* CDC says virus fairly widespread in U.S.
* Officials uncertain about how disease will progress
* Older children, young adults among those hospitalized (Updates with additional details from CDC briefing)
By Donna Smith
WASHINGTON, May 3 (Reuters) - The new H1N1 flu virus appears to be fairly widespread in the United States and seems to be hitting mostly younger people, with very few cases reported in people over 50, U.S. health officials said on Sunday.
"We think very few of the cases we have confirmed are in people over 50," the U.S. Centers for Disease Control and Prevention's Dr Anne Schuchat told reporters in a telephone briefing. "Whether this will pan out over the weeks ahead we don't know."
The CDC reported 226 cases of the new H1N1 swine flu virus and one death in 30 states. The CDC previously had confirmed 160 cases in 21 states.
Mexican officials say they believe the outbreak there is starting to ease, although they are still trying to get a full picture of just how far the disease has spread.
Schuchat said the virus is fairly widespread in the United States, meaning that most states have reported cases. New York has the most cases with 63, many linked to a school in the New York City borough of Queens. Texas has 40 cases.
U.S. health officials said they were encouraged by signs in Mexico that the number of cases are leveling off and that there was only one death in the United States -- a toddler visiting from Mexico.
Most cases in the United States have been reported to be mild. But 30 people, mostly older children and young adults, have been hospitalized with the disease, U.S. officials said.
Schuchat said that with seasonal flu, the elderly and very young are most likely to be sick enough to be hospitalized -- 200,000 a year on average.
"I don't think we are out of the woods yet," Schuchat said. "From what I know of influenza, I do know there will be more cases, more severe cases and more deaths."
Health and Human Services Secretary Kathleen Sebelius said on Sunday that flu vaccines for both the new strain of the H1N1 virus and the seasonal flu should be ready by autumn.
She told NBC's "Meet the Press" that the government is accelerating production of a vaccine against the seasonal flu, which is expected to infect millions of Americans, and is beginning laboratory work on the new H1N1 virus.
Companies already are making the vaccine for the autumn months with a mixture of three influenza viruses that was chosen this year before the new strain broke out.
They have a number of choices -- leaving the new strain out of the mix altogether, replacing the current H1N1 component with the new H1N1 strain, or making it a so-called quadrivalent vaccine that includes the new swine H1N1, the circulating seasonal H1N1, the H3N2 component and the influenza B strain.
It takes months to formulate influenza vaccines and they must be made fresh every ear, with new strains of the constantly mutating virus. (Additional reporting by Maggie Fox, David Morgan and Will Dunham; Editing by Eric Beech)