Study finds increased risk of serious blood clots up to six months after Covid-19.
A study from Sweden published in The BMJ finds an increased risk of deep vein thrombosis (a blood clot in the leg) up to three months after Covid-19 infection, pulmonary embolism (a blood clot in the lung) up to six months, and a bleeding event up to two months.
The findings also show an increased risk of events in patients with underlying conditions (comorbidities), patients with more severe Covid-19, and during the first pandemic wave compared to the second and third waves.
The researchers say these results support measures to prevent thrombotic events (thromboprophylaxis), especially for high-risk patients, and reinforce the importance of vaccination against Covid-19.
It is well known that Covid-19 increases the risk of serious blood clots (known as venous thromboembolism or VTE), but there is less evidence on the duration of the increase in this risk, whether the risk changed during pandemic waves, and if Covid-19. 19 also increases the risk of major bleeding.
To address these uncertainties, the researchers set out to measure the risk of deep vein thrombosis, pulmonary embolism and bleeding after Covid-19.
Using national registries in Sweden, they identified more than one million people with confirmed SARSCoV-2 infection (the virus responsible for covid-19) between February 1, 2020 and May 25, 2021, matched by age, sex and county of residence to more than four million people who had not tested positive for SARS-CoV-2.
They then performed two analyses: in the first, they calculated the rates of deep vein thrombosis, pulmonary embolism and bleeding in people with Covid-19 during a control period (before and long after Covid-19 diagnosis) and compared it with the rates at different time intervals after the diagnosis of covid-19 (days 1-7, 8-14, 15-30, 31-60, 61-90 and 91-180).
In the second analysis, they calculated the rates of deep vein thrombosis, pulmonary embolism, and bleeding during the period from 1 to 30 days after Covid-19 diagnosis in the Covid-19 group and compared them with the corresponding rates in the Covid-19 group. control.
The results show that, compared to the control period, the risks increased significantly 90 days after Covid-19 for deep vein thrombosis, 180 days for pulmonary embolism and 60 days for bleeding.
After taking into account a variety of potentially influential factors, the researchers found a five-fold increase in the risk of deep vein thrombosis, a 33-fold increase in the risk of pulmonary embolism, and a nearly two-fold increase in the risk of bleeding within 30 days of infection.
In absolute terms, this means that a first deep vein thrombosis occurred in 401 patients with Covid-19 (absolute risk 0.04%) and 267 control patients (absolute risk 0.01%). A first event of pulmonary embolism occurred in 1,761 patients with covid-19 (absolute risk 0.17%) and 171 control patients (absolute risk 0.004%), and a first event of hemorrhage occurred in 1,002 patients with covid-19 ( absolute risk 0.10%) and 1,292 control patients (absolute risk 0.04%).
The risks were higher in patients with more severe Covid-19 and during the first pandemic wave compared to the second and third waves, which the researchers said could be explained by improvements in treatment and vaccine coverage in older patients after the first wave.
Even among non-hospitalized mild Covid-19 patients, researchers found an increased risk of deep vein thrombosis and pulmonary embolism. No increased risk of bleeding was found in mild cases, but a notable increase was seen in more severe cases.
This is an observational study, so the researchers cannot establish cause and acknowledge several limitations that could have affected their findings. For example, VTE may have been underdiagnosed in patients with Covid-19, Covid-19 testing was limited, especially during the first pandemic wave, and information on vaccination was not available.
However, the results were largely consistent after further analysis and are in line with similar studies on the association between Covid-19 and thromboembolic events, suggesting that they withstand scrutiny.
As such, the researchers say their findings suggest that Covid-19 is an independent risk factor for deep vein thrombosis, pulmonary embolism and bleeding, and that the risk of these outcomes increases for three, six and two months after covid-19, respectively.
“Our findings possibly support thromboprophylaxis to avoid thrombotic events, especially for high-risk patients, and strengthen the importance of vaccination against covid-19,” they conclude.
In a linked editorial, researchers at the University of Glasgow point out that, despite the potential for new variants of concern, most governments are removing restrictions and shifting their approach to determining how best to "live with Covid."
However, they say this study “reminds us of the need to remain vigilant for complications associated with even mild SARS-CoV-2 infection, including thromboembolism.”
Reference : Risks of deep vein thrombosis, pulmonary embolism, and bleeding after covid-19: nationwide self-controlled cases series and matched cohort study doi: 10.1136/bmj-2021-069590
