WASHINGTON, Oct 23 (AFP) - The United States has seen "many millions of cases" of swine flu and more than 1,000 deaths -- or around a fifth of the world's total -- since the H1N1 outbreak began six months ago, a top health official said Friday.

"We have seen, since the beginning of the pandemic in April and May, more than 1,000 deaths from pandemic influenza and more than 20,000 hospitalizations in this country," Thomas Frieden, head of the Centers for Disease Control and Prevention (CDC), told a weekly news briefing.

"We have had, up until now, many millions of cases of pandemic influenza in the US, and the numbers continue to increase," Frieden said, lamenting the scarcity of H1N1 vaccine.

At least 4,735 people have died from swine flu infections worldwide since April, when an outbreak of H1N1 flu was first reported in Mexico from where it spread quickly to the United States, the World Health Organization has said.

Earlier this week, US Health Secretary Kathleen Sebelius confirmed to lawmakers that demand was outstripping supply of H1N1 vaccine.

The shortfall was evident at a clinic in a suburb of Washington on Wednesday, where several thousand people showed up in the hope of getting one of 200 injectable vaccines or 1,200 nasal spray doses.

As of Friday, the CDC had 16.1 million doses of swine flu vaccine ready for shipping, and more than 11 million doses have been sent out, said Frieden.

Around half of those were nasal mist, which can only be given to healthy people between the ages of two and 49, and excludes some high-risk groups such as pregnant women, people with chronic respiratory illness, like asthma, and young children aged six months to two years.

"We are nowhere near where we thought we'd be by now," said Frieden of the amount of vaccine available.

The shortfall was due in part to outdated vaccine production technology.

"The tools that we have available are not as modern as we would like or as rapid," said Frieden.

"Our planning anticipated a six- to nine-month period between emergence of a new virus and availability of vaccine against the virus. The six-month clock will be up at the end of October," he said.

Vaccine is produced using seed stocks grown made from the virus, and "even if you yell at them, they don't grow faster," said Frieden.

Meanwhile, as Americans waited for more shipments of vaccine, 46 of the 50 states were reporting widespread swine flu activity -- something that usually would not happen until the height of flu season, which in a normal year comes in January or February, Frieden said.