Updated at 6.49pm: New tests on six patients suffering from fever and upper respiratory infections at Tuen Mun Hospital show five of them have a form of common cold virus, a health expert said on Thursday. The five have tested positive for the human coronavirus OC43, a spokesman for the Department of Health and the Hospital Authority explained. The spokesman said coronavirus OC43 was a common cause for cold or influenza with mild symptoms. These included fever, runny nose and a sore throat, the spokesman said. Two of the patients remain in a serious condition. The six are all women, aged from 19 to 29. They developed fever and respiratory tract infection symptoms between November 20 and 24. The outbreak sparked fears of a possible return of the severe acute respiratory syndrome (Sars) outbreak in Hong Kong. Sars plagued the territory in early 2003 and killed 299 people. Tests have also shown that the six are not suffering from Sars or influenza. The tests were conducted by the Public Health Laboratory Centre under the Centre for Health Protection (CHP) of the Department of Health. Human coronavirus OC43 has an incubation period ranging from two to four days. It was usually transmitted by inhalation of droplets or aerosols generated by the coughs and sneezes of infected individuals, the spokesman added. No new cases were reported on Thursday afternoon.