Japan's health ministry said Wednesday that 345 people in 21 of the country's 47 prefectures were found to have been infected with novel coronavirus variants first detected in Britain, Brazil and South Africa.

Based on data by Tuesday, the ministry said domestic testing found 271 people were infected with a variant, while 74 were discovered similarly infected by going through airport quarantine checks.

Supplied electron micrograph shows a variant strain of the novel coronavirus originating from Britain. (Photo courtesy of Japan's National Institute of Infectious Diseases)(Kyodo)

The Japanese government is concerned about the risk that variants may spread rapidly through the country and that currently available vaccines may be less effective against them, even as its vaccine roll-out started recently.

Shigeru Omi, head of the government's COVID-19 subcommittee warned that variants would become the dominant strain in Japan "sooner or later," at a parliamentary committee session on Wednesday.

Most people infected with the variants in Japan are infected with the British version, but the Brazilian and South African variants are considered to be particularly contagious.

All of the 77 people who tested positive with variants after taking domestic tests since Saturday were infected with the British variant.

By prefecture, Osaka had the most people who were infected with the variants at 62, according to the ministry. It was followed by Saitama with 41, Hyogo with 38 and Niigata with 32.

The data were gathered by "HER-SYS," an information-sharing system on infected people developed by the government to monitor the pandemic.

The government has been stepping up efforts to enable municipalities to administer polymerase chain reaction tests to detect virus variants. There were 1,200 such tests conducted between Feb. 22 through Feb. 28, according to officials.

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