From migraines to fatigue, coronavirus patients say they are continuing to suffer debilitating symptoms months after first becoming infected, in what has become known as "long Covid."
Claire Twomey, 33, a social worker in County Meath, Ireland, told CNBC via telephone that it was in her first week back at work, around six weeks after she first became ill with the coronavirus, that her symptoms re-emerged.
She initially thought she had become re-infected with the virus when the headaches came back, followed by a fever, coughing and shortness of breath. But hospital tests found no underlying issues, she said.
Twomey said she felt "absolutely floored" when the symptoms re-emerged. "I was back in bed, I couldn't even read a book or watch TV for longer than half an hour."
More "insane, weird (and) strange" symptoms appeared in this relapse with the illness, including gastrointestinal issues, hair loss and skin rashes.
Twomey said she felt "frustrated" as the illness lingered, and worried about the future after being out of work for so long. "I've been on pause for six months," she said.
By mid-September, Twomey found she was having fewer "bad days" but knew that she still couldn't return to working as she had before.
Twomey applied for another part-time position in social care, but spent the eight days prior to the job interview bedridden with migraines. "I thought I was going to have to cancel the interview."
Fortunately, she was able to do the interview and got the job, which she is set to start in a few weeks.
Three health care bodies in the U.K. announced Monday that they were working on a formal definition of "long Covid" and how to identify symptoms, so that the National Health Service can officially recognize the illness. The "long Covid" guidelines are expected to be published by the end of the year.
In a paper published Monday by the Tony Blair Institute for Global Change on "long Covid," Tim Spector, a professor of genetic epidemiology at King's College London, warned so-called "long haulers … could turn out to be a bigger public-health problem than excess deaths from Covid-19."
The paper also highlighted new findings from the Covid Symptom Study, led by Spector, indicating that around 10% of people surveyed in the U.K. had suffered with "long Covid" symptoms for a month, while up to 2% were still experiencing them after three months.
With nearly 4.3 million downloading the study's app to record coronavirus symptoms, it is said to be the largest public science project of its kind in the world. There have been 532,779 confirmed cases of Covid-19 in the U.K. and 42,535 related deaths, according to data collected by Johns Hopkins University.
Based on extrapolated data, the researchers estimated that of those affected by the first wave of the virus in the U.K., 300,000 people would have experienced Covid-19 symptoms for a month, while 60,000 people would have suffered symptoms for three months or more.
And it isn't just people considered to be more vulnerable to catching the virus, such as those over the age of 70, that have been affected. The paper also cited another study in the U.S. that found one in five people aged 18-34, who didn't already have chronic medical conditions, said they had suffered from "long Covid" after initially becoming infected.
For Evie Connell, 23, a marketing and business student at Abertay University in Dundee, Scotland, prolonged illness meant she was unable to finish a summer internship. She's also been signed off from her part-time job at a convenience store.
Connell first started showing coronavirus symptoms in March but said that the initial period of the illness "wasn't that bad," as she mainly experienced fatigue and some breathlessness.
By week 15, Connell went to her doctor complaining of all-day chest pains. She was referred to a local Covid-19 rehabilitation team and was signed off work. In addition to chest pain, Connell continues to suffer from an erratic heart rate, breathlessness, as well as brain fog and chronic fatigue.
She only managed to do a few weeks of her digital marketing internship with a local business before finding that she no longer had the attention span to carry on. Connell has since returned to college for her third year but worries about her ability to concentrate on her studies.
"I could defer but then that's going to push me back another year of studying which I don't want to do," she told CNBC over the phone.
Connell said it had been "quite hard to come to terms" with just how much the virus had affected her, having been someone who went to the gym around four times a week to now finding herself breathless after climbing four flights of stairs.
Paul Garner, a professor of infectious diseases at the Liverpool School of Tropical Medicine, documented his own six-month struggle with "long Covid" on the British peer-reviewed medical journal the BMJ.
Garner told CNBC that he's also struggled with fatigue and slower cognition, forcing him to stop teaching. Besides two weeks when he initially fell ill with the virus, he has continued to work but admitted he "probably started back a bit too soon."
"I do think that the symptoms are frightening, unusual and often, people … can't quite believe it themselves and then they doubt whether it's something mentally wrong with them," he said.
Antibody testing of "long Covid" sufferers has been reported to give negative results, making it hard for long-haulers to prove their prolonged illness.
Garner said this is similar to the difficulties that those with chronic fatigue syndrome have long endured in being mis-diagnosed with psychosomatic illnesses. "People really do need some kindness and understanding," he added.
CNBC Make It is always looking to hear about the experiences of young people around the world. If you've got a story to share get in touch at Makeit@cnbc.com