Clinical trial results are the main source of share price appreciation for investors in development-stage biopharmaceutical companies. Last week, biopharmaceutical company Dynavax Technologies (NASDAQ:DVAX) reported solid results from the first of three phase 3 trials for its hepatitis B (HBV) vaccine, Heplisav. As a result of the positive Heplisav results, shares were up more than 20% for the week.

In this first phase 3 trial, Heplisav was tested in 400 people ages 40 to 70 to see how well the drug remained active inside people's bodies and protected them against the disease. Since older people have less active immune systems than younger folks, it's harder for vaccines of all types to provide what is called seroprotection against diseases in older people. After three doses, Heplisav appears to be more effective than a rival HBV vaccine offered by GlaxoSmithKline (NYSE:GSK).

Dynavax plans on testing Heplisav in several thousand more patients in the next phase III trials, with the trials expected to be completed in 2008. Right now the drug is not partnered, but Dynavax's management says that "it makes a lot of sense" to have a commercialization partner in place so that Heplisav will have better marketing resources behind it, if it can gain regulatory approval.

With an estimated 350 million carriers of HBV worldwide, there is obviously a pressing need to immunize people against the disease. Even though this sounds like a huge market, most of the people infected with this disease are in developing parts of the world. Thus, the sales potential for HBV vaccines in this market is not as large as for drugs that treat people in developed countries and can command a higher price.

Regardless, the market for existing HBV vaccines is currently around $400 million worldwide, with the "difficult to treat" population estimated to represent about a fourth of this total. If Heplisav can gain regulatory approval, even a couple hundred million in sales or the royalties from them would be meaningful for a company of Dynavax's size.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has a disclosure policy .