The world's first human trials of a vaccine to protect againt swine flu have begun in Australia, it has been announced.
It will be at least six weeks before the initial results show whether the vaccine is effective.
Meanwhile, Gordon Brown, the Prime Minister, has sought to reassure the British public as it emerged at least 31 people in the UK have now died after contracting swine flu.
More than 700 people worldwide are known to have died after getting the virus, which the World Health Organisation says is spreading faster than any previous flu pandemic.
Adelaide-based Vaxine, one of two pharmaceutical companies undertaking the vaccine trials in Australia, began tests on Monday with 300 adult volunteers.
Melbourne's CSL has 240 people enrolled in its seven-month trial, which started on Wednesday.
Vaxine said it would be six to eight weeks before results would verify whether any vaccine was effective.
"There is no guarantee any of these vaccines will work," he said. "Swine flu is a very peculiar beast, it's a very different virus that we're dealing with. But we are hopeful."
A spokeswoman for Baxter, a company which is to supply vaccine to Britain alongside the pharmaceutical giant GSK, said they expected to ship the first batches of its swine flu vaccine at the end of July or early August.
"Commercial production on the vaccine began in early June," she said.
"We expect to have a limited quantity of the vaccine for shipment at the end of this month or early August."
Baxter has contracts with five countries including the UK, Ireland and New Zealand.
Sir Liam Donaldson, the Government's Chief Medical Officer for England, has said the first batches of a vaccine should arrive by the end of August.
Andy Burnham, the Health Secretary has said the UK is at the "front of the queue" for supplies of the vaccine.