The recent respiratory infection has not spread into the community, the Secretary for Health, Welfare & Food Dr Yeoh Eng-kiong says.

He noted that all the pneumonia cases seem to have a particular subset, adding that there is a predisposition and predilection to affect healthcare workers and close family members.

Speaking to reporters at Central Government Offices this afternoon, Dr Yeoh said the cases may be caused by a viral infection which is transmitted by droplets.

In Hong Kong, there are about 1,500 to 2,000 cases of pneumonia each month and about half are bacteria-related.

Other cases are usually caused by viruses.

Dr Yeoh said the pattern has not changed and it is similar to that in other developed countries.

On the recent outbreak at hospitals, the Government is looking at whether this atypical pneumonia is due to a new virus or an existing virus that is behaving in a different way, or whether there is something in the environment that has changed it.

The Government is also investigating the four clusters of patients - from the Prince of Wales Hospital, from Princess Margaret Hospital which admitted the American businessman who had returned from Hanoi in serious condition and died, from Pamela Youde Nethersole Eastern Hospital, and from a private clinic - to see if there is a common cause.

To date, 43 medical staff have been hospitalised and put under observation. Of them, 29 had signs of pneumonia.

Dr Yeoh said they will update the media daily.