Geneva - A man who recently returned from Mexico has been confirmed as having Switzerland's first case of swine flu, medical officials said Thursday.

The announcement about the 19-year-old man's condition, which was deemed stable, was made by a hospital in the canton of Baden overnight following a laboratory confirmation.

The case had initially been misdiagnosed earlier in the week, media reports said, and the man was sent home from hospital only to return.

He was being held in isolated and treated with the anti-viral Tamiflu.

There were still 24 other suspected cases in Switzerland.

Late Wednesday, the World Health Organization raised its pandemic influenza alert level to the second highest, phase 5, indicating sustained human-to-human transmission and an outbreak that was spreading across countries.