Big Pharma races to get swine flu vaccines to market

But even if things proceed smoothly, delivery will not keep up with demand as the northern hemisphere enters the autumn and winter flu season, the World Health Organisation (WHO) has warned.

"We know that supplies will be extremely limited for some months to come," WHO chief Margaret Chan said on Friday.

Initial estimates that big pharma could crank out 94 million doses a week beginning in October have been slashed by 50 to 75 per cent due to poorer than expected yields from the so-called "seed virus" strains developed by WHO-approved laboratories.

According to the global health authority, 25 drug companies have announced their intention to make the vaccines.

Five of those firms will account for more than 80 percent of production: Sanofi-Pasteur in France, AstraZeneca and GlaxoSmithKline (GSK) in Britain, Baxter in the United States, and the Swiss group Novartis.

Significant numbers of vaccines should start to arrive within five to six weeks, they say.