"Current global manufacturing capacity... is inadequate to meet the expected global needs during a pandemic and cannot be rapidly augmented," the WHO said on Friday in a weekly bulletin on disease outbreaks and threats.
"Because the present global manufacturing capacity for influenza vaccine is limited, any decision to manufacture a pandemic vaccine in large quantities prior to the start of a pandemic would, of necessity, compromise the capacity to produce vaccines for seasonal influenza," the epidemiological bulletin said.
Currently, 90% of production capacity for all influenza vaccines, 300 million doses, is concentrated in Europe and North America.
Production there was likely to meet domestic demand for flu vaccines or treatments first even in an emergency, although those areas account for only 10% of the world's population, the WHO said.
Bird flu vaccination
The WHO's epidemiological bulletin welcomed the announcement earlier this month of successful first clinical trials of a vaccine to protect humans from the H5N1 strain of bird flu.
"Current global manufacturing capacity... is inadequate to meet the expected global needs during a pandemic and cannot be rapidly augmented"
The World Health Organisation
But it also highlighted a number of drawbacks in the production and distribution chain that could hamper the new vaccine's practical effectiveness.
The H5N1 virus has killed more than 60 people who were infected by chickens and birds in Asia since 2003.
The strain is regarded as the most likely current source of a more virulent form of the human flu virus that could mutate and spread rapidly around the world, potentially killing millions of people.
Any major production shift towards the H5N1 vaccine would also compromise protection against regular, seasonal influenza, the WHO added.
Seasonal epidemics of flu cause an estimated 250,000 to 500,000 deaths a year, according to the UN health agency.