The World Health Organisation moved to reassure the public about the safety of pandemic flu vaccines on Thursday, emphasising that rigorous precautions were taken despite the speed of their development.
"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 in a statement on its website.
Although many national regulators had fast-tracked procedures for approval, the swine flu vaccine was built on the same technology used to produce vaccines for frequently changing strains of seasonal influenza, the UN health agency underlined.
"During the 1957 and 1968 pandemics, vaccines arrived too late to be used as an effective mitigation tool during the more severe phases of the pandemics," the agency noted.
Influenza vaccines were not available in the deadliest flu pandemic on record in 1918, when an estimated 50 million people died.
The WHO advised all countries to carry on intensive testing and monitoring of vaccines even after they start to be administered.
While some serious adverse effects had been reported over the 60 years that flu vaccines have been used "these have been rare", it said.
Nonetheless, the WHO said "special safety issues will inevitably arise during a pandemic" because of the massive scale on which a vaccine would be administered.
"For example, adverse events too rare to show up even in a large clinical trial may become apparent when very large numbers of people receive a pandemic vaccine," the WHO added.
Countries have been ordering tens of millions of doses of vaccine for the A(H1N1) virus for mass vaccination campaigns, while clinical trials are usually carried out on thousands of volunteers.