GENEVA (AP) -- 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 previously predicted that companies would be able to make up to 5 billion doses each year.
"Virtually everyone is susceptible to infection by a new and readily contagious virus," WHO said, noting that 3 billion doses wouldn't be enough to cover the entire global population of 6.8 billion people in the first year.
Still, the World Health Organization admits that not everyone may need vaccination.
"Most people will do well without the vaccine," WHO vaccine chief Marie-Paule Kieny told reporters.
She said most people infected with the pandemic strain of the H1N1 virus have a mild illness and recover by themselves.
At the moment, WHO says the priority should be to vaccinate health workers -- who make up about 2 percent of the world's population -- as well as certain high-risk groups, including young children and pregnant women.
Altogether these groups would likely be covered by the 3 billion doses available each year, especially as early clinical data show that healthy adults and older children require only a single dose of vaccine rather than two, as some experts had predicted.
WHO said ensuring poorer countries receive sufficient vaccines remains one of the main challenges 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.
Kieny said large-scale vaccination programs would probably detect some cases of severe reaction following the vaccination, but that those would likely have occurred anyway without vaccination.
The agency is urging countries to monitor the vaccination procedure for possible further side effects.
In the long term, the H1N1 virus, which has caused the pandemic, will evolve, Kieny said, requiring adjustments to the vaccine.
WHO and vaccine manufacturers will be closely watching the virus, but so far it has remained stable, she said.
Meanwhile WHO 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.