Japan may have entered its ninth wave of COVID-19 infections, according to an expert who has served as the government's top coronavirus adviser, who on Monday stressed the importance of protecting vulnerable elderly individuals from the disease.

"A ninth wave may have started," Shigeru Omi told reporters after meeting with Prime Minister Fumio Kishida to discuss the recent increase in infections nationwide after the government eased countermeasures, including downgrading the legal status of the disease in May to the same category as seasonal influenza.

"As people have been increasingly in contact with others, (the rise in infections) is as expected in a sense," Omi also said.