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A vaccine appears highly effective for protecting against the most common form of epidemic viral encephalitis in the world, a finding researchers hope will lead to its approval in the United States.

Doctors from Walter Reed Army Institute of Research in Washington, D.C., reported Wednesday in The New England Journal of Medicine the vaccine was 91 percent effective in protecting children from Japanese encephalitis in Thailand.Japanese encephalitis, a deadly disease that causes the brain to swell, is common in China and many parts of Asia, such as Thailand, Vietnam, Nepal and India. It is also causing some concern in the United States because the Asian tiger mosquito, which can carry the viruscausing the disease, has recently been found in this country.

A vaccine from a virus that has been killed has been used since the 1960s, but it has never been tested to confirm its effectiveness, said Dr. Thomas Monath, of the federal Centers for Disease Control in Fort Collins, Colo.

"This vaccine has never been proven to prevent disease," said Monath, who wrote an editorial accompanying the new study. "This is really the first time this vaccine was found to prevent disease."

The researchers studied 65,224 children in Thailand who received the vaccine or a phony substitute between November 1984 and March 1985. They found the vaccine was 91 percent effective in preventing the disease.

"We conclude that . . . inactivated Japanese encephalitis vaccine . . . protect(s) against encephalitis due to Japanese encephalitis virus," the researchers said.

Monath said he hoped the findings would persuade the Food and Drug Administration to approve the vaccine, which would encourage American drug firms to produce it and make it readily available to Americans traveling abroad.