Covid) criteria.”
It should include continuous symptoms after confirmed SARS-CoV-2 infection and take into consideration baseline characteristics, including “physical and mental health, which may contribute to an individual’s post Covid experience.”
The study was led by University of California, San Francisco and also involved the University of Southern Denmark as well as researchers from Public Health England and St George’s University of London.
In future, research should include properly matched control groups, the paper said, as well as “sufficient” follow-up time after infection.
They also called for “internationally established” diagnostic criteria and called for studies to avoid the “umbrella term” and instead “narrowly define” certain post-Covid symptoms.
“Ultimately, biomedicine must seek to aid all people who are suffering,” the team added. “In order to do so, the best scientific methods and analysis must be applied.
“Inappropriate definitions and flawed methods do not serve those whom medicine seeks to help.
“Improving standards of evidence generation is the ideal method to take long Covid seriously, improve outcomes, and avoid the risks of misdiagnosis and inappropriate treatment.”
However, Dr Adam Jacobs, the senior director of biostatistical science at Premier Research, said that while much of the research on long Covid was “imperfect” it was hard to conclude that the most optimistic estimates of its prevalence were likely to be correct.
Highlighting the most recent survey by the Office for National Statistics in spring which counted 1.9 million people in the UK with long Covid, he said even if this was wrong by a factor of 10 there would be more people with the condition than with Parkinson’s disease.
Last week, a study looking at the impact of Covid-19 found that nearly a third of patients who were admitted to hospital with the virus had abnormalities in multiple organs five months after infection.
MRI scans of patients in the study by Oxford University showed more abnormal findings involving the lungs, brain and kidneys compared to a group of people who had not had the virus.