A 23-year-old army conscript died on Tuesday, one day after he received a second dose of the AstraZeneca vaccine.
Khanti Anantasiri, who was based with the army in Tak province, died at his home in Chom Bung district of Ratchaburi province.
He had earlier been inoculated with the Sinovac vaccine.
He took leave to get a second dose, and was given the AstraZeneca vaccine at a gymnasium in Muang district of Ratchaburi on Monday, the Thai media reported yesterday.
After receiving the second dose at about 2.45pm, Khanti returned home.
His father Chaidet Anantasiri, 54, said that around 10pm that day his son complained of itchiness and fever and took two pills to bring his temperature down.
On Tuesday morning, he went to his son's room and asked if he wanted anything to eat. Khanti replied he was not hungry. He told his son to buy food at a shop in front of the house if he changed his mind.
Around noon, his mother brought him some food and said he looked normal.
Mr Chaidet said relatives went to check on his condition in the afternoon before going off to work.
Around 3.45pm Khanti's mother went to see him and was shocked to see her son with his lips turning green. An hour later dark spots appeared on his chest.
Mr Chaidet said his youngest son died about 24 hours after receiving his second dose of vaccine. Khanti had no underlying health conditions.
The father said he believed his son died as a result of the vaccination. His son's body was sent to Ratchaburi Hospital for an autopsy.
Khanti's elder brother, Santisuk, told reporters he had filed a form seeking compensation for his brother's death.