Two women who arrived in the country in June and July were infected.
It is noted that a 40-year-old woman arrived at Narita Airport from the UAE in late June. A 50-year-old passenger flew from the UK to Haneda Airport in early July.
The WHO notes that Covid-strain Mu has mutations that scientists believe may reduce immunity after vaccination.
This new variant of the coronavirus was first detected in Colombia in January and has since been confirmed in at least 39 countries.
Earlier. it was reported that WHO monitored a new Covid-19 strain Mu that can be resistant to vaccines.