COVID-19 and Blood Clots: What the Evidence Shows

Among the less commonly discussed effects of COVID-19 is its impact on blood clotting, an area where the evidence has become considerably clearer since the early pandemic. Understanding this risk matters both for people currently unwell with COVID-19 and for anyone recovering from a past infection.

The basic mechanism

There is mounting evidence that COVID-19 causes abnormalities in blood clotting, which can in turn cause heart attacks and strokes. Patients with severe COVID-19 infection appear to be at greater risk of developing blood clots in both the veins and arteries, a pattern that's been documented consistently across research since early in the pandemic.

The specific types of clots involved

Blood clots can form deep in the veins of the leg, a condition called deep vein thrombosis or DVT and these clots can potentially move to the lungs, causing a pulmonary embolism that restricts blood flow and oxygen and can, in serious cases, be fatal. Separately, blood clots forming in arteries can cause heart attacks by blocking blood supply to the heart, or strokes by blocking oxygen supply to the brain. These are genuinely serious potential complications, which is why clotting risk has received sustained research attention.

How big is the overall population impact?

Research has estimated the COVID-19 pandemic may have led to an additional 10,500 cases of heart attacks, strokes and other blood clot complications, including DVT, in England and Wales during 2020 alone. It's worth being clear that this is a population-level figure; the excess risk to any single individual remains comparatively small and that risk reduces over time following infection.

How long does the increased risk last?

Research has found clotting-related risk remained elevated for some complications up to 49 weeks, nearly a year, after infection. At that point, the risk of deep vein thrombosis specifically was found to be nearly double in people who'd had COVID-19 compared with those who hadn't. This is a longer persistence of risk than many people might expect, extending well beyond the acute illness itself.

Does long COVID play a role too?

Yes, research has specifically linked long COVID to an increased risk of abnormal blood clotting, suggesting the connection between COVID-19 and clotting risk isn't limited to the acute infection period. This adds blood clot risk to the growing list of ways long COVID can affect the cardiovascular system beyond the more commonly discussed symptoms like fatigue and brain fog.

Who faces the highest risk

People with severe COVID-19 illness, particularly those hospitalised, face the clearest elevated risk, though milder cases haven't been shown to be entirely without risk either. People with pre-existing risk factors for blood clots, including certain genetic conditions, obesity, or a personal or family history of clotting problems, may face a compounded risk when combined with a COVID-19 infection.

Warning signs worth knowing

Symptoms of DVT include swelling, pain, warmth or redness, usually in one leg rather than both. Pulmonary embolism can cause sudden breathlessness, chest pain that may worsen with breathing and a rapid heartbeat. Given how serious these complications can be, any of these symptoms following a COVID-19 infection, even weeks or months later, deserves prompt medical attention rather than being dismissed as unrelated. NHS guidance on deep vein thrombosis sets out these symptoms in more detail and is worth reading if you want a fuller picture of what to watch for.

Hospital prevention measures

If you're hospitalised with COVID-19, clotting risk is something hospital teams actively manage rather than simply monitor passively. Measures commonly used include blood-thinning medication given as a preventative measure during your stay, compression stockings to improve circulation in the legs and encouraging movement and mobility as soon as it's medically appropriate, all specifically aimed at reducing clot risk during a hospital stay. If you're discharged still at elevated risk, you may be sent home with a short course of blood-thinning medication and clear guidance on the specific symptoms that should prompt you to seek urgent care.

Balancing awareness with proportion

It's worth balancing genuine awareness of this risk with a sense of proportion. The large majority of people who have COVID-19, including many with fairly significant illness, don't go on to develop a blood clot and the population-level figures reflect a real but comparatively small increase in risk across a very large number of infections. The goal of knowing the warning signs isn't to create anxiety about every ache or twinge after COVID-19, but to make sure genuine warning signs get the prompt attention they deserve if they do occur.

A quick recap

COVID-19 has been clearly linked to an increased risk of blood clots, including DVT, pulmonary embolism, heart attacks and strokes, with this risk remaining elevated for many months after infection and connected to long COVID too. While individual risk remains relatively small, knowing the warning signs and seeking prompt medical attention if they appear is a sensible precaution following any COVID-19 infection.

Frequently asked questions

Does the COVID-19 vaccine also increase blood clot risk?

A rare clotting condition was linked to certain adenovirus-vector COVID-19 vaccines early in the vaccination programme, though this risk has been extensively studied, is very rare and is considerably smaller than the clotting risk associated with COVID-19 infection itself.

Should everyone who's had COVID-19 be screened for blood clots?

Routine screening isn't recommended for everyone who's had COVID-19; screening or further investigation is generally reserved for people who develop symptoms suggestive of a blood clot.

Does mild COVID-19 carry the same clotting risk as severe illness?

Severe illness carries a clearer, more pronounced elevated risk, though research hasn't found mild infections to be entirely without any clotting risk either, just a comparatively smaller one.

Can lifestyle changes reduce clotting risk after COVID-19?

General measures that reduce clotting risk more broadly, staying active as your recovery allows, staying well hydrated and not smoking, are sensible steps, though they aren't a guaranteed way to eliminate any COVID-19-related increase in risk.

How long after COVID-19 should I stay alert to clotting symptoms?

Given research showing elevated risk up to around a year after infection, it's worth staying aware of the warning signs for a similarly extended period, particularly if you had a more severe initial illness.

Are certain ethnicities or age groups at higher clotting risk from COVID-19?

Some research has suggested a degree of variation in clotting risk across different demographic groups, broadly similar to wider patterns already well documented in COVID-19 severity more generally, though individual personal risk factors such as pre-existing underlying health conditions typically matter considerably more overall than demographic group membership on its own truly does.

Does having a COVID-19 vaccine reduce the clotting risk from infection?

Since vaccination reduces the risk of severe COVID-19 illness generally and clotting risk is linked to illness severity, vaccination is reasonably expected to reduce clotting risk indirectly by reducing the chance of a severe infection in the first place.

Should I take aspirin or another blood thinner as a precaution after COVID-19?

No, starting any blood-thinning medication should only ever be done on specific individual medical advice from a qualified doctor, since these medications carry their own separate risks and aren't recommended as a routine, unsupervised self-directed precaution to take following a typical COVID-19 infection.