One of the perils of COVID-19 is the lingering aftereffects of the illness – including fatigue, headaches, “brain fog,” breathlessness, and many more symptoms – that can continue to lurk for months after the initial infection. Known as long-COVID, it’s thought millions upon millions of people worldwide are suffering from the condition, but the underlying cause of the symptoms remains uncertain. 

Previous explanations have ranged from persistent organ damage to "dead" relics of the virus sparking an immune response, but a new theory suspects another culprit: inflammatory micro clots clogging up the blood.

In August 2021, a team of researchers from Stellenbosch University in South Africa published a study in the journal Cardiovascular Diabetology that found the blood of individuals with long-COVID contains various inflammatory molecules trapped in micro clots.

Blood has to strike a fine balance when it comes to clotting – it needs to thicken and coagulate to prevent blood loss after an injury, but it also must break down the fibrin in the coagulated blood to prevent clots from blocking up blood vessels. In COVID long-haulers, this balance appears to be upset. The team found that the blood of COVID-19 patients and people suffering from long-COVID had micro clots that trapped vital inflammatory clotting proteins such as fibrinogen, as well as alpha(2)-antiplasmin (α2AP) protein, which prevents the breakdown of blood clots. Compared to healthy people, α2AP levels were almost eight times higher in long-COVID patients and over nine times higher than in those with acute COVID-19.

The strong presence of this protein reduces the body’s normal ability to break down the clots. An increase in micro clots raises the risk of blockages in the microcapillaries, which could potentially deprive parts of the body of oxygen, leading to symptoms like tiredness and breathlessness.  

Writing in an op-ed for the Guardian this week, the lead researcher on this project, Resia Pretorius, a distinguished research professor in the physiological sciences department at Stellenbosch University, explains that traditional blood tests do not pick up on this micro clot formation and the presence of inflammatory molecules. In their research, they were only detectable under a fluorescence or bright-field microscope, which can be a time-consuming and pricey process. 

If their theory is correct, it could point towards possible treatments for long-COVID. For instance, it further backs up the idea that antiplatelet and “blood thinner” medicines could be used to treat chronic long-COVID. Alternatively, patients could potentially receive a dialysis-like treatment that filters micro clots and inflammatory molecules from the blood.

For now, none of this is set in stone and it’s likely that there are a variety of explanations behind long-COVID. A solid explanation can’t come soon enough, however. Some health experts are concerned that the millions of cases of long-COVID created during the pandemic could lead to a major global health problem in the years and decades to come.