Half of those hospitalised with the new H1N1 virus are under 25, a clear illustration that the pandemic is affecting the young disproportionately, according to US health officials.

They said reports from 27 US states show 53 per cent of people sick enough to be hospitalised with H1N1 flu are under the age of 25, with only seven per cent of hospitalisations among people 65 and older.

"This is really, really different from what we see with seasonal flu," the CDC's Anne Schuchat said. "With seasonal flu, about 60 per cent of hospitalisations occur in people 65 and over."

She stressed the report was incomplete but said that if anything, it was underestimating the extent of the pandemic. An analysis of 292 deaths from 28 states showed that younger people than usual are also dying, she said.

"Almost a quarter of deaths are occurring in young people under the age of 25. Specifically, 23.6 per cent of the deaths are in that age group. About 65 per cent of the deaths are in people 25 to 64 years of age," added Dr Schuchat.