SYDNEY - Australia ordered 21 million courses of swine flu vaccine Tuesday - enough to cover the country's entire population - as the World Health Organization (WHO) warned the pandemic was "unstoppable".

With Australia already the worst-hit nation in the Asia-Pacific region, federal chief medical officer Jim Bishop expressed concern that the "hard-edged" virus was now infecting young and healthy people.

He said Australia hoped to launch a major immunisation program in October to combat the progress of the virus, which has hit more than 9,000 people and been linked to 19 deaths.

"We are seeing a few people now thrown up that have other medical conditions being unwell, (but) also we are seeing a few people that were previously well but are severely affected by the disease," Bishop said.

"Almost all of those swine flu deaths have been in people with prior medical conditions which have been exacerbated but, as I said, there will be now some experience with unfortunately people who have been previously well," he added.

Six people younger than 40 who had otherwise been healthy were on life support in Sydney, with A(H1N1) influenza damaging their lungs so badly they were put on special machines to oxygenate their blood, officials said.

The WHO said Monday a vaccine should be available as early as September, warning that the pandemic was now "unstoppable," with more than 90,000 cases reported, including 429 deaths.

Bishop said 21 million doses would be enough to vaccinate everyone in Australia once over, or half the population if two courses were required, ahead of a "second wave" of infections.

"(Swine flu) does have a hard edge ... it's not like seasonal flu, it's a bit more worrying than that,"' he said.

"We're more concerned about the second wave of this thing, which might be more severe than the first."

A US study released this week warned A(H1N1) could pick up genes from other flu strains that would enable it to be both highly virulent and contagious, or acquire mutations enabling it to be resistant to the anti-viral drug Tamiflu.