After a Covid infection, there is an increased risk of developing a serious blood clot for the next six months, a study from Sweden suggests. The research found people with severe Covid, and those infected during the first wave, had the highest clot risk. This highlights the importance of being vaccinated against the virus, the researchers say. Blood clots can also occur after vaccination but the risk is far smaller, a major UK study found. People who have had Covid-19 are more likely to develop a blood clot – particularly patients who have needed hospital treatment. Scientists wanted to find out when that risk returns to normal levels. The researchers tracked the health of just over one million people who tested positive for Covid between February 2020 and May 2021 in Sweden, and compared them with four million people of the same age and sex who had not had a positive test.