The health ministry began testing around 10,000 people for coronavirus antibodies Monday to get a better grasp of the nationwide scale of infection after the state of emergency was lifted last week.
The tests, which look for specific proteins made by the immune system in response to infection, are being held in Tokyo and in Miyagi Prefecture. Osaka will follow suit on Wednesday.
The government's aim is to find out roughly how many people were actually infected in the first wave, including those who showed no symptoms, to see if Japan achieved "herd immunity."