The number of doses produced in a year will therefore fall short of the 4.9 billion doses the global health body previously hoped could be available for the pandemic, WHO spokesman Gregory Hartl told reporters in Geneva.
Production will be lower because some manufacturers are still turning out vaccines for seasonal flu - an illness that can be serious in sick and elderly people, Hartl said.
Production problems also have reduced the weekly output of pandemic vaccine, he said.
The United States, France, Britain and six other countries announced Thursday that they will share part of their vaccine supply with poorer countries.
WHO welcomed the move Friday, saying it "demonstrates the commitment of these countries to fairness in sharing of scare resources."
"Current supplies of pandemic vaccine are inadequate for a world population in which virtually everyone is susceptible to infection by a new and readily contagious virus," it said.
WHO says that in theory all the world's 6.3 billion people should receive at least one dose of vaccine against the pandemic strain of H1N1 - also known as swine flu - to ensure comprehensive protection against the disease, which has so far killed some 3,500 people.