WASHINGTON (Reuters) - An experimental vaccine for a common virus that can cause mental retardation and deafness in some newborns yielded encouraging results in guinea pigs and could lead to a human vaccine, scientists said on Wednesday.

Cytomegalovirus, or CMV, is transmitted to a fetus through the mother's placenta. Women can carry the virus without being aware. Congenital CMV is the second-most common cause of mental retardation in infants, behind only Down syndrome, and also can cause deafness.

The vaccine was given to female guinea pigs before they got pregnant. Vaccinated animals had fewer dead offspring and were less likely to transmit the CMV infection to their babies than unvaccinated guinea pigs, the researchers found.

The animals that got the vaccine gave birth to 28 live babies and four dead ones, a death rate of 13 percent. Those that did not get the vaccine had nine live babies and 12 dead ones, a death rate of 57 percent.

Dr. Mark Schleiss of the University of Minnesota School of Medicine, who led the research, said the study marks a key step toward developing a human vaccine.

"It demonstrates for the first time in a relevant animal model of congenital CMV infection that a vaccine strategy that's being considered for human clinical trials is successful in ameliorating disease in newborns," Schleiss said in a telephone interview.

The so-called vector vaccine, developed along with North Carolina-based AlphaVax Inc., uses an altered virus to deliver one gene from the viral DNA to the recipient's cells.

The study, funded in part by the U.S. National Institutes of Health, appears in the Journal of Infectious Diseases.

"An effective CMV vaccine for women of childbearing age could greatly reduce the disability caused by the virus," Dr. Duane Alexander, director of the NIH's National Institute of Child Health and Human Development, said in a statement.

'PUBLIC HEALTH CRISIS'

"Congenital cytomegalovirus infection is a major public health crisis in the United States today, and the average person on the street doesn't know anything about it," Schleiss said.

"I have these moms come into my practice," Schleiss said, "and invariably what they say to me is, 'I've never heard of this before it happened to me and my baby.'"

About 10 to 15 percent of newborns with congenital CMV end up with a long-term disability like mental retardation, cerebral palsy and deafness. CMV also can damage the placenta, leading to miscarriage.

About 1 percent of U.S. babies are born with the virus, amounting to 40,000 per year, researchers said. Medical experts advising U.S. policymakers have deemed development of a vaccine to prevent cytomegalovirus during pregnancy a high priority.

Cytomegalovirus is found widely around the world and is a type of herpes virus. According to the NIH, CMV infects between 50 to 80 percent of all U.S. adults by age 40. There is no cure for it, although commonly it causes few if any symptoms.

It can cause blindness in AIDS patients.

It is transmitted through close personal contact like kissing or sex or sharing eating utensils, and through blood transfusions and nasal secretions.

AlphaVax said it was seeking U.S. Food and Drug Administration approval for the vaccine.

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