The World Health Organisation (WHO) restated its confidence in the H1N1 flu vaccine on Tuesday, calling it the most important tool against the pandemic. Mild adverse side effects such as muscle cramps or headache are to be expected in some cases, but everyone who has access to the vaccine should be inoculated, it said.

Mass vaccination campaigns against the swine flu virus are underway in China and Australia and will be starting soon in the United States and parts of Europe, WHO spokesman Gregory Hartl said.

"It is important to remember that the vaccines, which have already been approved, have been used for years and years and years in their seasonal vaccine formulation and have been shown to be among the safest vaccines that exist," he told a news briefing.

Hartl, asked whether WHO was concerned by reports that some people were reluctant to be injected with the new vaccine, said: "Certainly we have seen the reports. Again, we would restate that the most important tool that we have to fight this pandemic is the vaccine."

It was doubly important that health care workers be vaccinated, as it protects them as well as patients, he added. "We would hope that everyone who has a chance to get vaccinated does get vaccinated," Hartl told Reuters.

The United Nations agency declared in June that the H1N1 virus was causing an influenza pandemic and its collaborating laboratories have provided seed virus to drug makers worldwide to develop vaccines.

GlaxoSmithKline won a further 22 government orders for its H1N1 swine flu vaccine in the last two months, taking the total number of doses ordered to 440 million worth some $3.5 billion.