Suspected cases of human-to-human transmission in Indonesia have set off international alarm bells. Six family members from a remote farming village on Sumatra died after testing positive for the H5N1 virus -- the world's largest reported family cluster.

Shu Yuelong, director of the National Influenza Center under the Chinese Center for Disease Control and Prevention, said suspected cases of human-to-human transmission had also been reported in Vietnam, Thailand and other countries.

But there has been no conclusive evidence showing that H5N1 had evolved into a human-to-human transmission virus, Shu was quoted by China Population News as saying.

Both epidemiological and etiological evidence was needed to determine whether a virus could be transmitted between humans, Shu said.

The H5N1 virus had acquired the ability to infect and kill mammals, but the number of human infections was still small, Shu said.

Research of the National Influenza Center showed the virus extracted from Chinese bird flu patients was genetically different from those in Vietnam, Thailand and Indonesia.

No trace of human influenza had been found in the gene of the virus extracted from Chinese patients of bird flu, Shu added.

The H5N1 virus remained mainly a virus of birds, but experts fear it could change into a form easily transmitted from person to person and sweep the world.

So far, most human cases can be traced to direct or indirect contacts with infected birds.

The virus has killed 130 people around the world since 2003, according to the World Health Organization (WHO). Some 200 million birds have died or been culled.

So far, China has recorded 19 human cases of bird flu with 12 deaths. More than 30 outbreaks of bird flu have been reported in China since last October. The latest outbreak occurred in north China's Shanxi Province, said sources with the Ministry of Agriculture.

The Chinese government should review the strategies and effects of the bird flu control efforts of the past two years and improve them to cope with the epidemic, which was still a serious threat, said Chinese bird flu control expert Liu Xiufan.

"When, and to what extent, the current avian influenza virus could evolve into a human pandemic is unpredictable. We should do our best to reduce the risk of a human pandemic influenza breaking out and make necessary preparations before such a risk becomes reality," said Liu.

Some changes in the H5N1 strain have taken place recently. It had increased its virulence in ducks, and the available vaccines were ineffective in protecting poultry, said Liu.

The H5N1 strains isolated from 2004 to 2006 had increased their ability to replicate in mammalian cell culture. The transmission mode of the viruses is changing from fecal-oral to aerosol, he said, adding the virus had increased resistance to the environment, especially to temperature.

China faced an enormous challenge in eradicating the virus because it had been circulating in poultry in China for some time.

He said in some Asian countries, such as Japan, the Republic of Korea and Malaysia, outbreaks of bird flu in poultry did occur several times in 2004 and 2005.

However no human cases were reported because the outbreaks were stamped out very quickly, Liu said.

Currently the transmission efficiency of H5N1 from birds to humans was very low, as the species barrier still existed. However, the virus was changeable, and might acquire the ability to cross the species barrier to transmit infection to mammals and humans through gene mutation and reassortment, Liu said.

"Therefore, the better avian flu in poultry is prevented and controlled, the quantity and lifespan of the H5N1 virus is reduced, thus lessening the risk that avian flu might evolve into a human pandemic," Liu said.