Almost a fifth of people who develop no symptoms while infected with the coronavirus go on to endure long COVID, research suggests.
Not everyone who overcomes the coronavirus returns to a clean bill of health. In the UK alone, more than 1 million people who had the infection are said to have endured lingering complications while testing negative for the virus.
Whether the coronavirus itself was mild or severe, long COVID patients have reported everything from brain fog and fatigue to palpitations and even organ damage.
To better understand long COVID's prevalence, a team from the non-profit organisation FAIR Health in New York analysed nearly 2 million Americans from February 2020 to February 2021.
Results reveal 19% of the Americans who were asymptomatic for the coronavirus endured long COVID, defined as complications that persist for 30 days after testing positive for the infection.
Watch: What is long COVID?
"Even as the COVID-19 [the disease caused by the coronavirus] pandemic wanes, long-haul COVID persists as a public health issue affecting many Americans," said Robin Gelburd, president of FAIR Health.
In the UK, official guidance recommends long COVID patients set realistic goals and join support forums.
Long COVID's symptoms can be broad, with each patient having to be treated on a case-by-case basis.
While most people fully recover within 12 weeks, some patients are enduring long COVID more than a year after overcoming the coronavirus itself.
In the largest long COVID study to date, the FAIR Health team analysed the insurance claims made by 1.96 million Americans.
Nearly one in five (19%) of the asymptomatic coronavirus carriers endured long COVID up to a month later, rising to just over a quarter (27.5%) among those who showed signs of the infection but did not require hospital care.
Half (50%) of those who were hospitalised with coronavirus complications endured long COVID for up to 30 days, the results – published in a white paper – show.
Overall, pain was the most common long COVID symptom. This was followed by breathing difficulties, raised cholesterol, general discomfort or fatigue, and high blood pressure.
The frequency with which certain complications occurred varied according to the patient's age, however. Among the younger individuals, intestinal issues were more common than high cholesterol.
Long COVID was generally found to be more common among the female participants than the males. In March 2021, scientists from the University of Leicester reported white middle-aged women are most at risk of complications five months after being discharged from hospital.
In the FAIR Health study, heart inflammation – a relatively common issue – affected the men more than the women, however.
The coronavirus is said to be mild in four out of five cases, but it can become significantly more severe among the elderly. When it comes to long COVID, however, a quarter of the asymptomatic FAIR Health participants who later developed heart inflammation were aged 19 to 29.
Some of the asymptomatic coronavirus patients also endured mental health complications down the line, with anxiety being the most common disorder, followed by depression.
Adjustment disorders – when unprecedented stress causes "significant problems" in a person's life – and tic disorder – fast, repetitive muscle movements that affect the body and speech – were also reported.
The scientists have stressed they did not compare long COVID patients against healthy individuals who never had the coronavirus. It is therefore unclear whether some of the complications would have occurred without catching the infection.
The study has also not shed any light on how long COVID comes about.
"Theories include persistent immune activation after the acute phase, initial damage from the virus – such as damage to nerve pathways that is slow to heal – and persistent presence of low-level virus," according to the scientists.
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