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OLYMPIA, Wash. — Amid a measles outbreak that has sickened more than 50 people in the Pacific Northwest, Washington lawmakers heard testimony Friday on a bill that would remove parents' ability to claim a personal or philosophical exemption to opt their school-age children out of the combined measles, mumps and rubella vaccine.

Hundreds of people opposed to the measure lined up more than an hour before the start of the hearing before the House Health Care and Wellness Committee, many wearing stickers with the bill number, HB 1638, within a crossed out circle.

The bill comes as health officials have reported at least 52 known cases in Washington state and four in Oregon. Most of the Washington cases are concentrated in Clark County, just north of Portland, Oregon. The measure is sponsored by a lawmaker from that region, Republican Rep. Paul Harris of Vancouver, and has the support of the state medical association and Gov. Jay Inslee. Inslee declared a state of emergency last month.

Harris said people in his area are "concerned about our community, its immunity and the community safety."

The measure does allow proof of disease immunity through laboratory evidence or history of disease to substitute for immunization.

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Currently, the state allows school-vaccination exemptions for children at public or private schools or licensed day-care centers based on medical, religious and personal or philosophical beliefs. Unless an exemption is claimed, a child is required to be vaccinated against or show proof of acquired immunity of nearly a dozen diseases — including polio, whooping cough and mumps — before they can attend school or a child care center.

John Wiesman, the secretary of the state Department of Health, said the effort to limit exemptions is "about safe schools and protecting vulnerable children."

Wiesman told the panel that compared to other outbreaks in the state in the past decade, "the outbreak we are dealing with right now is larger and infecting people faster than recent history."

Opponents testifying against the bill included environmental activist Robert F. Kennedy, Jr., who questioned safety standards around vaccines.

Susie Corgan, with Informed Choice Washington, said after the hearing that parents who are worried about their children having adverse reactions to vaccines have a right to philosophical exemptions.

"Where there is risk, there must be choice, and there is risk with this vaccine as there is with any other medical procedure," she said.

Four percent of Washington secondary school students have non-medical vaccine exemptions, according to the state Department of Health. Of those, 3.7 percent of the exemptions are personal, with the remainder being religious exemptions. In Clark County, 6.7 percent of kindergartners had a non-medical exemption for the 2017-18 school year, according to health officials.

Washington is among 17 states, including Oregon, that allow some type of non-medical exemption for vaccines for "personal, moral or other beliefs," according to the National Conference of State Legislatures.

Legislation introduced in Washington state in 2015 that would have removed the personal or philosophical belief allowance for an exemption to childhood vaccines never made it to the House floor for a vote after it faced stiff opposition.

Both the WSMA and Inslee also support a broader bill that was introduced in the Senate earlier this week. That measure, which has not yet been scheduled for a hearing, would not allow personal or philosophical exemptions to be granted for any required school vaccinations.

California removed personal belief vaccine exemptions for children in both public and private schools in 2015, after a measles outbreak at Disneyland sickened 147 people and spread across the U.S. and into Canada. Vermont also abandoned its personal exemption in 2015.