Neither firm has a contract with the UK government, which expects the first vaccine batches by the end of August.
But Vaxine research director Nikolai Petrovsky said: "There is no guarantee any of these vaccines will work. Swine flu is a very peculiar beast, its a very different virus that we're dealing with. But we are hopeful."
Meanwhile, drugs giant GlaxoSmithKline says it expects to triple capacity of its flu treatment Relenza to 190 million treatments per year.
Mr Brown admitted swine flu was putting the health service in the UK under strain.
But the prime minister said some of the pressure would be relieved by the National Flu Service, which is being launched in England later this week.
The phone and internet service will allow people with swine flu to get access to anti-flu drugs without needing to consult a doctor.
Mr Brown said: "I want the public to be reassured that we have been preparing for the possibility of a pandemic for a number of years.
Gordon Brown: "Robust plans are in place"
"The NHS is continuing to cope well, but as swine flu cases have started to increase we have needed to be able to give anti-virals more quickly.
"From the end of this week the National Flu Service will be up and running. This will free up GP and NHS time."
GPs have started to raise concerns about the number of calls they are getting about flu with every region of England apart from Yorkshire and the Humber seeing "exceptional" levels of demand.
Under contingency plans, non-emergency operations can be cancelled and doctors moved around the health service to help tackle hotspots.
It has not reached that stage yet, but ministers have been forced to set up the flu service.
We saw in the snow that many schools recognised that their school website was a good communication channel
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