The risk of bird flu evolving into a more transmissible agent in humans remains high due to the widespread distribution of the H5N1 virus in poultry and the continued exposure to humans , according to a study by the UN health agency.
The overall case-fatality rate was 56 per cent with the highest rate in persons aged 10 to 39 years, according to the study by the World Health Organisation (WHO), which analysed epidemiological data of all cases reported so far.
Cases have occurred all year round, but human cases peaked during the period roughly corresponding to winter and spring in the northern hemisphere. If this pattern continues, an upsurge in cases could be anticipated starting in late 2006 or early 2007, the WHO says.
In case of seasonal influenza, the mortality is highest in the elderly. Half of the cases occurred in people under the age of 20 and 90 per cent of cases in people under 40 years.
This was the first analysis released by the agency encompassing all cases reported between December 2003 and April 2006.
The analysis calls for better collection of essential data to understand and refine case management of H5N1, which experts fear could, in a worst case scenario, mutate into a deadly human pandemic.
'The sharing of data may be seen as part of an early warning system that will collectively defend all countries against a common threat,' the WHO said.