'Countries especially in the developing world, where populations are most vulnerable, should prepare to see more than the present small number of severe cases,' Ms Chan told the 193 member states at the end of the WHO's annual assembly here.

The spread in impoverished nations was one of the signals that the WHO was keeping an eye on before it declares a pandemic, instead of simply relying on geographical spread under its influenza rulebook.

'What we will be looking for are something that will signify a substantial increase in the risk of harm to people,' interim Assistant Director General Keiji Fukuda told journalists.

'This could be change in the clinical severity, it could be another kind of signal such as the impact on the southern hemisphere,' he added.

Ms Chan underlined that the new virus had been found and tackled in countries that could mobilise the best detection, marking the first time that the world could follow 'the conditions conducive to a pandemic.'

'The strength of a country's health system will make the biggest difference in sickness and survival during an influenza pandemic,' Chan underlined.

The WHO director general also highlighted the risk that the A(H1N1) virus could intermingle with seasonal flu during the impending winter season in the southern hemisphere.

That meant they could 'possibly exchange genetic material in unpredictable ways,' she added, referring to the risk that the new virus could become more dangerous in the process. -- AFP