A medical facility worker in his 30s was taken to hospital on 12 May after receiving the jab on 27 April. He developed a headache on 9 May before falling seriously ill, and also suffered muscle cramps.
Jeong Eun-kyeong, director of the Korea Disease Control and Prevention Agency (KDCA), said in a statement: “His condition has improved since then and we understand there is no big problem though he still needs monitoring.”
The symptoms of blood clotting are curable if detected early and treated properly, she added.
About 3.27 million people in South Korea have been given at least one dose of the AstraZeneca vaccine so far. Seoul limited its use to the over-30s following the initial reports of blood clotting elsewhere in the world.
Those reports have mainly concerned a type of blood clot called cerebral venous sinus thrombosis. The condition has been seen in combination with low blood platelet levels, called thrombocytopenia.
Medical regulators in the US and Europe have concluded that there appears to be a link between AstraZeneca's vaccine, as well as one made by Johnson & Johnson, and the clots, but they say the shots are safe and that the benefits of being vaccinated outweigh the risks.
AstraZeneca has said it is working to understand individual cases and “possible mechanisms that could explain these extremely rare events”, while J&J has said that no clear causal relationship has been established between the vaccine and the clots.
The KDCA reported 430 new coronavirus cases as of Sunday, for a total of 140,340 cases with 1,959 deaths.
Additional reporting by Reuters