There is an increased risk of developing deep vein thrombosis (DVT) for six months after a COVID, a study published in the BMJ suggests, as well as increased risk of pulmonary embolism and bleeding for shorter periods. The risk was particularly evelated for those with severe COVID as well as those infected during the first wave.
This highlights the importance of COVID vaccination, the researchers said. While there has been concern over the risk of blood clots after vaccination, the risk is far smaller, according to a large study last year.
It had previously been observed that people who had COVID had an increased risk of blood clots, and the researchers wanted to find out when that risk returns to normal levels.
The researchers tracked the health of just over one million people in Sweden who tested positive for COVID between February 2020 and May 2021 in Sweden, comparing them against four million people age- and sex-matched non-infected individuals.
Adjusting for confounding factors such as comorbidities, cancer, surgery, long term anticoagulation treatment, previous venous thromboembolism, or previous bleeding event, the researchers found an increased risk of:
- first DVT, for up to three months
- first pulmonary embolism, for up to six months
- first bleeding event, such as a stroke, for up to two months
Comparing blood clot risk after COVID to the normal level of risk, the results showed that:
- four in every 10 000 COVID patients developed DVT compared with one in every 10 000 non-infected individuals
- about 17 in every 10,000 COVID patients had a blood clot in the lung compared with fewer than one in every 10,000 non-infected individuals
The authors wrote that the increased risk of blood clots was higher in the first wave than later waves, probably because treatments improved during the pandemic and older patients were starting to be vaccinated by the second wave.
Pulmonary embolism risk in people with severe COVID was 290 times greater than normal, and seven times higher than normal after mild COVID. However, there was no increase in bleeding risk in mild cases.
“For unvaccinated individuals, that’s a really good reason to get a vaccine – the risk is so much higher than the risk from vaccines,” said principal study investigator Anne-Marie Fors Connolly, from Umea University in Sweden.
While COVID’s causing the blood clots cannot be proven in this study, the researchers have a number of theories on the mechanism. It could be the direct effect of the virus on the layer of cells which line blood vessels, an exaggerated inflammatory response to the virus, or the body making blood clots at inappropriate times.
Though vaccines are very effective against severe COVID, but less so against infection, especially with the Omicron variant. This means repeat symptomatic infections are commonplace.
Source: BBC News
