More than 22 million doses of swine flu vaccine are available in the United States (US) now, and most Americans should soon find it easier to get their dose, US health officials said on Tuesday.

"We're beginning to get to significant increases in the availability," said Dr Thomas Frieden, director of the US Centers for Disease Control and Prevention, at a briefing.

Last week, there were just 14 million doses on hand, despite predictions that as many as 120 million doses would be ready by mid-October. The slow supply trickle has frustrated Americans, who have stood in line for hours, in some parts of the country.

Increased demand

The shortage has probably increased demand, Frieden said.

"It's quite likely that that too little vaccine is one of the things that's making people more interested in getting vaccinated, frankly. When we have shortages, we see an increase in demand," he said.

The vaccine is grown in eggs in a reliable but slow process, and smaller amounts of it were being produced per egg than expected. There were other snags too, but health officials say manufacturers have overcome most of those and are making the vaccine more speedily.