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 Organisation said.
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, Ms 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 per cent 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 Ms Kieny also confirmed obesity as a risk factor for severe complications.
"The (WHO group of experts) recommended first that healthcare workers should be immunised 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, Ms Kieny said countries might also want to prioritise 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 per cent of seasonal vaccine production will be completed by the end of this month.
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.
Ms 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 Ms Kieny said it was not possible yet to give a revised forecast.
Seasonal influenza is deadly enough - each year it is involved in 250,000 to 500,000 deaths globally. But most are the elderly or those with some kind of chronic disease that makes them more vulnerable to flu, such as asthma.
The elderly seem to have some extra immunity to this new H1N1, which is a mixture of two swine viruses, one of which also contains genetic material from birds and humans. It is a very distant cousin of the H1N1 virus that caused the 1918 pandemic that killed 50 million to 100 million people.
A study published in the journal Nature confirmed that the blood of people born before 1920 carried antibodies to the 1918 strain, suggesting their immune systems remember a childhood infection.
The work by Dr Yoshihiro Kawaoka also supports other studies that this new H1N1 strain does not stay in the nose and throat, as do most seasonal viruses.
"The H1N1 virus replicates significantly better in the lungs," Dr Kawaoka said. Other studies have also shown it can cause gastrointestinal effects, and that it targets people not usually thought of as being at high risk.
On Friday, a team at the US Centres for Disease Control and Prevention and the University of Michigan reported that nine out of 10 patients treated in an intensive care unit there were obese. They also had unusual symptoms such as blood clots in the lungs and multiple organ failure.
The CDC estimates at least a million people are infected in the US alone and clinics everywhere are advised not to test each and every patient, so keeping an accurate count of cases will be impossible. The US has documented 211 deaths and WHO counted 429 early last week.