Long COVID may be caused by the virus changing a patient's blood cells, research suggests.
Not everyone who overcomes the virus returns to a clean bill of health.
Regardless of the severity of the infection itself, former coronavirus carriers have reported everything from fatigue to organ damage, despite supposedly clearing the virus from their body.
The latest Office for National Statistics data suggests close to one million people in the UK are enduring symptoms more than four weeks after their initial infection.
Long COVID's cause is something of a mystery. In certain patients, the coronavirus may cause the immune system to trigger excessive inflammation, leading to lasting damage. The infection may also linger in bodily tissues, outside of the airways.
Writing in the journal Biophysical Journal, a team of German scientists has found the coronavirus can change the size and stiffness of some patients' blood cells.
With blood cells involved in a person's immune response and transporting oxygen around the body, these deformations may trigger lingering complications.
In severe incidences, the coronavirus can make a patient's blood stickier, leading to clots and even strokes.
With blood complications linked to the infection itself, the German scientists set out to uncover whether these cells may also be involved in long COVID's onset.
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Scientists from the Max-Planck-Zentrum für Physik und Medizin analysed more than four million blood cells.
These were extracted from 17 long COVID patients, 14 people who had recovered from the coronavirus and 24 healthy volunteers, who acted as the control group.
In a process called real-time deformability cytometry, up to 1,000 blood cells were sent through a narrow channel per second.
This stretched the blood's immune cells and red blood cells, which carry oxygen around the body. Finally, a camera recorded the cells' size and any deformation.
Results reveal the red blood cells of the long COVID participants "deviated strongly" from those of the healthy control group, suggesting cell damage.
"We were able to detect clear and long-lasting changes in the cells, both during an acute infection and even afterwards," said co-lead author Professor Jochen Guck.
This may explain how the infection increases the risk of blocked blood vessels and clots in severe cases.
How oxygen is transported around a patient's body may also be impaired.
The results also reveal immune cells were "softer" in the long COVID patients than those of the volunteers' - an indicator of an overactive immune response.
"We suspect the cytoskeleton of immune cells, which is largely responsible for cell function, has changed," co-author Markéta Kubánková.
Overall, the long COVID patients' cells remained "drastically altered" seven months after the initial infection.
"While some of these changes recovered to normal values after hospitalisation, others persisted for months after hospital discharge, evidencing the long-term imprint of COVID-19 on the body," wrote the scientists.
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