But an expert group said it could not be determined if the doctor's disorder was directly caused by the vaccine.

A doctors' group said the case led to a decline in the number of people receiving vaccinations yesterday.

Controller of the Centre for Health Protection, Thomas Tsang Ho-fai, said it is impossible to say whether the man's illness was caused by the flu jab.

"The [expert] group thinks it is not possible to differentiate with reasonable certainty whether the relationship between the vaccine and patient's symptoms is causal or coincidental, meaning that we are unable to conclude one way or the other based on a single case," Tsang said.

"We have vaccinated over 100,000 people in Hong Kong and if we look at overseas experience, the vaccine has not been associated with an increased rate of Guillain-Barre syndrome."

He added: "So we are of the view that for high-risk groups the benefit of receiving the vaccine definitely outweighs the risk and we recommend members of such groups to receive the vaccine."

The cardiologist, Kwok Hong-tai, who has an underlying medical condition, received the vaccine on December 24 and began feeling weakness in his lower limbs on December 28, Patrick Li Chung-ki of Queen Elizabeth Hospital said.

Kwok was admitted to Queen Mary Hospital on January 2 where he remained in serious but stable condition yesterday.

A nurse at the cardiologist's clinic in North Point declined to confirm whether the doctor is the one suffering from Guillain-Barre syndrome.

Li said the patient has shown weakness in all four limbs and his symptoms were "compatible with Guillain-Barre syndrome."

The rare disease occurs after receiving vaccines, while other patients develop it after viral or bacterial infections.

For some patients, there is no underlying cause but is "incidental," Li said. Hong Kong Doctors Union president Henry Yeung Chiu-fat said there were significantly fewer people who came forward to be vaccinated at private clinics.

But government figures showed at least 8,872 were vaccinated in the 24 hours ending 1pm yesterday. Of those, 1,717 went to private doctors, compared with 2,171 the day before.

Infectious disease specialist Lo Wing-lok said the case is more likely related to the vaccine "because of the time sequence and because of various clinical observations and tests done."

He said the case is a "very strong deterrent" for people not to be vaccinated.