One in eight people who have had Covid-19 have problems such as breathlessness and general tiredness 90 to 150 days after their coronavirus infection, a new study set to be published in the The Lancet journal early on Friday said, providing what may be the most reliable estimates of Long Covid prevalence yet.

Long Covid, a loosely defined set of conditions that people report after having caught Covid-19, is yet to be properly understood, either for the biology involved or its estimated prevalence.

The new study from The Netherlands seems to answer at least the second question. Participants were asked to report symptoms of what they were feeling regularly from early on in the pandemic, giving researchers the ability to determine how commonly certain conditions are present in people who weren’t infected and in those who were.

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This allowed identification of what may be the core symptoms of Long Covid: chest pain, difficulties breathing, pain when breathing, painful muscles, loss of taste and smell, tingling extremities, lump in throat, feeling hot and cold, heavy arms and/or legs, and general tiredness. “There is urgent need for data informing the scale and scope of the long-term symptoms experienced by some patients after Covid,” said Judith Rosmalen from the University of Groningen, lead author of the study, in a release.

“However, most previous research into Long Covid has not looked at the frequency of these symptoms in people who haven’t been diagnosed with Covid-19 or looked at individual patients’ symptoms before the diagnosis of Covid-19,” Rosmalen added.

Of the participants who submitted pre-Covid symptom data, the researchers found that 21.4% (381 out of 1,782) of Covid-19-positive participants experienced at least one of the above mentioned symptoms to at least moderate severity. In comparison, only about 8.7% (361 out of 4,130) had these symptoms in the control group, which can be the result of any number of illnesses, including chronic conditions. This allowed for the researchers to remove conditions that may not be linked to Covid-19. When compared, it translates into a 12.7% higher prevalence of Long Covid symptoms, which seemed to last three to five months after an infection.

“These core symptoms have major implications for future research, as these symptoms can be used to distinguish between post Covid-19 condition and non-Covid-19-related symptoms,” said first author of the study, Aranka Ballering, according to the statement.

“Post-Covid-19 condition, otherwise known as Long Covid, is an urgent problem with a mounting human toll. Understanding the core symptoms and the prevalence in the general population represents a major step forward for our ability to design studies that can ultimately inform successful healthcare responses to the long-term symptoms of Covid-19,” she added.

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A variety of factors, including possible long-term effects on the brain and the cardiovascular system are being investigated to determine the biological underpinnings of Long Covid. The authors of the new study said future research must also look at mental health effects.

“Future research should include mental health symptoms (e.g. depression and anxiety symptoms), along with additional post-infectious symptoms that we could not assess in this study (such as brain fog, insomnia, and post-exertional malaise). We were unable to investigate what might cause any of the symptoms observed after Covid-19 in this study, but we hope future research will be able to give insights into the mechanisms involved,” Rosmalen said, adding that due to the study’s timing — it was carried out from March 2020 to right before The Netherlands began widely vaccinating people a year later — the effect of Covid-19 vaccination and different Sars-CoV-2 variants on Long Covid was yet to be understood.