GENEVA – Annual production of H1N1 vaccine will be "substantially less" than the 4.9 billion doses the World Health Organisation (WHO) previously forecast was possible, a spokesman said on Friday.

WHO spokesman Gregory Hartl said that based on clinical test results from some 25 drugmakers, weekly production was less than 94 million doses, although it seemed one dose would be enough to give immunity against the swine flu strain.

"The real figure will be substantially less than 4.9 billion," Hartl told reporters.

The WHO previously forecast one third of the world's nearly seven billion people could be affected by the H1N1 pandemic, but so far the vast majority of victims are suffering only mild symptoms.

Hartl gave no exact new forecast but noted WHO's previous estimate of 4.9 billion had been based on drugmakers switching all production of seasonal flu vaccine to the new vaccine against the H1N1 strain and getting good yields.

In a statement, the WHO said current supplies of pandemic vaccine are "inadequate for a world population in which virtually everyone is susceptible to infection."