Four travelers from Brazil's Amazonas province, Japan's Health Ministry reports, have found a new coronavirus variant, the first new virus mutation discovered.

The effectiveness of vaccines against the new strain, which varies from the highly contagious strains first identified in Britain and South Africa that have caused an increase in cases, has now been investigated.

Takaji Wakita, head of the National Institute of Infectious Diseases, told a health ministry briefing on Sunday, "At the moment, there is no evidence showing the latest variant found in those from Brazil is high in infectiousness."

Nevertheless, the Brazilian Ministry of Health reported that the Japanese authorities had confirmed that the new variant had 12 mutations, one of which had already been identified in the variants found in the United Kingdom and South Africa.

"It implies a potential for greater infectiousness of the virus," it said.

A man in his forties had difficulty breathing, a woman in her thirties had a headache and sore throat, and a man in his teens had a fever, while a woman in her teens showed no symptoms, the health ministry said, of the four travelers who arrived at Tokyo's Haneda airport on January 2.

All travelers at Tokyo's airport are in quarantine, Brazil's Ministry of Health said.

Japan announced a state of emergency for Tokyo and three neighboring prefectures in the capital on Thursday after seeing a sharp increase in coronavirus cases.

There were approximately 289,000 cases nationwide, with 4,061 deaths, public broadcaster NHK said.