Manufacturers may run into challenges in efforts to produce a vaccine against the new H1N1 flu strain depending on how the virus acts, the U.S. Food and Drug Administration's chief scientist told Reuters on Wednesday.

Dr. Jesse Goodman said officials were moving quickly with initial steps in case a vaccine was needed but faced too many unknowns to predict how many doses could be made or whether it would even be necessary.

Total production will depend in part on how fast the virus grows and how much of the key protein it produces to stimulate an immune response, he said.

"If it behaves, I think we're well-positioned," Goodman, the FDA's acting chief scientist and deputy commissioner for scientific and medical programs, said in an interview.

The FDA announced earlier on Wednesday a boost in U.S. flu vaccine capacity with the approval of a new plant operated by Sanofi-Aventis SA . It was cleared to make seasonal influenza shots but could also be used for to make an H1N1 vaccine, officials said.

Still, "the other big uncertainty is how will the body's immune system respond" and what dose is needed to provide effective protection, Goodman said.

The avian flu virus that was a focus of concern in recent years "really took people by surprise in that it required a higher dose to get a good immune response," he said.

Global health officials are debating whether the threat from the H1N1 virus is serious enough to ask manufacturers to mass-produce a vaccine for it.

The new flu, a mixture of swine viruses and elements of human and bird flu, has brought the world to the brink of a pandemic. The virus appears to act like seasonal flu but has confused doctors as it has killed some young and apparently healthy adults in Mexico, not the usual pattern for influenza.

Goodman said "we still have uncertainties about how this epidemic will evolve" and cannot predict if the virus will mutate and change its behavior for better or worse.

While health officials and manufacturers are working quickly, officials are following the same process used to develop the seasonal flu vaccine each year, which should help ensure the vaccine's safety, Goodman said.

"We are following methods that are very tried and true," he said.