A group of 21 people at Japan's national infection research center attended a farewell party in mid-March, when a state of emergency over the novel coronavirus was still in place in Tokyo and neighboring areas, according to health ministry officials.

"Although measures against infections were appropriately taken, it aroused public mistrust," health minister Norihisa Tamura told a press conference on Friday, referring to the party at the National Institute of Infectious Diseases, news of which came to light late Thursday.

The participants, including officials of the institute and doctors in training, were at the party, during which food and beer were served, but none of them have complained of poor health, according to the ministry.

The ministry-affiliated institute has played a key role in Japan's fight against the COVID-19 disease caused by the virus.

Takaji Wakita, the head of the National Institute of Infectious Diseases, speaks at a House of Councillors committee session in Tokyo on Feb. 2, 2021. (Kyodo)

Takaji Wakita, chief of the Tokyo-based institute, said the party, which lasted about one hour from 6 p.m. on March 18, was a "deed that led to misunderstanding" as he had instructed officials to avoid dining with others.

But Wakita said there was no problem in the way it was done because the participants were wearing face masks during the party and only 15 minutes were allocated for eating and drinking.

Wakita, who also leads the ministry's coronavirus countermeasures advisory panel, added he has given verbal warnings to the attendees.

The ministry is also facing criticism after 23 officials at its Division of the Health for the Elderly held a late-night party on March 24, despite the Tokyo metropolitan government's request for shorter business hours to curb the spread of the virus.

Three of the participants have contracted the virus, according to the ministry.