A third of countries which have drawn up flu pandemic plans have failed to set out how they would distribute medical treatment, a report has found.
Almost half of the countries prioritised antiviral treatments
Researchers at the John Hopkins Bloomberg School of Public Health and Ben Gurion University Israel studied 45 national pandemic plans.
They warned resources would be scarce, so decisions on who should get drugs or vaccines should be made in advance.
They said prioritising treatment could help reduce death and disease.
The World Health Organisation (WHO) has urged every country to develop and maintain a national plan on bird-flu.
It also recommends nations prioritise the allocation of pharmaceutical resources among the population.
Researchers looked at 19 plans from developed nations and 26 from developing countries. In total, these represented around two-thirds of the world's population - 3.8bn people.
The countries included the US, Norway, Australia, India, China, Serbia, Bahrain, Israel, South Africa, UK, Mexico, Venezuela, Hong Kong, Thailand and Singapore.
The report, Priority Setting for Pandemic Influenza: An Analysis of National Preparedness Plans, found almost half of the plans they examined favoured antiviral medications, such as Tamiflu, while 62% prioritised giving citizens a flu vaccine.
This was an unexpected finding, researchers said, as antiviral treatment may be the only pharmaceutical intervention available in some countries.
"We cannot expect to vaccinate more than 14% of the world's population within a year of pandemic."
Lead author of the study, Lori Uscher-Pines, said resources would need rationing if there was a flu pandemic.
"We learned that individual countries have not consistently prioritised population groups for vaccines and antivirals.
"No countries prioritised population groups to receive ventilators, face masks and other critical resources," she said.
The study also found allocation decisions for different groups varied widely from country to country.
Most countries - including the UK - prioritised health care workers for the vaccine and antiviral treatment, but policies varied on other groups such as the elderly, essential service workers and children.
It also found almost half of the countries studied had prioritised children, despite a WHO recommendation against it.
The report said it was also unclear why vaccine and antiviral medications had been singled out for treatment when there is likely to be a shortage of many resources.
It said further guidance was needed to address whether schemes for ventilator masks and hospital beds are necessary.
It is the first study to look at prioritisation decisions in developed and developing countries.
Professor John Oxford, a virologist at Queen Mary School of Medicine, London, said he was not surprised at the variations in plans betweens countries.
"Only once the disease is moving will we know who it's targeting. There's going to be an element of 'wait and see'."
He said antivirals, vaccines and hygiene levels were all key to intervening in a pandemic.
He added the study could potentially be a "huge contribution" towards helping countries further develop their plans.
The Department of Health said it has taken steps to prepare.
It has purchased 14.6m courses of Tamiflu and 2.5m doses of H5N1 vaccine. PCTs have also been asked to draw up local contingency plans to respond to a pandemic, which the department will audit.
"The World Health Organisation has said that the UK is at the forefront of preparations internationally for a pandemic influenza," a spokesperson said.
"Preparing for a pandemic is ongoing and we are constantly reviewing and developing our plans as new information emerges."