At least four more people and 70 per cent of poultry in Hong Kong have been exposed to a new strain of bird flu virus which infected two girls in March, it was announced yesterday. But health officials insisted influenza A H9N2 posed 'no imminent threat' to people and medical experts stressed the chances of the virus causing an outbreak in humans were very slim. There has been no new H9N2 human infection case since the virus of pure avian nature attacked two girls, aged one and four, in March. Both infected girls recovered after running a high fever and showing typical flu symptoms. Another strain of bird flu, H5N1, infected 18 people, killing six of them in 1997. Studies revealed yesterday showed two per cent of 100 poultry workers tested were found to have antibodies to the H9N2 virus, compared with only 0.6 per cent of the 500 members of the general population surveyed. The studies, jointly conducted by the Department of Health and United States Centres for Disease Control and Prevention, tested 733 people in Hong Kong to find out if they had been exposed to the virus. They found 70 per cent of poultry sold locally had been exposed to the virus, renewing the call for central slaughtering in the SAR. A nurse, who was not involved in the care of the two H9N2 patients, a blood donor and two poultry workers were found to have the antibody to the virus. Five mainlanders were infected with the virus before it infected the two girls in Hong Kong. The mainlanders showed only very mild flu symptoms. The Department of Health's community medicine consultant, Dr Mak Kwok-hang, warned yesterday that although the main mode of transmission of the virus was from birds to humans, the possibility of human-to-human transmission remained open. '[The] H9N2 virus causes relatively mild illness in humans and poses no imminent major threat to public health. There is no cause for undue alarm. 'However, we should not be complacent as the influenza virus is capable of changing.' Only one out of 233 people who might have come into contact with the two girls infected in March has the antibody. None of the two girls' relatives, classmates or teachers has the antibody. Dr Mak said it was safe to consume poultry as long as it was thoroughly cooked. The four-year-old girl was believed to have contracted the virus through 11 days of contact with live chickens at her grandparents' home. But the source of infection for the one-year-old girl remains a mystery. University of Hong Kong microbiologist Professor Yuen Kwok-yung said last night the chances of H9N2 causing an outbreak were minimal. Professor Yuen said to cause infection, the virus had to invade human cells after reacting with human enzymes. Gene sequencing results show the structure of the H5N1 virus means it can easily get into human cells.