The production capacity of bird flu vaccines has tripled since 2007 but it could still take up to four years to meet global demand if a pandemic were to occur, a WHO-backed study said on Tuesday.
Even in the best case scenario, 18 months would be required for sufficient doses of medication to be produced to cover the worldwide population, the study said.
Marie-Paule Kieny, who heads the World Health Organization's initiative for vaccine research, expressed concern that production facilities might cut back because supply had more than met demand for regular, seasonal vaccines.
"We still don't have enough production capability to cover the whole world in the early months of the pandemic... and what is worrying especially is that the capacity to make seasonal vaccines ... is now much larger than the demand for these vaccines."
Kieny noted that global demand at the moment for regular flu vaccines stands at 500 million doses per year, but capacity is running at 800 million doses a year and climbing up to 1.7 billion doses in 2014.
"We are now building a surplus capacity. What will the manufacturers do with it? Either they maintain it... or they close some of them down," she said.
Stephen Gardner from pharmaceutical giant GlaxoSmithKline warned: "You cannot create vaccine capacity and then mothball this capacity until a pandemic outbreak."
He pointed out the need to use surplus capacity to produce stockpiles of H5N1 bird flu vaccines, as well as to encourage more seasonal vaccination.
In Europe, for instance, only 20 percent of the population gets seasonal vaccination, Gardner said.
Some 408 people have been infected by the bird flu since 2003, with 255 of them dying, most of them in Asia, according to official figures.
The H5N1 virus typically spreads from birds to humans via direct contact, but experts fear that it could mutate into a form easily transmissible between humans, with the potential to kill millions in a pandemic.
The study was conducted by the WHO, consulting group Oliver Wyman and industry group the International Federation of Pharmaceutical Manufacturers and Associations.