A possible link between Covid-19 and the risk of thrombosis - the formation of blood clots - in patients has led to some concerns that people who do not know they have been infected may still be at risk of the more serious consequences of these blood clots.
Dr Sriram Narayanan, senior consultant vascular and endovascular surgeon at Gleneagles Hospital's Harley Street Heart and Vascular Centre, said he is concerned about those who may have been exposed to the coronavirus but were never diagnosed with Covid-19.
He said the evidence so far points to such patients being at a higher risk of thrombosis, even after they recover from Covid-19.
Thromboembolism, which refers to the blockage of a blood vessel by a blood clot that has formed and travelled from another part of the body, can have severe consequences.
Assistant Professor Christine Cheung, principal investigator at the Laboratory of Molecular and Vascular Medicine at Nanyang Technological University's Lee Kong Chian School of Medicine, said victims are at risk of heart attack, stroke and lung embolism, where blood clots block major arteries supplying oxygen and nutrients.
"If the clot is large and stops blood flow to these vital organs, large extent of tissue damage can be deadly," she said.
While clots can form in various body parts, they occur most commonly in the veins. This is known as venous thromboembolism (VTE).
Dr Sriram said: "Veins carry or hold about 70 per cent of our blood volume at any one time, and they take blood back to the heart. So, when clots form in the veins in the legs, if they travel to the heart and lungs, it could potentially be fatal - or if they go to the brain, they can cause a stroke."
He said that before the Covid-19 outbreak, those most at risk of VTE were patients who had major surgery or passengers on long-haul flights. Even then, the risk was less than 4 per cent and 1 per cent, respectively.
Covid-19 patients appear to be at a much higher risk. Dr Sriram noted that globally, about 30 per cent to 50 per cent of such patients in intensive care units have developed VTE.
This is exacerbated by the fact that many people with chronic conditions that increase their risk of thrombosis - such as obesity, diabetes and heart disease - are avoiding medical check-ups during the current outbreak, he added.
Dr Shawn Vasoo, clinical director of the National Centre for Infectious Diseases (NCID), said: "Covid-19 has been associated with a tendency to form blood clots. This seems to occur in persons with more severe disease where clotting factors are increased."
Professor Dale Fisher, a senior infectious diseases expert at the National University of Singapore's Yong Loo Lin School of Medicine, said: "In some cases, the low oxygen levels appear out of proportion and suggest that clots in the arteries to the lungs could be playing a role."
How to lower risk of developing blood clots
People could have been exposed to the coronavirus and not know it, running a higher risk of developing blood clots. Dr Sriram Narayanan of Gleneagles Hospital's Harley Street Heart and Vascular Centre offers three tips to protect yourself.
• First, stay mobile and drink enough water, even though you may be going out less often. Walking about keeps your blood moving, lowering the risk of clots forming, while staying hydrated reduces the chances of your blood thickening.
• Second, if you have a chronic condition such as heart disease, hypertension or diabetes, you should continue to have it monitored, whether in person with a doctor or via teleconsultation. You should also stick to your medication routine. This is because such conditions are linked to an increased risk of thrombotic events.
• Finally, see a doctor if you develop symptoms such as a mild chest pain, minor limb swelling or aches in limbs that did not previously hurt. These could be signs of clot formation.
Last month, neurologists at University College London found that some patients had markedly raised blood levels of a protein fragment associated with abnormal clotting.
Scientific journal Nature also reported this month that studies from France and the Netherlands had found that clots might occur in 20 per cent to 30 per cent of critically ill Covid-19 patients.
Research into the exact link between Covid-19 and thromboembolism is ongoing.
Prof Cheung said the ACE2 receptor, which the coronavirus uses to enter cells, helps to moderate inflammation and the constriction of blood vessels. She said one theory is that the virus may disrupt the effect the receptor has on blood vessels, promoting the formation of clots.
Measures are in place here to protect hospital patients from developing such complications.
Dr Vasoo said critically ill patients in NCID may be assessed for certain laboratory markers that could be a sign of clotting. High-risk patients may be given blood thinners, while imaging studies may be done if a clot is suspected.
Prof Fisher said that in general, Singapore looks to give blood thinners to Covid-19 patients in hospital as a preventive measure, while looking for evidence of clots which would require bigger doses and possibly different blood thinners.
"We are learning more about treatment every day and certainly, the role of blood clots and the need for blood thinners will be a part of the story," he added.
Dr Sriram advised people to stay mobile and hydrated, monitor any chronic conditions and see a doctor if they develop symptoms such as mild chest pain or minor limb swelling.
"Please don't ignore these conditions - they could be markers of something else."