A doctor prepares a syringe in a municipal vaccination centre in Nice, southeastern France, September 9, 2009. REUTERS/Eric Gaillard

Drug makers can only produce enough H1N1 vaccine each year for half the planet because they lack factory capacity, the World Health Organization said on Thursday.

The H1N1 vaccine looks to be as safe as the regular flu shot, the WHO said in a statement, adding that drug makers worldwide can produce an estimated 3 billion doses per year and a single dose should be enough to give immunity to healthy adults and older children.

But it said companies had limited, inadequate and not readily augmented capacity to increase output to cover the planet's 6.8 billion population.

The WHO's previous projection last May was that global production capacity would be close to 5 billion doses, but its new estimate was made on the basis of results from clinical trials and confidential data provided to the U.N. agency.

There is not enough production capacity worldwide to vaccinate everyone, WHO spokesman Gregory Hartl told Reuters.

New production capacity takes a long time to come on line. Any new single plant for vaccine production takes about five years to build, test and get approval, he added.

It was not immediately clear whether the WHO's new estimate of 3 billion doses per year implied switching all production from seasonal flu vaccine to H1N1 pandemic vaccine.

Marie-Paule Kieny, director of WHO's initiative for vaccine research, is due to give a teleconference at 1500 GMT on Thursday.

Outcomes of trials completed to date suggest that pandemic vaccines are as safe as seasonal influenza vaccines, WHO said.

However, even very large clinical trials will not be able to identify possible rare events that can occur when pandemic vaccines are administered to many millions of people, it said.

Pandemic vaccines are most effective as a preventive strategy when given before or near the peak incidence of cases in an outbreak, it said.

The WHO advised countries to closely monitor the vaccine's safety and report adverse events. This was vital to determine whether changes in vaccination policies were needed.

Side effects are expected to be similar to those with seasonal flu vaccines, including soreness or swelling at the point of injection and possible fever, headache, muscle or joint aches, according to the United Nations agency.

In almost all people, these symptoms should be mild and last 1-2 days, it said.

Most rich nations have contracts with drug makers to obtain enough vaccine to cover their entire populations, it said. 

But most low- and middle-income countries lack the financial resources to compete for an early share of limited supplies, which in such countries would depend mainly on donations.

The WHO said it would begin an initial distribution of some 300 million doses of vaccine donated by rich nations to more than 90 developing countries from November.

Leading flu vaccine makers include Sanofi-Aventis, Novartis, Baxter, GlaxoSmithKline and Solvay.

Regulatory authorities have licensed pandemic vaccines in Australia, China, Hungary and the United States, soon to be followed by Japan and several countries in Europe, the WHO said.