The World Health Organization has warned doctors to brace for a possible wave of very sick H1N1 swine flu patients.

TheU.N. health agency said Friday the virus can cause life-threateningviral pneumonia that may be particularly dangerous to otherwise healthyyoung people.

Health officials urged doctors to treat suspected cases quickly with antiviral drugs.  

Thewarning comes as U.S. health officials announced deliveries of swineflu vaccines will be delayed. The Centers for Disease Control andPrevention says there will only be 28 million to 30 million doses ofthe vaccine available by the end of October instead of a projected 40million.

The CDC says there is wide spread flu activity in 41U.S. states, an unprecedented level for this time of year. They say forthe first week this fall, the level of influenza and pneumoniamortality in the U.S. is above the epidemic threshold.

The WHOhas declared the swine flu outbreak a pandemic. The agency says morethan 4,700 deaths attributed to H1N1 influenza have been reportedworldwide.