(Reuters) - Dynavax Technologies Corp’s experimental hepatitis B vaccine is effective, but there were more deaths and serious heart problems in patients given the vaccine than in those given a rival product, according to a preliminary Food and Drug Administration report published on Wednesday.

However, overall numbers and rates of such cardiac events were low, the review concluded, and there were limitations in the evaluation of cardiac events that “limits the causal interpretation of these observations.”

The review, posted on the FDA’s website, comes two days ahead of a meeting of outside advisors who will discuss the vaccine, Heplisav-B, and recommend whether it should be approved. The FDA is not bound to follow the advice of its advisory panels but typically does so.

The FDA has twice rejected the product citing unresolved safety concerns. The second rejection, in November, sent Dynavax’s shares plunging roughly 70 percent to a year low of $3.20. They have since bounced back and were trading down 0.8 percent at $9.83 on Wednesday.

Anupam Rama, an analyst at J.P. Morgan, currently estimates the likelihood of approval at 40 percent given the mixed opinions in the FDA’s briefing documents and previous rejections, a position that may change following the advisory committee meeting, he said in a research note on Wednesday.

If the vaccine is approved, Dynavax’s shares could rise into the “mid-to-high teens,” he added.

Dynavax argues that the higher number of heart problems, including heart attack, recorded with Heplisav-B is due to fewer than expected instances occurring in the group of patients who took GlaxoSmithKline Plc’s Engerix-B.

One cardiologist consulted by the FDA said the argument “appears plausible” and suggested the imbalance should be noted on the product’s label and safety monitoring should continue after it reaches the market. Two other cardiologists were less sure.

“While the finding could be attributable to chance, I cannot confidently say that there is no increased cardiovascular risk with Heplisav,” said one.

The other stated that Dynavax “cannot/does not fully eliminate the notion that this is a “real” observation worth further investigation. I agree.”

Heplisav-B is designed to enhance the body’s immune response to the hepatitis B virus, which can lead to cirrhosis of the liver, cancer and death. The virus is spread through sexual contact, sharing of needles and from mother to child during pregnancy.