The first case of novel coronavirus infection within the imperial family was confirmed after Princess Yoko tested positive and was hospitalized, the Imperial Household Agency said Feb. 8.

No other imperial family member has been identified as a close contact, but her older sister, Princess Akiko, will take a polymerase chain reaction (PCR) test as a precaution, the agency said.

Yoko is the second daughter of the late Prince Tomohito, a cousin of Emperor Emeritus Akihito.

According to the agency, Yoko developed a sore throat on Feb. 7 and took a PCR test the following afternoon at the Akasaka Imperial Palace in Tokyo.

The positive result came back in the evening.

She is being treated at the Imperial Household Agency’s hospital on the premises of the Imperial Palace.

Other than a sore throat, she has not developed any symptoms. The source of infection remains unknown, the agency added.

Yoko visited the Imperial palace at the end of January to attend an imperial rite.

Agency officials said it is unlikely that Emperor Naruhito and other imperial family members who attended the event contracted the virus.