LAURAN NEERGAARD AP Medical Writer WASHINGTON (AP) — U.S. scientists hope to have a key ingredient for a swine flu vaccine ready in early May, but are finding that the novel virus grows slowly in eggs — the chief way flu vaccines are made. Even if all goes well, it still will take a few months before any shots are available for the first required safety testing, in volunteers. "We're working together at 100 miles an hour to get material that will be useful," Dr. Jesse Goodman, who oversees the Food and Drug Administration's swine flu work, told The Associated Press. Using samples of the new swine flu, taken from people who fell ill in Mexico and the U.S., scientists are engineering a strain that could trigger the immune system without causing illness. "We're about a third of the way" to that goal, Dr. Ruben Donis of the Centers for Disease Control and Prevention said in an interview Tuesday. The hope is to have that ingredient — called a "reference strain" in vaccine jargon — to manufacturers around the second week of May, so that they can begin their own laborious production work, Donis said. But, "this is biology, not mathematics," he cautioned. To further speed the vaccine hunt, the CDC has shipped a raw sample of the new virus to one manufacturer — Gaithersburg, Md.-based MedImmune LLC, which sells the only flu vaccine given via a nasal spray instead of a shot. MedImmune thus uses a slightly different approach to creating influenza strains suitable for that spray, Donis explained. Health authorities are struggling to rein in the swine flu epidemic that has sparked a global crisis since discovery of the never-before-seen strain just last week — and the world learned that travelers to Mexico, where dozens may have died, were carrying the bug home. Standard anti-flu drugs can treat the illness. But the world has no vaccine that prevents this new strain, a mix of pig, human and bird viruses that people presumably have little natural immunity to. And if the virus ultimately spreads enough to spark a pandemic — which hasn't happened yet and may not — a vaccine would be key to mitigating the disaster. Vaccine manufacturers are just beginning production for next winter's regular influenza vaccine, which protects against three human flu strains. Monday, the World Health Organization said factories should stay with that course for now — it won't call for mass production of a swine flu vaccine unless the outbreak worsens globally. Think of flu viruses as wearing coats, changeable proteins on their surface that trigger the immune system to mount an attack. Those proteins give flu strains their main identity: This new swine flu is part of the Type A/H1N1 family — the "H' being a version of the protein hemagglutinin and the "N' is the protein neuraminidase. Matching those H and N components forms the basis of a vaccine. First researchers had to grow enough virus samples, culled from a handful of patients, to work with. Influenza virus traditionally is grown by injecting it into fertilized chicken eggs, but this novel virus didn't grow easily there. There's an alternative, growing it in vats of cells instead, but most flu vaccine manufacturers today still rely on eggs. "There is a little bit of concern there," said CDC's Donis, whose laboratory eventually created three samples that did grow in eggs, just slowly. More work is under way to try to improve that. Next, using a technique called reverse genetics, scientists are selecting genes for the swine flu's H and N antigens to create a customized strain and look for signs that it will prompt a good immune response. Then manufacturers would get the strain to start their own production supply, which could take another two months. "It's worth taking that time at the very beginning to really make sure you've got exactly what you want," said MedImmune senior director Dr. Kathleen Coelingh. "We've got to get this right." But those initial pilot lots will go straight into human safety tests already being planned, Goodman said. Flu vaccine in general is very safe. But in 1976, thousands claimed side effects from a swine flu vaccine administered after an outbreak at Fort Dix, N.J., that never spread. For now, manufacturers are studying production options. Sanofi Pasteur, the world's largest flu vaccine maker, just opened a new U.S. factory but if necessary could keep its older one open as well just for swine flu vaccine production, said spokesman Len Lavenda. It also produces vaccine at a factory in France. At Novartis AG, spokesman Eric Althoff said the company is studying which of its two technologies for vaccine-making would be better. The Swiss drugmaker can make vaccines both in eggs and in cell culture.