A new state law is making it more difficult for parents to send their children to school without first getting immunized against diseases.

The bill, signed into law by Gov. Chris Gregoire on Monday, would require parents to receive a written certificate of exemption by a licensed health care provider to enroll their children in public schools without the recommended battery of vaccinations designed to protect children.

“For too long Washington has had an easy way for parents to opt out,” said Secretary of Health Mary Selecky. “All it took was a simple check on a form.”

This extra requirement for parents opposed to vaccines for their children is expected to bolster the state’s vaccination rates, which have historically lagged behind most other states.

Some parents oppose vaccines on religious grounds. Others continue to believe that vaccines are harmful despite a multitude of studies supporting the safety of vaccines for children.

Health officials are counting on medical professionals, including physicians, physician assistants, osteopaths, naturopaths and advanced registered nurse practitioners, to stress the importance of immunizing children against many diseases.

Selecky noted that pertussis – commonly called whooping cough – along with measles are two diseases that have spread among populations of children who have not been vaccinated.

Keeping a registry of immunized children at schools helps staff protect vulnerable students in case a contagious disease is diagnosed.

Washington’s immunization rates have gained in recent years, although it ranks 34th among the states. The state’s historically poor showing has been especially evident in Eastern Washington, where vaccination rates have been among the lowest. It’s much the same in North Idaho.

Health officials, though, say the bill follows recent progress.

For example, the standard 15-dose series vaccination rate for Washington toddlers ages 19 months to 3 years old is now 70.3 percent, bumping the state up 15 spots to 17th.

“We want parents to really think about the health of their children,” Selecky said.