Two Japanese who had been subject to home quarantine in South Korea for Middle East respiratory syndrome were back in Japan as of Monday, a source close to Japan-South Korea relations said.

They are believed to be among 20 to 30 foreign nationals who senior South Korean health ministry official Kwon Jun-wook said have been subject to home quarantine. None showed symptoms and all have tested negative for the virus, according to the South Korean official.

It remains unknown why the two Japanese could be repatriated when South Korea has banned people under quarantine to leave the country. The pair may have left before they were notified of the quarantine measure.

An official at the Health, Welfare and Labor Ministry confirmed the information regarding the two Japanese subject to home quarantine.

“We are aware of such people, but they have shown no symptoms and pose no contagion risks,” the official said.

The official said South Korea “considerably expanded the scope” of home quarantine and its criteria do not necessarily match those for Japan’s “health monitoring” measures.

The health ministry has stepped up Japan’s border checks for quarantine and adopted response measures since June 4 because of the MERS outbreak in South Korea.

People arriving or returning from South Korea will be subject to “health monitoring” even if they have exhibited no symptoms, such as fever, if they had contact with a MERS patient or stayed in the same home or hospital as a patient. They will be required to have their temperatures taken twice a day for the virus’s 14-day incubation period.

If a fever is detected in the monitoring period, a virus detection check will be conducted on the person at the nearest regional public health institute and at the National Institute of Infectious Diseases in Tokyo. If confirmed positive, an announcement will be made, officials said.