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What happened

After the Food and Drug Administration announced earlier today that it had canceled an advisory committee meeting to evaluate its hepatitis B vaccine, shares in Dynavax Technologies Corp. (DVAX -1.71%) dropped 31.6% by market close.

So what

Dynavax Technologies' investors were dealt a blow in April when the FDA opted to delay an approval decision on Heplisav-B, the company's hepatitis B vaccine, and the FDA dealt another blow to the company today when it canceled the advisory committee meeting scheduled to consider the vaccine.

In August, Dynavax Technologies reported that the FDA's Vaccines and Related Biological Products committee would debate Heplisav-B's pros and cons on Nov. 16, yet only weeks later, the FDA has determined it needs more time "to review and resolve several outstanding issues."

The cancellation puts in jeopardy the FDA's ability to issue an approval decision on Heplisav-B's Dec. 15 PDUFA date.

Now what

The FDA did say that its evaluation is continuing and that it would schedule another advisory committee in the future, so this could simply be a delay and not an indication that the FDA has found something new and worrisome in the Heplisav-B data it's reviewing. The FDA previously rejected Heplisav-B, in 2013, because of worry that it could cause autoimmune disorders.

If Heplisav-B does eventually make it across the finish line to market, the opportunity could be big, because it offers an arguably better dosing schedule than GlaxoSmithKline's Engerix-B, the leading hepatitis B vaccine. Heplisav-B is given via two doses in one month, while Engerix-B is given via three doses over six months.

Nevertheless, Heplisav-B's past FDA setbacks make this company too risky for me to buy. Instead, I'm content to focus on other ideas that may be less risky.