Advisors to the US Food and Drug Administration have backed the efficacy of Dynavax' hepatitis B vaccine Heplisav but expressed concerns about its safety profile.

The agency's Vaccines and Related Biological Products Advisory Committee voted 13 to one that the data presented by Dynavax adequately demonstrated immunogenicity. However, they also voted eight to five with one abstention that there was insufficient data to adequately support the safety of Heplisav.

The vote was no great surprise, following documents released earlier in the week by FDA staff concerning trials which compared Heplisav with  GlaxoSmithKline's older vaccine Engerix B. The staffers had noted that while the incidence of autoimmune events in Heplisav-Engerix B studies were low, all of them occurred in the Heplisav arms.

The panel was also concerned about the lack of representation of Asian-Americans, African-Americans and Hispanics in the studies which involved nearly 5,850 patients. The committee suggested that the database of patients may need to be double that.

Dynavax said it will continue working with the FDA as it completes its review ahead of the scheduled Prescription Drug User Fee Act date for Heplisav of February 24. Investors fear the worse, however, and Dynavax shares sank over 60% in extended trading.