OLYMPIA — Starting next year, children entering schools and child-care facilities in the state will have to be immunized for chickenpox, the State Board of Health unanimously voted yesterday.

The requirement takes effect next July 1 and applies to children under age 13. But state health officials proposed that the requirement be phased in over several years, beginning with children in child care, preschool, kindergarten, and sixth grade.

The schedule for phasing in the rule will be determined later.

The decision makes Washington among the last states to require the vaccination for entry to school or child care. The state ranks near the bottom for chickenpox vaccinations and immunization in general.

A federal Centers for Disease Control and Prevention advisory committee recommended requiring chickenpox vaccines in 1999. Idaho, Montana and Wyoming have no such requirement, while Arizona has a regulation in the works.

"It is an area where we're playing catch up," said Dr. Tom Locke, who chairs the Washington state Board of Health. "Most states have seen the wisdom and seen the benefits."

Locke and other health officials said many physicians wanted to wait a few years to offer the vaccine, which came on the market in 1996, to make sure it was safe and effective. The vaccine also requires special, refrigerated storage.

Board of Health members yesterday voiced concerns about vaccine distribution because of the storage, but state health officials said the system could handle it

The State Board of Education and the Washington Chapter of the American Academy of Pediatrics endorsed the new requirement.

Chickenpox, also called varicella, is caused by a highly contagious virus. In children, it is usually mild, resulting in a rash, itching, fever and fatigue.

But about 10 percent of children suffer from complications, including infected skin lesions and other infections, dehydration from nausea or vomiting, aggravation of asthma or, in the worst cases, pneumonia, brain damage or death.

The risk of complications is 10 to 20 times higher in unimmunized adults, who can contract the illness from children. Washington had 1,045 hospitalizations and 16 deaths related to chickenpox in 2003, almost all in elderly people, according to the state Department of Health.

The chickenpox vaccination joins a list of nine other immunizations required for entry into school or child care: diphtheria, tetanus, whooping cough, polio, measles, mumps, rubella, hepatitis B and haemophilus influenza type B.

The requirement can be waived for medical reasons or by parents who object to immunization for religious or philosophical reasons.

About 77 percent of the state's children are current with their immunizations. That ranks the state 46th in the nation. The national rate is 80.5 percent.

About 73 percent of children in Washington have received the chickenpox vaccine. And that ranks the state next to last in the nation. The national rate is 86.2 percent.

Concerns have been raised by some parents that thimerosal, a mercury-containing preservative used in some vaccines, could cause neurological problems, including autism. But the CDC has said that scientists have not verified any such link.

Except for some flu vaccines, the agency said, thimerosal is not used in vaccines given to young children. Janna Bardi, immunization program manager for the Department of Health, verified that the chickenpox vaccine does not contain thimerosal.

Recommended childhood vaccinations are financed with federal and state funds. Doctors may charge an administration fee, usually about $15. Bardi estimated the state's portion of the cost resulting from the new requirement will be about $1 million a year.

Warren King: 206-464-2247