The risk of deep vein thrombosis (DVT) is increased for up to three months after Covid-19 infection, while the risk of pulmonary embolism is increased for up to six months and bleeding for up to two months, a large observational study has found.

Patients with underlying conditions and those who have experienced a severe Covid-19 infection, especially those who had been admitted to intensive care, were at a higher risk of an event, according to the study of more than one million people who had been infected with Covid-19 and four million case-matched controls who had not had the virus.

There was also a higher risk of events in the first wave of the pandemic, compared with the second and third waves. The researchers suggested this could be due to treatment improvements, the widespread use of thromboprophylaxis after the first wave as well as increased vaccine coverage in older adults.

Writing in the BMJ, the researchers said the results supported thromboprophylaxis in people who have had Covid-19, particularly in high-risk patients, and strengthened the importance of vaccination against the virus.

The study, which examined data from national registries in Sweden, used a self-controlled case series and a matched cohort analysis, with the increased risks consistent across both study methods.

After adjusting for other potential risk factors - such as comorbidities, cancer, surgery - the researchers found a fivefold increase in risk of DVT, a 33-fold increase in risk of pulmonary embolism, and an almost twofold increase in risk of bleeding in the 30 days after infection.

Even non-hospitalised Covid-19 patients were at increased risk of DVT and pulmonary embolism, but the increased risk of bleeding was only observed in more severe cases.

A linked editorial stressed that the study findings remained relevant even though 65% of the world’s population had now received at least one vaccine dose.

‘Current vaccines are highly effective against severe Covid-19 but confer only moderate protection against infection with the omicron variant,’ the editorial said.

‘Although many infections with the omicron variant are mild, the new study confirms an increased risk of venous thromboembolism even among those with milder infections who do not require admission to hospital.’

Despite the potential for new variants of concern, most governments were removing restrictions in an effort to live with Covid-19, so the study was a reminder of the need to remain vigilant for complications associated with even mild SARS-CoV-2 infection, including thromboembolism, the editorial concluded.

The study comes after a UK Health Security Agency (UKHSA) review in February found vaccination against Covid-19 also protects against long Covid.