Drugmakers are on track to have a vaccine against the new H1N1 strain of flu ready for the northern hemisphere autumn after receiving seed virus samples, company officials said on Wednesday.

Sanofi-Aventis, GlaxoSmithKline, Novartis and Solvay all said their vaccine teams had obtained the new influenza A (H1N1) seed virus within the past fortnight, enabling them to begin the production process.

What is still unclear, however, is how much vaccine they will be able to manufacture, since this depends on how easily the new virus strain grows within a commercial production environment.

"It will probably take a couple of weeks to ascertain the yields before we get into large-scale manufacture," a Glaxo spokesman said.

The World Health Organisation (WHO) said on Tuesday it was on the verge of declaring the first influenza pandemic in more than 40 years.

It is concerned at the sustained spread of the H1N1 strain - including more than 1,000 cases in Australia - following major outbreaks in North America, where it emerged in April.

Confirmed community spread in a second region beyond North America would trigger moving to phase 6 on the U.N. agency's six-step disease scale, signifying a full-blown pandemic.

Recent investment in flu vaccine capacity means companies are in far better shape to meet the challenge of a pandemic than in the past.

They are also well advanced in manufacturing supplies of the normal seasonal flu shot, allowing them to switch some capacity to making a pandemic H1N1 vaccine over the coming months.

As a result, there could be windfall sales and profits this year for leading suppliers, some of which have already won orders for H1N1 vaccine from governments in Europe and North America.

Although the H1N1 flu strain seems mild at present, health officials are worried it might return in a more virulent form in the northern hemisphere winter.

The WHO estimates vaccine makers could produce 4.9 billion pandemic flu shots a year in the best-case scenario, though this will still not be enough for the entire world population of more than 6.5 billion, particularly if it turns out that people need more than one injection to gain immunity.

The 4.9 billion estimate would be significantly lower if there was no wholesale switch from seasonal to pandemic flu vaccine-making.

The H1N1 vaccine now being developed by companies must be tested first on ferrets and then on humans in clinical trials before regulatory authorities can approve it.