A member of the Japanese government's panel of experts on the coronavirus says the rapid spread of the highly transmissible BA.5 Omicron sub-variant may be one of the factors behind the surge in infections nationwide.

Toho University Professor Tateda Kazuhiro spoke to NHK after a daily caseload of more than 110,000 was recorded in Japan for the first time since the start of the pandemic on Saturday.

Tateda said people have become more active since the start of the month, resulting in more contact between them. He said the waning effects of third vaccine shots may also be a contributing factor.

Tateda said the number of new cases is doubling from a week earlier, and the daily tally may well exceed 200,000 in the coming week.

He said the number of severely-ill patients has yet to increase sharply. But he said caution is advised, because as caseloads surge, a certain proportion of patients will develop severe COVID-19.

As Japan is in a middle of a three-day weekend, Tateda said many people might be planning to travel. But he added that those who feel sick with cold-like symptoms should stay at home, or undergo testing for the coronavirus.

He said people feeling unwell should also avoid personal contact, especially with the elderly who face a higher risk of falling seriously ill.