Another method of creating a candidate virus also mixes wild virus with a second, fast-growing strain, but instead of growing them in eggs, lets the viruses mingle in a special solution while scientists manipulate them through a complex technique called reverse genetics.

CNN: Candidate virus for H1N1 vaccine arrives at CDC

Webby has been working to produce a candidate virus against H1N1 as well, but told CNN the virus is growing slower than expected, and that it would likely be a few weeks before a suitable strain is ready to be sent to the CDC.

CNN: Candidate virus for H1N1 vaccine arrives at CDC

The incremental process involves finding a good vaccine candidate, reassessing and testing the virus, developing seed vaccines and ensuring their safety.

CNN: CDC: Cases of new swine flu strain rising