The highest level, phase 6, is defined by community-level outbreaks in at least one other country in a different WHO region, according to the agency.

CNN: WHO raises pandemic alert to second-highest level

The WHO's "Phases of Pandemic Alert, " which has been in existence for five years, characterizes phase 5 as a human-to-human spread of the virus into at least two countries in one WHO region, signaling that a pandemic is imminent.

CNN: WHO raises pandemic alert to second-highest level