Further studies are needed to ensure the coronavirus that has caused the SARS epidemic in human beings is 100 per cent similar to that carried by animals, said an official from the World Health Organization (WHO) yesterday.

WHO's global research network pools efforts from laboratories in various countries and is striving to find all possible sources of the virus, said Alan Schnur, team leader of communicable disease control under the WHO representative office in China.

He made the remarks in an exclusive interview with China Daily on the sidelines of the ASEAN (the Association of Southeast Asian Nations), China, Japan and the Republic of Korea High-Level Symposium on SARS yesterday in Beijing.

Scientists from South China's Guangdong Province and the Hong Kong Special Administrative Region jointly announced in late May that the virus is 99 per cent similar to one carried by the masked palm civet.

Leading Chinese groups under the Ministry of Science and Technology that are researching potential sources of the SARS virus have also confirmed recently that the virus comes from animals.

But there are many unknown aspects of the virus that are hindering efforts to control it, said Li Liming, director of Chinese Centre for Disease Prevention and Control.

Latest research in China cannot yet prove conclusively the virus can be spread by air or that it can be transmitted by excrement and urine or oral grume, Li said. It is not certain that the virus' communicable ability changes according to the season.

No person has been found with immunity from birth against the SARS virus up to now. And it is still unclear whether people who recover from SARS have immunity or not, Li noted.

Beijing learned yesterday that if it does not have any new SARS cases for 20 days then it will not be regarded as part of the affected areas by the WHO, Henk Bekedam, WHO's representative in China, told China Daily.

The WHO has not considered lifting the travel advisory against Beijing as it is too early. The capital is still reporting new SARS cases or suspected cases most days and many patients remain in hospital, the representative added.

"What we want to see is the level of risk being controlled, then we can lift them, maybe in weeks," Bekedam said.

He also gave four criteria for lifting the travel advisory: number of cases currently treated in hospitals to be fewer than 60, number of new patients to be fewer than five for a number of days, no local transmission and no exporting of cases to other regions.

Editor: Liao Ming