Shares in US group Dynavax Technologies fell through the floor after US regulators refused to approve its flagship hepatitis B vaccine Heplisav-B.

The company's stock plummeted more than 70 percent after investors shrank back on news that the US Food and Drug Administration issued a complete response letter rejecting the jab.

The regulator is asking for more information relating to several areas, including clarification on specific adverse events of special interest (AESIs), a numerical imbalance in a small number of cardiac events in a single study (HBV-23), new analyses of the integrated safety data base across different time periods, and post-marketing commitments.

However, it has acknowledged a review of responses received from Dynavax in early October, including those relating to AESIs and the numerical imbalance in cardiac events, is not yet complete, and presumably the company is hoping that this may answer some of the outstanding questions.

The responses included an extensive analysis that included independent expert consultation supporting the firm's view that the imbalance was driven by an unexpectedly low number of events in the comparator arm, Dynavax said.

The FDA has not requested any additional clinical trials and there are no apparent concerns with rare serious autoimmune events. Dynavax is expecting a Class 2 designation for a resubmission of the application, which would result in a target review period of six months, but it may need outside help to complete this.

"The CRL is consistent with our opinion that Heplisav-B is approvable and we are seeking to meet with the FDA as soon as possible," said chief executive office Eddie Gray. "However, the time and resources that will be required to gain approval leads us to consider that we may not be able to advance this program on our own and we are moving swiftly to identify a potential pharmaceutical or financial partner".