Can tiny blood clots cause Long COVID

Nature: Could tiny blood clots cause long COVID’s puzzling symptoms?

This is a very comprehensive review in Nature of the debate into whether micro-clots are involved in the causation of Long Covid – and possibly ME/CFS as well. Contains opinions from both sides of the debate.

Dr Charles Shepherd

Nature has an article examining whether Long COVID could be caused by micro-blood clots. The full article can be read from the button below.


Researchers are baffled by long COVID: hundreds of studies have tried to unpick its mechanism, without much success. Now some scientists, and an increasing number of people with the condition, have been lining up behind the as-yet-unproven hypothesis that tiny, persistent clots might be constricting blood flow to vital organs, resulting in the bizarre constellation of symptoms that people experience.

But many haematologists and COVID-19 researchers worry that enthusiasm for the clot hypothesis has outpaced the data. They want to see larger studies and stronger causal evidence. And they are concerned about people seeking out unproven, potentially risky treatments.

When it comes to long COVID, “we’ve now got little scattered of bits of evidence”, says Danny Altmann, an immunologist at Imperial College London. “We’re all scuttling to try and put it together in some kind of consensus. We’re so far away from that. It’s very unsatisfying.”

Many researchers find it plausible and intriguing that micro-clots could be contributing to long COVID. And the hypothesis does seem to fit with other data that have emerged on clotting. Researchers already know that people with COVID-19, especially severe disease, are more likely to develop clots. The virus can infect cells lining the body’s 100,000 kilometres of blood vessels, causing inflammation and damage that triggers clotting.

Those clots can have physiological effects. Danny Jonigk, a pathologist at Hanover Medical School in Germany, and his colleagues looked at tissue samples from people who died of COVID-19. They found micro-clots and saw that the capillaries had split, forming new branches to try to keep oxygen-rich blood flowing10. The downside was that the branching introduces turbulence into the flow that can give rise to fresh clots.

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