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The World Health Organization has raised its pandemic alert for a new strain of swine flu by one, to level four.

A Mexican soldier wears a surgical mask as he stands guard at Mexico City's international airport Benito Juarez April 27, 2009. [Felipe Leon?/REUTERS]

The phase 4 alert means there is sustained human-to-human transmission in at least one country and is two levels short of declaring a full pandemic.

The WHO said the evolving crisis could easily push the pandemic alert higher or lower in the coming days.

Meanwhile, WHO spokesman Gregory Hartl says US health officials have taken the first steps toward producing a vaccine against the virus.

Gregory Hartl, WHO Spokesman. said, "We understand that the USCDC, for example, has already taken wild virus and has taken a vaccine virus strain and is currently growing this virus in eggs, which is the first stage in a vaccine production process, and we've seen media reports like you that some vaccine manufacturing companies are already preparing to produce a vaccine against this strain of A1H1N1, so we certainly know that there the work has begun to produce a vaccine."

(CCTV April 28, 2009)