The World Health Organisation's clarification on the definition of active treatment of SARS has led to a major breakthrough in the process of persuading the organisation to lift its travel advisory against Hong Kong.

Reciting the long discussion between the organisation and the Government, Secretary for Health, Welfare & Food Dr Yeoh Eng-kiong said he sought clarification from the organisation on the definition of active treatment before he left Geneva on Tuesday.

On the basis of what the Government understood from WHO on the active treatment, Hong Kong health officials came back to look at the cases in Hong Kong.

"Of the 140-odd cases that we have, that reported as under active treatment, 77% were admitted before May 8. So most of our patients were really long staying patients that have been there for a long time," Dr Yeoh said.

Among them, a lot of patients met with the WHO's criteria of convalescing because they do not have fever for 48 hours, there is improvement in the X-ray chest in pneumonia, and there is improvement in the blood counts and other improvements, Dr Yeoh said.

The Government asked hospitals yesterday to report back on the nature of the cases and it was found that there were actually only 59 active cases.

The other criteria which Hong Kong did not completely meet was to be able to link up all the previous cases to identify the source of infection.

"Of these 1,400-odd cases, we have been able to account for 91.3% of all the cases. There were 8.7% that we could not trace the source of infection," Dr Yeoh said.

"But we were also able to demonstrate that of this 8.7%, 57% were not due to the coronavirus that causes SARS because there is a lot of background pneumonia which can be very confusing. So it meant that there are about 4% of the cases that cannot be accounted for the mode of transmission."

The WHO accepted that it is impossible to account for a 100% of the cases to contact tracing in an outbreak.

Welcoming WHO's decision to lift its travel advisory against Hong Kong, Dr Yeoh said the whole community needed to work towards the ultimate objective of eliminating the SARS related coronavirus in Hong Kong.

"To be able to do that, we need to make sure that we have no cases and the requirement usually is that we have no cases for two incubation periods, that is, in 20 days."