The World Health Organisation is warning the global capacity to make anti-flu vaccines would not be flexible or large enough to counter a threatened pandemic that could rapidly kill millions of people around the world.
"Current global manufacturing capacity... is inadequate to meet the expected global needs during a pandemic and cannot be rapidly augmented," the WHO said in a weekly bulletin on disease outbreaks and threats.
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.
But it also highlighted a number of drawbacks in the production and distribution chain that could hamper the new vaccine's practical effectiveness.
The health agency has been giving countries advice about a range of preparative measures to guard against a pandemic, including stocks of anti-viral drugs and public health measures.
The bulletin said those measures took into account "that adequate supplies of vaccine will not be available at the start of a pandemic in any country".
The H5N1 virus has killed more than 60 people who were infected by chickens and birds in Asia since 2003.
Deadly potential
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.
"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.
"In the current situation, the capacity to respond to seasonal influenza must be balanced against preparations for pandemic influenza," it added.
Ninety per cent 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 per cent of the world's population, the WHO said.
"Pandemics thus throw into sharp relief inequities in global access to vaccines and other medical interventions during an emergency," the bulletin underlined.
The WHO bulletin also highlighted problems with the formulation of the H5N1 vaccine tested by French vaccine company Sanofi Pasteur and scientists in the United States.
It contains a higher dose of antigens than common flu vaccines while antigen supplies are limited. The WHO said preliminary results of tests on formulations that could use less antigens would be available in three months.
