US health authorities on Wednesday acknowledged a swine flu vaccine shortage in the United States and, that manufacturers would likely not catch up until December.

US Health and Human Services Secretary Kathleen Sebelius told the Senate Homeland Security Committee that currently there is not enough H1N1 vaccine made to inoculate every American who wants to receive the shot.

"Right now we are at a point where the demand is ahead of the yield," Sebelius told lawmakers worried about the spread of the sometimes deadly disease and closely watching the US government's response.

Independent Senator Joseph Lieberman, the committee's chairman, noted that experts had previously forecast there would be 40 million doses available by late October but that expectations now were for roughly 30 million.

"There are now very unsettling reports of growing vaccine shortages," said Lieberman. "We're asking ourselves if enough vaccine will be produced in time for all who will need it as we continue to experience the spread of H1N1 flu."

At least 4,735 people have died from swine flu infections since April, when an outbreak of H1N1 flu was first reported in Mexico, the World Health Organization has said.

Earlier, US Homeland Security Secretary Janet Napolitano told CNN television that vaccine makers "will be caught up somewhere around December."

"This is a delay. It's not a shortage. There, ultimately, is going to be vaccine for everyone who wants to be vaccinated," said Napolitano, who urged Americans to take "common-sense" precautions.

"It's keeping your child home from school if he or she is sick, staying home from work yourself if you're sick. It's coughing properly into your arm, not into your hand. It's frequent hand washing.

"You know, those may sound kind of low-tech, which they are, but they're remarkably effective things when it comes to slowing the transmission of the virus," she said.