Healthcare workers should get priority access to H1N1 flu vaccinations to ensure health systems keep functioning as the swine flu pandemic spreads around the globe, the World Health Organization said on Monday.

Marie-Paule Kieny, WHO director of the Initiative for Vaccine Research, said the agency's experts had concluded every country in the world would need access to vaccines, which drug companies are now racing to produce.

Disappointingly, Kieny said, yields from growing the new virus in chicken eggs -- the mainstay of flu vaccine production -- had so far been poor, at only 25 to 50 percent of that achieved with normal seasonal flu strains.

Alternative strains are now being studied that will hopefully increase output, she added.

The WHO raised the global flu alert to the highest level on June 11, declaring a pandemic is underway from the H1N1 virus strain commonly known as swine flu.

The new disease has so far caused mild symptoms in most patients, though at least 429 have died and experts fear the death toll will climb steeply without a widespread immunization programme.

Pregnant women have been vulnerable to serious effects and Kieny also confirmed obesity as a risk factor for severe complications.

The (WHO group of experts) recommended first that healthcare workers should be immunized in all countries in order to retain a functional health system as the virus evolves, she told reporters in a conference call.

In terms of the other groups it depends on the strategy a country wants to follow.

In addition to pregnant women and people with underlying diseases, Kieny said countries might also want to prioritize children, since they can transmit the virus rapidly at school and at home.

The WHO said countries should continue with their normal vaccination programs against seasonal flu and there would be substantial supplies for this since 90 percent of seasonal vaccine production will be completed by the end of July.

The seasonal flu formula is a cocktail of three different strains -- the seasonal H1N1 strain, a very distant cousin of the pandemic strain; an H3N2 virus; and an influenza B virus.

Sanofi-Aventis, Novartis, Baxter, Schering-Plough's Nobilon, GlaxoSmithKline, Solvay, CSL and AstraZeneca's MedImmune are among those working on flu vaccines.

Kieny said the small number of manufacturers like MedImmune who used so-called live attenuated viruses for their vaccines were achieving yields similar to that with seasonal vaccine production.

The WHO has previously estimated that the world could have as many as 4.9 billion doses of H1N1 swine flu vaccine ready for the next flu season -- but this assumes people only need one shot and production yields are similar to seasonal vaccine.

The WHO is now reviewing its numbers in light of the latest yield results and Kieny said it was not possible yet to give a revised forecast.