Patients with mild symptoms are believed to be playing an important role in spreading the new coronavirus that causes COVID-19, a government panel of experts has said.

As most patients above age 10 and below 40 only suffer mild symptoms of the virus and people in that age group interact with others frequently, they may be causing further infections unknowingly, the panel said Monday.

It made the comments after analyzing data mainly from Hokkaido, where a state of emergency was declared in relation to the virus last week.

The panel said that 80 percent of people who contracted the virus in Japan have not spread infections to other people.

There have been cases in which one virus carrier created a cluster of patients in a closed indoor space, such as a gym or a club, it said.

Referring to the situation in Hokkaido, the panel said it is possible that travelers from China who visited the prefecture for a snow festival caused infections in urban areas and that young people who became infected then caused further infections in other regions.