People who have recovered from Covid-19 may be at risk of developing blood clots due to a 'lingering and overactive immune response', according to a study.

Those with pre-existing cardiovascular conditions including high blood pressure, are said to be most at risk.

A team of researchers collected and analysed blood samples from 30 patients, a month after they had recovered from the infection and were discharged from hospital.

They found that all recovered patients had signs of blood vessel damage.

They had twice the normal number of circulating endothelial cells (CECs) - which line all blood vessels -  that had been shed from damaged blood vessel walls. 

The study also found that recovered patients continued to produce high levels of cytokines – proteins produced by immune cells that activate the immune response against pathogens – even in the absence of the virus.

Unusually high numbers of immune cells, known as T cells, that attack and destroy viruses were also present in the blood.

READ MORE: Covid has created 'perfect storm' for increased heart deaths 

The presence of both cytokines and higher levels of immune cells suggest that the immune systems of recovered patients remained activated even once the virus was gone.

Researchers from the Lee Kong Chian School of Medicine in Singapore hypothesise that these persistently activated immune responses may attack the blood vessels of recovered Covid patients, causing even more damage and increasing the risk of blood clot formation further.

In some cases, they are at increased risk of heart attack, stroke or organ failure when blood clots block major arteries to vital organs.

A study is underway at the University of Glasgow looking at the impact of Covid on blood pressure. 

It will look at routinely collected health records for people in the West of Scotland who attended hospital or had a positive test for Covid-19 between April 2020 and April 2021.

READ MORE: Heart function may play key role in Covid deaths, Scots cardiology experts say 

Assistant Professor Christine Cheung, who led the Singapore research, said: "With more people recovering from Covid-19, we started hearing from clinicians about patients returning with blood clotting issues after they had been discharged and cleared of the virus.

"This makes a strong case for the close monitoring of recovered Covid-19 patients, especially those with pre-existing cardiovascular conditions like hypertension and diabetes who have weakened blood vessels.

“Those with cardiovascular conditions need to be more cautious since their underlying conditions already weaken their vascular systems.

"It’s a double blow with Covid-19."

The study’s key findings could help inform guidelines for post-hospitalisation care of patients who might be susceptible to ‘long Covid' symptoms, said the research team.

Meanwhile, doctors in the UK say patients worried about blood clots after having the AstraZeneca vaccine are swamping hospital A&E departments.

Last week, the Joint Committee on Vaccination and Immunisation (JCVI), the UK Government's vaccine advisory body, issued updated guidance advising that under-30s should be offered an alternative to the AstraZeneca Covid-19 vaccine, if available.

It followed a review by the MHRA, which found that by the end of March, 79 people in the UK had suffered rare blood clots after vaccination - 19 of whom had died.

The death of a young patient in Northern Ireland two weeks after they received the  vaccine is now being investigated to establish whether the jab played a role in the tragedy.

The patient died last month with a blood clot on their brain and a low platelet count.

The research was published on March 23 in the peer-reviewed scientific journal eLife.