The new contract with French vaccine maker Sanofi-Pasteur marks a major scale-up in US preparation for the possibility that the worrisome virus could spark an influenza pandemic.

While the vaccine is still experimental, preliminary results from the National Institutes of Health's first testing in people suggest the inoculations spur an immune response that would be strong enough to protect against known strains of the avian influenza, sparking the new investment.

How Many Doses?

But just how many doses the $100 million will buy isn't yet clear.

That's because there is contrasting research on just how much antigen much be in each dose to provide protection, explained Sanofi spokesman Len Lavenda. The range is huge -- from 15 micrograms of antigen per dose to 90 -- and the protective amount likely will wind up somewhere in between, he said.

Previously, the government has said it has stockpiled 2 million doses of bird flu vaccine.

Sanofi stored that vaccine in bulk, and the 2 million estimate assumed a single 15-microgram dose per person, Lavenda said. In contrast, the preliminary NIH research suggested it may take two 90-microgram shots to provide protection.

Simple math suggests that means the $100 million purchase could provide enough doses to protect anywhere from 1.7 million people -- "we're quite sure it's going to be a lot more than that," Lavenda said -- to a maximum of 20 million people.

A study now under way in France pairs the vaccine with an immune booster, called an adjuvant, that may help stretch doses. Sanofi expects results later in the year.

Deadly Worldwide Outbreak

Regardless of the ultimate number, clearing the way for mass production now is a big step. Sanofi's factory in Swiftwater, Penn., can produce bird flu vaccine in September and October -- months not occupied making vaccine for regular winter flu -- and separate bulk lots into agreed-upon doses later.

The government's ultimate goal is to stockpile 20 million vaccine doses, a first wave of protection if the H5N1 bird flu strain eventually sparks a pandemic.

It's a quest gaining urgency. The virus has now killed or led to the slaughter of millions of birds, mostly in Asia but in parts of Europe, too. Although it has killed only about 60 people, mostly poultry workers, that's because so far it doesn't spread easily from person to person.

If that changes -- and flu viruses mutate regularly -- it could trigger a deadly worldwide outbreak, because H5N1 is so different from the flu strains that circulate each winter that people have no residual immunity.

The nation also plans to stockpile 20 million doses of anti-flu medication, and the government announced Thursday it was purchasing enough of the drug Relenza, from maker GlaxoSmithKline, to treat 84,300 people.

Already in stock is enough of a competing drug, Tamiflu, to treat 4.3 million. Tamiflu is a pill, while Relenza must be inhaled, a drawback. The government still is planning additional Tamiflu purchases.

"These countermeasures provide us with tools that we have never had prior to previous influenza pandemics," said Health and Human Services Secretary Mike Leavitt.