The World Health Organisation on Thursday released details of swine flu samples sent to drug companies this week for use in making a vaccine against the new strain of influenza .
The three candidate reassortant vaccine seed viruses were developed by laboratories in Britain and the United States, according to documents published on the WHO's website.
"So far we have three candidate vaccine viruses that have been sent to companies," a WHO spokeswoman said.
Candidate or seed viruses act as references to allow pharmaceutical companies to develop and produce vaccines.
WHO interim Assistant Director General Keiji Fukuda said last week that the pharmaceutical industry should be ready to produce a preventive vaccine against influenza A(H1N1) by the end of June or early July.
Output of up to 4.9 billion doses a year would be possible, according to a forecast presented to about 30 vaccine makers this month.
However, experts are still mulling whether to give the go ahead for production as this may reduce or halt the manufacture of vaccines for seasonal flu.
Other issues including dosage and the cost still have to be ironed out as well.