Why are some Covid-19 survivors at risk of blood clot? Thursday, April 15, 2021 IWK Bureau People who have recovered from Covid-19, especially those with pre-existing cardiovascular conditions, may be at risk of developing blood clots due to a lingering and overactive immune response, according to a study. The study, led by researchers from the Nanyang Technological University in Singapore, found that recovered Covid-19 patients had twice the normal number of circulating endothelial cells (CECs) that had been shed from damaged blood vessel walls. The elevated levels of CECs indicate that blood vessel injury is still apparent after recovering from viral infection.