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WASHINGTON, July 17 (Reuters) - A DNA-based vaccine against avian influenza can safely stimulate the immune system to levels expected to protect against flu, San Diego-based Vical Inc <VICL.O> reported on Thursday.

It said that in a study of 100 volunteers, 67 percent of the patients receiving the higher dose had immune responses that could protect against infection with the H5N1 avian flu virus, with no serious adverse reactions after two injections.

The Phase I clinical trial, meant to demonstrate the vaccine is safe, was continuing, Vical said in a statement.

"The preliminary results from this Phase 1 trial indicate for the first time that ... DNA vaccination against H5N1 influenza is well-tolerated and can induce impressive antibody responses," Dr. Robert Belshe of Saint Louis University School of Medicine, who monitored safety, said in a statement.

"Successful development of a safe and effective DNA vaccine will help address the potential public health threat of pandemic influenza."

The company says a DNA-based vaccine could be made in six to eight weeks, compared with four to six months for influenza vaccines made the current way, using bits of the actual virus grown in chicken eggs.

DNA vaccines use bits of genetic material called plasmids. They are meant to generate an immune response against a specific bit of the virus and are designed to last longer than standard vaccines under conditions of heat and cold.

The H5N1 avian flu virus has become entrenched among birds in much of Asia and parts of Europe, Africa and the Middle East. It rarely infects people but it has killed 243 people out of 385 infected in 15 countries, according to the World Health Organization.

Experts fear the constantly mutating virus could change into a form easily transmitted from person to person, perhaps sweeping the world and killing millions.

At least 16 companies are working on vaccines against H5N1. No one knows if they will work against whatever strain might eventually cause a pandemic, but makers agree it is better to be prepared.

"The ability to manufacture DNA vaccines in weeks rather than the months required for conventional vaccines may provide a significant advantage when dealing with an emerging infectious disease such as pandemic influenza," Vijay Samant, Vical's president and chief executive, said in a statement.

The company will present the interim Phase I results at the IBC Life Sciences Next Generation Vaccines conference being held near Washington. (Reporting by Maggie Fox; Editing by Peter Cooney)

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