US regulators have temporarily suspended clinical development of Dynavax Technologies and partner Merck & Co’s investigational hepatitis B vaccine Heplisav so that a serious adverse event observed in one patient given the vaccine can be fully investigated.

The US Food and Drug Administration has placed a clinical hold on the two Investigational New Drug applications for Heplisav after one subject given the vaccine in a Phase III study outside the US developed Wegener's granulomatosis, a rare condition characterised by inflammation of the blood vessels.

Consequently, the groups said they have stopped giving the vaccine to participants in the only study of Heplisav in which injections were still being administered, a Canadian Phase II trial assessing its potential in patients with end stage renal disease, and that no new studies will be started until the issue has been resolved.

All parties in involved have now turned their attention to the medical history of the patient who developed the condition “to understand better the timing and onset of the disease symptoms, including whether it was a pre-existing condition or was related to vaccine administration”, Dynavax said in statement.

Confident in safety profile?
But the group seemed to remain confident in the vaccine’s safety profile, pointing out that, over the last seven years, around 2,500 individuals have been vaccinated with more than 5,000 doses of Heplisav in 10 clinical trials, with no previous reports of Wegener’s granulomatosis.

Even so, investor confidence took a serious hit on the news causing Dynavax shares to plummet nearly 42% in after-hours trading on the Nasdaq, particularly as the vaccine is the only product in the company’s late-stage development pipeline. Merck, however, was able to brush off the news, its shares remaining flat.