Novartis and Baxter, two of the world’s biggest flu vaccine manufacturers, warned on Thursday that yields in the production of their swine flu vaccines were below those for seasonal flu, raising the prospect of fresh international shortfalls.

Raymund Breu, chief financial officer of Novartis, said yields so far were just 30-50 per cent of those for seasonal flu, adding to concern that there will be far more competition for already limited global manufacturing capacity.