It urged all countries to keep an eye out for unusual developments related to the disease, such as clusters of infections or fatalities.

The group noticed that H1N1 has been characterised by its relative mildness, but also by its speed of transmission. 'In past pandemics, influenza viruses needed more than six months to spread as widely as the new H1N1 virus has spread in less than six weeks.'

In its last report July 6, the WHO recorded 94,512 cases worldwide with 429 deaths. Experts say actual figures could be higher.