CAIRO (Reuters) – Twelve asymptomatic cases of the coronavirus have been registered on board a Nile cruise ship, the World Health Organization (WHO) and Egypt’s health ministry said on Friday.

Asymptomatic cases are those who carry the virus but display no symptoms of illness such as fever. There were no further details from the WHO about the 12 individual cases mentioned in a joint statement issued by the WHO and the health ministry.

WHO epidemiologist Maria van Kerkhove said on Thursday the organization did not believe transmission of the disease by asymptomatic people was a major factor fuelling its spread.

“We know that is possible but we do not believe that is a major driver of transmission,” she told a Geneva news briefing…If we look at the actual epidemics and how these epidemics are unfolding, if they were a major driver of transmission it would have caused much larger numbers of cases.”

Through tracing people exposed to the virus in some countries, asymptomatic carriers are being identified, she said. “So these are people who are pre-symptomatic, so it is a few days before they develop symptoms,” she added.

The cruise ship was heading to the southern Egyptian city of Luxor from Aswan, further upstream on the Nile, the joint statement said. It is unclear how many people were on board at the time.

The 12 cases were diagnosed after a Taiwanese-American woman who had been on the cruise tested positive for the coronavirus upon her return home, the statement said. She was believed to be the individual who spread the virus to the others.

The 12 new cases will be transferred to an isolation facility and all others who had been in contact with them will be placed under a 14-day quarantine, the ministry said.

The cruise ship, named River. A, was towed to an area approximately 20 km (12 miles) away from Luxor and placed under quarantine as well, state-owned online newspaper Al-Ahram Gate reported, citing a security source.

The paper also said the ship had arrived in Luxor on Thursday after a two-day Nile voyage, passing by tourist sites such as the Kom Ombo temple and Edfu.

Egypt had until now diagnosed only three people with the virus, one of whom it said had fully recovered after receiving treatment. The virus surfaced in China latest last year and has spread to over 85 countries, with more than 100,000 infections and 3,300 deaths.

(Reporting by Omar Fahmy in Cairo and Stephanie Nebehay in Geneva with additional Reporting by Hesham Abdul Khalek in Cairo; writing by Nadine Awadalla; editing by Mark Heinrich)