Although the virus has killed more than 100 people around the world, it is not spread easily between people, said Zeng Guang, chief epidemiologist of Chinese Centre for Disease Control and Prevention.

So far 16 cases of human infections have been reported in China.

"We have found that some of the patients had immune deficiencies and two were pregnant," Zeng said.

The reason why they were infected is still unclear, but "we know that human beings are not generally susceptible to the virus," Zeng told China Daily at the APEC Symposium on Emerging Infectious Diseases.

Most of the human cases in China have had direct contact with diseased poultry.

Although realistically large numbers of people must have come into contact with diseased poultry, only "a very small number" of these people were infected by the virus.

This means the virus isn't spread by airborne particles, said Henk Bekedam, WHO Representative in China.

If it could be spread through the air, it is impossible that there are less than 200 human cases in the world, Bekedam noted.

After taking samples from healthy people living with infected people, "we found the virus in respiratory systems of some of those tested," Zeng said.

However, the virus in the respiratory system "cannot really infect the person," he added.

This is because people are not susceptible to the virus now, whether it is carried by poultry, animals or human beings, he said.

Both Zeng and David Nabarro, the United Nations System Senior Co-ordinator for Avian and Hunan Influenza, who visited China yesterday, said there are not people who have been infected with the virus but showed no symptoms.

But Bekedam asked scientists to keep researching on the mutation of the virus which might become communicable among humans.

Vaccine not enough

He also warned that the global vaccine production capacity is far less than what will be needed to control the bird flu pandemic, given that recent studies show only high dosages of a vaccine can induce immune responses in human bodies.

Late last month, a research team with the US-based National Institute of Allergy and Infectious Disease published its latest discovery in a clinical trial of bird flu vaccine for human use, which says that a proper antibody response can only be stimulated by a dose of 90 micrograms, five times more than a regular dose for seasonal influenza.

"It means the global capacity is one twelfth of what we thought it was," Bekedam said.

Nabarro added some research is underway to work on an adjuvant that can be taken with the vaccine to help lower the dosage needed. And advice also came from US scientists last month at the China-US Dialogue on Emerging Science and Technology in Beijing.

Arnold S Monto, a US expert in epidemics said it is advisable that governments start encouraging seasonal vaccine production. "It is unknown when the pandemic will occur.

But even if it doesn't, at least we have the capacity to deal with seasonal flu, which claims a huge number of lives too," Monto said.

Bekedam said that WHO may seek co-operation with China to help other countries in vaccine supplies.

"China has a great capacity to develop vaccine, especially for poultry. Although it is not the same, but we want to see whether it can be converted for human use," he said.