For a select group of coronavirus patients known as "long-haulers", the onset of symptoms is the beginning of an extended battle. Many COVID-19 patients develop weeks- or months-long illnesses that researchers now call "long-COVID".

These individuals are difficult to study, since not all received a proper diagnosis initially due to testing shortages or the abnormal nature of their symptoms. Some may simply not report lingering ailments, making them difficult for researchers to track.

But a spate of preliminary studies are beginning to pinpoint the early signs that a patient won't recover right away.

A recent study from King's College London, which is still awaiting peer review, examined more than 4,000 coronavirus patients across Sweden, the UK, and the US by asking them to record their symptoms in an app.

About 20 percent said they still weren't feeling better after four weeks - the threshold at which the researchers mark a case of long-COVID. By eight weeks, around 190 patients reported lingering symptoms. And by 12 weeks, nearly 100 patients said they hadn't recovered yet.

Patients who experienced more than five symptoms during the first week of their illness were significantly more likely to develop long-COVID, the study found. That was true across sex and age groups.

The researchers also identified five symptoms that predicted a case of long-COVID more than others: fatigue, headache, difficulty breathing, a hoarse voice, and muscle or body aches. This could offer clues about targets for future COVID-19 treatments.

"It's important we use the knowledge we have gained from the first wave in the pandemic to reduce the long-term impact of the second," Dr. Claire Steves, the study's senior author, said in a statement.

"Thanks to the diligent logging of our contributors so far, this research could already pave the way for preventative and treatment strategies for long-COVID."

Nearly 98 percent of patients with long-COVID in the study reported fatigue, while 91 percent reported a headache.

"We know that fatigue is a huge component, so I'm really glad that their research captured that," Natalie Lambert, an associate professor of medicine at Indiana University who wasn't involved in the study, told Business Insider.

Lambert is also looking at patterns of symptoms among long-COVID patients. All of the roughly 1,500 long-haulers she surveyed in July said they'd experienced fatigue at some point in their illness.

Roughly two-thirds said they had experienced muscle or body aches. The same amount said they had difficulty breathing, and around 58 percent said they had developed a headache.

The results of the King's College study, Lambert said, square with her observations so far.

Age, gender, and BMI could also predict long-COVID cases

By far the strongest predictor of a long-COVID case, according to the King's College study, was age. Around 22 percent of participants ages 70 and older reported long-term symptoms, compared to 10 percent of people ages 18 to 49.

Participants with a higher body mass index (BMI) were also more likely to develop long-COVID.

Though sex wasn't as strong of a predictor of a long-COVID case, women in younger age groups were found to be more likely to suffer this outcome than men. Around 15 percent of women in the study had long-term symptoms compared to nearly 10 percent of men.

That finding is unexpected, since men are on average more vulnerable to severe COVID-19 outcomes than women. Scientists haven't determined exactly why, but studies have shown that women may develop a more robust T-cell reaction or quicker immune response to the virus.

Other scientists have pointed to behavioural factors like men eating less nutritiously than women do, being more likely to smoke cigarettes, or being reticent to wear masks or wash their hands.

One explanation for the surprising trend when it comes to long-lasting cases, however, could simply be that more women than men logged their symptoms into the app in the first place.

"I've had the same experience where many more women who have long-term symptoms took my survey than men by a huge margin," Lambert said. "Is it because more women are experiencing long-term symptoms? Is it because women are more likely to take these surveys and share their health experiences? We won't really know until we get enough data about everybody."

It's important to note, she added, that anyone is vulnerable to long-lasting symptoms.

"It can happen to absolutely anybody, no matter how healthy they were beforehand," Lambert said.

Non-hospitalized patients are still under-studied

Surveys that ask people to report their own symptoms are imperfect, since people may have trouble remembering each symptom or they might associate it with something other than the virus.

"With COVID, the symptoms are so numerous and wide-reaching that sometimes people don't recognise it as something related to COVID until you ask them about it," Lambert said. "We've found that with things like blurry vision."

But even imperfect data can be useful, she added, since so little is known about the virus' long-term effects.

Most coronavirus studies have focused on hospitalized patients, who may be more likely to develop certain symptoms, like a fever. The King's College London study, for instance, found that fever was a strong predictor of a hospital visit.

But in Lambert's latest survey of roughly 4,000 symptomatic coronavirus patients, only 8 percent of patients reported a fever in the first 10 days of their illness.

To better understand the effects of the virus, Lambert said, more research should track non-hospitalized patients, including people who are asymptomatic.

"On the one hand, it's amazing that scientists and researchers all over the world are finding each other and working on this stuff, but at the same time, we kind of feel like a ragtag team," Lambert said.

"These are questions that we really need to answer now."

This article was originally published by Business Insider.

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