Two of the new patients, all of whom were Thai nationals, had returned from vacation in Japan's northern island of Hokkaido and came into contact with the third boy, said Sukhum Kanchanapimai, permanent secretary at the Public Health Ministry.
The boy's grandfather, a 68-year old patient, had not disclosed his travel history when he first sought medical attention and risked becoming a super-spreader, an incident of large transmission from a single source.
The patient came into contact with 30 medical personnel at the hospital, who later tested negative for the virus and are in self-quarantine at home, the statement said.
Passengers, cabin crew and students in the boy's class were all being tested, officials said. The boy's school has been closed for 14 days and will be cleaned, a statement from the Don Mueng District Office said.