Monday’s news that Japan detected its first case of the highly transmissible XE variant of the coronavirus adds another concern for the government, which is struggling to raise vaccination rates among young people as infection levels remain high nationwide and are surging in mostly rural prefectures.

The health ministry announced Monday evening that the XE variant was detected in a woman in her 30s who arrived at Narita Airport from the United States on March 26. The woman, whose nationality has not been released, was asymptomatic, treated at a facility for infected individuals and released after her quarantine period ended, according to the ministry.

XE is among a new set of highly transmissible variants known as “recombinant variants.” A recombinant variant occurs when an individual becomes infected with two or more variants at the same time, leading to a mixing of their genetic material within the patient’s body.