BEFORE THE PANDEMIC, Lyth Hishmeh—a 26-year-old living in Camberley, England—was always looking for something to keep himself busy. He was working as a software engineer and doing research into AI on the side while making plans to start a new company. He was juggling four to five textbooks at once. “I could not sit still,” he says. 

All that came to a halt on March 13, 2020, when he was sent home from work with a suspected case of Covid-19. His symptoms were mild but familiar: a cough, fever, shortness of breath. Within two weeks, they had subsided, and so Hishmeh went to buy groceries. At the shop, his heart began racing; he felt dizzy and out of breath. “It felt like some sort of heart attack.” He ignored it and boarded the bus home. But the same feeling came back again, this time worse. He stopped the bus, got off, and flagged down a police car before falling to the ground. He was brought to the hospital, where he had an ultrasound, which indicated Covid-19 pneumonia. The consultant said he was fine, and he was discharged. 

But Hishmeh wasn’t fine. Over the next few months, he developed all the strange and debilitating symptoms that have come to characterize the condition known as long Covid: brain fog, severe fatigue, heart palpitations. Just going to the bathroom was a struggle. Hishmeh was housebound for months, until October 2020. In the worst days of his long Covid, he couldn’t even watch a film all the way through. He went to the emergency room more than 10 times. “I would cry and beg, ‘Just fix me—do something,’” he says. 

Today, 16 months after being infected, Hishmeh can leave the house, but he still isn’t fully recovered. He hasn’t been able to return to work, and he has new food allergies. He also has postural tachycardia syndrome, where his heart races when he stands up. “I’m nowhere near fully recovered,” he says. “It was so awful that where I am now is a huge improvement. But where I am now to a normal person would probably be the end of the world.” 

Hishmeh is one of the estimated millions of people around the world who have long Covid. They’re stuck in a life-limiting limbo while scientists scramble to understand the mysterious condition. But as long Covid patients like Hishmeh continue to struggle with their illness, health authorities are struggling with some of the most basic questions about long Covid.  

To get a grasp on how big a problem long Covid is, we need to know how many people out there are stuck in situations like Hishmeh’s. That number is surprisingly difficult to pin down. The figures mentioned in the media vary wildly, depending on which study is cited. So what is the real figure?

Some estimates have ranged on the more conservative side. One data set, collected as part of the Covid Symptom Study using the app ZOE Covid from researchers at King’s College London, took a survey of 4 million people between March 25, 2020, and June 30, 2020. The results indicated that 4.5 percent of people with Covid-19 reported symptoms after 8 weeks, and only 2.3 percent of people after 12 weeks—a pretty low estimate. However, the study has faced criticism from long Covid sufferers and researchers alike. There are a few reasons why this estimate might be on the low side. First, the study most likely missed out on a number of long Covid sufferers who were too fatigued to log all their symptoms on the app on a regular basis. Also, if the patient had fewer than five symptoms on the last day they used the app, it counted them as recovered. 

This study was deliberately designed to give a conservative estimate, says Claire Steves, one of the authors. Because they wanted to establish without doubt that long Covid existed, they purposefully took a skeptic’s eye, applying stringent criteria; it included only people whose positive test for Covid-19 was confirmed by a PCR test. “I wouldn’t say that the Covid Symptom Study gives you the most accurate definition of everybody who might have this syndrome,” she admits. 

Other studies returned much higher numbers. One report from Imperial College London, called React-2, gives a much higher estimate of the prevalence of long Covid. The study surveyed over 500,000 participants between September 2020 and February 2021, asking them whether they thought they had had Covid-19, whether they had any symptoms from a list of 29, and if so, for how long. It estimated that almost 40 percent of people who definitely had or thought they had Covid-19 still had at least one symptom lasting 12 weeks or more. That’s an estimated 2 million people in England living with the condition between those two dates. 

Asking people to retrospectively count symptoms isn’t a fool-proof method, however. The study may overestimate the prevalence of long Covid, Steves says, as it doesn’t account for other conditions that can cause similar symptoms, such as diabetes and heart disease, not to mention the stress of living through a global pandemic. And the symptoms that respondents could select range widely, from a blocked nose or a hoarse voice to severe fatigue. But in fact, the varying symptomatology of long Covid is a big problem in itself. Devising diagnostic criteria is complicated by the sheer number and diversity of symptoms that sufferers report. One study into Covid-19 came back with a range of over 200 symptoms, including memory loss, menstrual cycle disruptions, and hallucinations. 

To rectify this, a review by the UK’s National Institute for Health Research (NIHR) proposes that long Covid could be subdivided into at least four different syndromes. These included post-intensive-care syndrome, long-term organ damage, post-viral fatigue syndrome, and a novel one: a long-term syndrome caused by a continuation of Covid-19 symptoms. It may also be that some people are suffering from more than one of these syndromes. 

Even defining exactly what long Covid is is a tough undertaking. There’s no universally accepted definition; even the name differs from country to country. Health authorities don’t agree on what specific conditions would merit a long Covid diagnosis. According to NICE guidelines, a patient should have symptoms persisting 4 to 12 weeks after the start of acute Covid-19 that can’t be explained by an alternative diagnosis, or symptoms that have not resolved 12 weeks after the start of acute Covid-19. The cutoff point for which week an acute case of Covid-19 becomes a case of long Covid remains controversial. “The time point at which you estimate the prevalence is really important,” says Elaine Maxwell, the lead author of the NIHR review. “I’m not sure it’s particularly helpful to be looking at it before 12 weeks.” 

Falling somewhere in between the big figures from the React-2 study and the more modest number from the King’s College London study is the data from the UK’s Office of National Statistics. The ONS estimates that 13.7 percent of a sample of over 20,000 people who had tested positive for Covid-19 still reported symptoms after at least 12 weeks. They then were compared against a control group of a similar size to make sure that this didn’t include people who had symptoms that weren’t related to Covid. This is the number that Maxwell backs most, but the ONS warns that these figures are just experimental and may not be final.

Working out the prevalence of long Covid is also a moving target. New variants and mass vaccination will inevitably have some effect on the condition that could muddle the numbers. “What’s true for the first and second wave may not be true for Delta, and certainly won’t be true in the context of vaccination,” says Steves. 

It’s easy to get caught up with the limitations of each study and survey, but we still know so little about the condition that, at this point, the more research we have, the better. “We’re at the stage of understanding long Covid, where it is more important to be open-minded, because we don’t know what we don’t know,” says Maxwell. “We need to keep looking at some of these smaller self-reported studies, because otherwise, we won’t know what to look for in the bigger, more controlled studies.”

Hishmeh just wants some answers, and some relief. “I’m 26—these are supposed to be my golden years. And I feel like I’m 80,” he says. “It really feels like at some point, the pandemic is going to end, and the world is going to move on. And we’re just going to be stuck here.”