Procedures to fast-track approvals of new vaccines to combat H1N1 influenza do not reduce safety, the World Health Organisation said on Thursday.
The WHO said vaccines had to be available quickly and in large quantities to have the greatest impact.
"The public needs to be reassured that regulatory procedures in place for the licensing of pandemic vaccines, including procedures for expediting regulatory approval, are rigorous and do not compromise safety or quality controls," it said.
The WHO has said the current outbreak of H1N1, declared a pandemic on June 11, is the fastest ever pandemic and could eventually affect 2 billion people.
A WHO statement said concerns had been expressed about the safety of vaccines to fight H1N1, known as swine flu.
But the United Nations agency said vaccines had arrived too late in the 1957 and 1968 flu pandemics to be of much use, and flu vaccines had not yet been developed in the 1918 "Spanish flu" pandemic which killed an estimated 50 million people.
It said it had been working with government regulators and vaccine makers in 2007 to look at ways of speeding up the approval of a new vaccine if a fresh virus strain emerged.
In some cases the process could be faster because a vaccine was not really new, as it was based on the technology for existing seasonal flu vaccines, it said.
But the WHO said safety issues could arise during a pandemic when a vaccine is administered on a mass scale, even if they did not show up in safety trials.
"For these reasons, WHO advises all countries administering pandemic vaccines to conduct intensive monitoring for safety and efficacy, and many countries have plans in place for doing so," it said.
The WHO's top vaccines expert, Dr Marie-Paule Kieny, is due to give a briefing at 8:30 a.m. EDT in which she will provide an update on H1N1 vaccines.
Leading flu vaccine makers include Sanofi-Aventis, Novartis, Baxter, GlaxoSmithKline and Solvay.
Novartis has started human testing of H1N1 swine flu vaccine candidates while Sanofi-Aventis, the world leader in flu shots, will commence within days, company officials said on Tuesday.
(For the full WHO statement go to http://www.who.int/csr/disease /swineflu/notes/h1n1_safety_vaccines_20090805/en/index.html )