Dynavax (DVAX 2.39%) is in a free fall. Shares have been cut in half today following a negative outcome from yesterday's Food and Drug Administration advisory committee review of its hepatitis B vaccine, Heplisav. The company's top drug candidate received an 8-5 vote against the sufficiency of its safety data, leading to huge doubts around its chances of approval next year. It's not clear how the FDA will react, and the possibility certainly still exists for an approval. However, as health care bureau chief Brenton Flynn explains in the video below, buying shares of Dynavax has become a gamble given the huge uncertainties that exist.