US scientists hope to have a key ingredient for a swine flu vaccine ready in early May. But they say that the new virus grows slowly in eggs, which is a key step in flu vaccine production.

Even if all goes well, it will still take months before any shots are available for the necessary safety testing in volunteers.

The researchers must engineer a strain that could trigger the immune system without causing illness.

The US Food and Drug Administration's swine flu chief, Doctor Jesse Goodman, says scientists are working "at 100 miles an hour" to create good raw material to deliver to vaccine manufacturers.

And the Center for Disease Control and Prevention says work is about a third completed.

(CCTV April 29, 2009)