Japan on Wednesday confirmed its first cases of the new mu variant of the coronavirus in two people arriving from abroad through airport screenings.

The health ministry announced that the mu variant was detected in a woman in her 40s who arrived on June 26 from the United Arab Emirates. The other case was with a woman in her 50s who arrived on July 5 from the U.K. Both women were asymptomatic upon arrival.

All travelers to Japan are required to take a PCR test upon arrival and wait for results — if they test positive, they are quarantined in a designated facility or hospitalized depending on their symptoms. Even if they test negative, they are required to quarantine for 14 days at home or a designated facility depending on where they have come from.

The ministry said it would continue to take measures to prevent the spread of the variant by closely monitoring the situation in other countries.

On Monday, the World Health Organization designated the mu variant, also known as B.1.621, as a variant of interest, the second-highest level in its classification of variants.

It remains unclear, however, how transmissible the mu variant is or whether it is resistant to vaccines.

But in a statement released Tuesday, the WHO warned that the variant “has a constellation of mutations that indicate potential properties of immune escape.” This means that vaccines or antibody treatments may not work as well against the mu strain as they do against the original version of the coronavirus.

In its weekly pandemic bulletin, the WHO added that preliminary data showed reduced effectiveness of vaccines against mu similar to that seen for the beta variant, which was discovered in South Africa and is considered to be more contagious than the original coronavirus.

At present, the WHO designates alpha, beta, gamma and delta as variants of concern, the highest classification.

The mu strain was first detected in Colombia in January and currently accounts for about 40% of cases in the country, according to the WHO. The mutation has so far been detected in at least 40 countries but makes up less than 0.1% of all cases worldwide, the organization said.

The WHO also said that since the variant was first identified, there have been a few sporadic reports of infections with the mu variant and even of some larger outbreaks in other countries in South America and in Europe.

The health ministry’s announcement follows a recent report about a new sublineage of the delta variant that emerged in Japan in mid-August. Delta, which originated in India, has become the most prevalent variant in Japan.

So far, the alpha, beta, delta, gamma, kappa, lambda and mu variants have been confirmed in Japan.

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