Pharmaceutical companies will be able to produce about 3 billion doses of swine flu vaccine a year, much less than previously expected, the World Health Organization said Thursday.
The U.N. agency had predicted that companies would be able to make 5 billion doses each year.
"Virtually everyone is susceptible to infection by a new and readily contagious virus," WHO said, noting that the available doses wouldn't be enough to cover the entire global population of 6.8 billion people in the first year.
There are questions over how high actual demand for the vaccine will be, as most people who are infected experience only mild symptoms. Several million people who have already contracted the disease would also be expected to have built up immunity to it, at least until a significant mutation occurs.
"WHO continues to recommend that health workers be given high priority for early vaccination," it said.
Early clinical data show that healthy adults and older children will only require a single dose of vaccine rather than two as some experts had predicted, WHO said.
The agency said ensuring poorer countries receive sufficient vaccines remains a challenge because much of the global supply has been reserved by rich nations.
It praised a group of nine countries including the United States which last week agreed to donate a share of their pandemic vaccine supply to developing nations.
"WHO will be coordinating the distribution of these donated vaccines," it said, starting with an estimated 300 million doses in November.
Addressing concerns about the safety of the pandemic vaccine, WHO said trials to date suggest it is as safe as a regular seasonal flu shot.
"Side effects are expected to be similar to those observed with seasonal influenza vaccines," it said, listing soreness, swelling, redness, fever, headache and muscle or joint aches.
"In almost all vaccine recipients, these symptoms are mild, self-limited and last 1-2 days," WHO said.
It urged countries to monitor the vaccination procedure for possible further side effects.
WHO's Director-General Dr. Margaret Chan repeated Thursday her recommendation that governments keep up their guard against swine flu but refrain from closing borders or restricting trade.
She also advised continued monitoring of unusual flulike illnesses and cases of severe pneumonia, to help track the spread of the disease, its severity and possible mutations of the virus.
She made her recommendations after meeting with top international flu experts Wednesday.
Chan was to lead a meeting on the swine flu pandemic at the United Nations in New York later Thursday.