Federal health officials were elated last week after preliminary results from swine flu vaccine tests defied their expectations. Scientists found just one dose instead of two would offer effective protection against the H1N1 virus, meaning there would likely be more vaccine supply to go around.

Yet, the folks at the National Institutes of Health are still planning to go through with a study starting today on an immune-boosting substance known as adjuvant that could be added to the vaccine. Using an adjuvant would mean giving the shot in smaller doses, further stretching supply.

It's unlikely it will be needed this fall, especially if further tests show that one standard shot is good enough to protect people from the virus. But using adjuvant could prove helpful in future years, or if the flu took a turn for the worst, said Dr. Wilbur Chen, a vaccinologist at the University of Maryland's Center for Vaccine Development, who is leading the NIH-sponsored tests.

The trials are start here in Baltimore on adults and elderly with an experimental adjuvant manufactured by GlaxoSmithKline. Chen thinks the adjuvant, essentially an oil-and-water mixture added to the vaccine -- could be particularly helpful in strengtheninng the response in the elderly, since flu vaccines work less well in older people.

Thing is, adjuvant is added to seasonal flu shots in Europe, but it's never been licensed here in the states. Using it here would require emergency declaration by the Food and Drug Administration. That's enough to make some people worried.