The World Health Organization says it is more concerned about bird flu than severe acute respiratory syndrome or SARS. China's state-run Xinhwa News Agency, quoting Shigeru Omi, director of the WHO Regional Office for Western Pacific, said Sunday that a bird flu pandemic is likely to occur unless effective measures are taken. Omi said in Shanghai that bird flu was circulating far more widely than initially predicted. The WHO official said virtually nobody will have an immunity against the new virus if it is proven to have developed through a process of intermingling human and avian forms of the virus. He added, however, that there has not yet been a scientifically proven case of human-to-human infection of avian influenza. Omi warned that if the situation continues for many years to come, there is a greater possibility that the virus will eventually gain the potential to transmit itself on a human-to-human basis. To prevent the avian virus from jumping from animals to humans, Omi urged countries to improve their systems for reporting cases of the disease. China, Thailand, Vietnam, Indonesia and Malaysia have all reported bird flu outbreaks since July, after an earlier crisis this year. SARS killed 349 people last year in mainland China and spread to some 30 countries, causing the deaths of nearly 800 people worldwide. So far, 28 people are known to have died from bird flu this year.