UPDATE 1-Dynavax, Merck say hepatitis vaccine still on hold

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Oct 21 (Reuters) - Dynavax Technologies Corp (DVAX.O) and Merck & Co (MRK.N) said the U.S. health regulators have asked for additional information on the companies' experimental hepatitis B vaccine and maintained the clinical hold on the drug, sending Dynavax's shares down 28 percent.

The U.S. Food and Drug Administration, which placed a clinical hold on the drug Heplisav in March, has advised the companies that the balance of risk versus potential benefit no longer favors continued clinical evaluation of Heplisav in healthy adults and children.

A clinical hold is an order issued by the FDA to delay a proposed clinical trial or suspend an ongoing clinical trial.

Heplisav was placed on hold after a patient receiving the vaccine was preliminarily diagnosed with Wegener's granulomatosis, a disease in which blood vessels get inflamed.

Dynavax and Merck are evaluating the FDA's response in considering next steps.

Shares of Dynavax fell to 89 cents in trading after the bell. They closed at $1.23 Tuesday on Nasdaq. (Reporting by Esha Dey in Bangalore; Editing by Amitha Rajan)