LONDON, April 27 (Reuters) - The World Health Organisation will raise its pandemic alert level to phase 4 over the deadly swine flu virus, indicating the infection can spread between humans to cause community-level outbreaks, a source familiar with the decision said on Monday. The decision came after experts held hours of emergency talks on whether to to raise the alert level from phase 3 due to the outbreak, which has killed up to 149 people in Mexico and spread to the United States, Canada and Europe.

Phase 4 is characterised by human-to-human spread of a new virus able to cause community-level outbreaks, according to the six-point scale the WHO revised earlier in the day. The level also indicates there is a significant increased risk of a pandemic but not that this is a foregone conclusion.

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