This is still two steps short of declaring a full-blown pandemic in humans.
WHO says the phase 4 alert means sustained human-to-human transmission is causing outbreaks in at least one country. It signals a significant increase in the risk of a global epidemic, but doesn't mean a pandemic is inevitable.
Dr Keiji Fukuda, assistant director for health security and environment, said the WHO committee discussed the level very carefully to consider whether moving to phase 4 or phase 5 was most appropriate, and "it was clearly felt we were seeing human-to-human transmission, and that put us into phase 4."
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