Many charter schools in Oregon have such low student vaccination rates for measles that they'd be at risk if the bug – once declared eliminated in the United States – infected anyone in their school.

An analysis by The Oregonian/OregonLive shows that nearly 65 percent of the state's public charter schools lack what scientists call herd immunity against measles, meaning not enough children are immunized to prevent the disease from sweeping through their immediate community.

Those charter schools serve nearly 13,000 students across the state, from Portland to Grants Pass and Silverton to Baker City.

A much smaller percentage of traditional public schools fall into the same category, but they have more students, so the potential exposure is greater. Just over 60,000 students attend those schools, also spread across the state.

The risk worries health officials. Measles is highly contagious, potentially fatal and has made a resurgence in recent years.

It's the one disease, among all of those on the list for schoolchildren, that most needs such immunity to protect against an outbreak, said Dr. Paul Cieslak, medical director of the infectious disease and immunization programs at the Oregon Public Health Division.

The virus spreads easily by coughing or sneezing and can live for up to two hours in an area where an infected person has been. But the measles-mumps-rubella vaccine is considered highly effective against measles. Two doses will prevent 97 percent of people who are exposed from getting sick.

When people get vaccinated, they also shield those who aren't – providing a safeguard for the "herd."

There hasn't been a measles eruption in an Oregon school yet, but Cieslak said: "If you drop a case of measles into one of those schools, it would spread like wildfire. So far, we've been lucky."

OUTBREAKS IN RECENT YEARS

Measles used to infect about 4 million people a year in the United States, hospitalizing 50,000 and killing 500.

But widespread vaccination starting in the 1960s led the federal Centers for Disease Control and Prevention to declare in 2000 that the disease was eliminated, no longer endemic in the United States.

It's made a comeback with a rise in skepticism about vaccines and as Americans travel to countries where the disease runs rampant bring it back. In 2015, nearly 150 people became infected with the measles in seven states, including Oregon. One woman in Washington state died.

The year before, almost 670 people nationwide contracted measles in dozens of outbreaks, with nearly 400 infected in an Amish community in Ohio.

In both years, most of those who got sick weren't vaccinated, according to the Centers for Disease Control.

Oregon historically has had among the lowest overall vaccination rates in the country.

For the 2016-17 school year, the latest nationwide data available, 6.7 percent of Oregon kindergartners claimed at least one exemption, according to the Centers for Disease Control. Only Alaska had a higher rate.

Besides measles, vaccinations required by Oregon schools include diphtheria, tetanus, pertussis (whooping cough), mumps, rubella, hepatitis A and B, polio and varicella (chickenpox).

Children who aren't fully vaccinated by Feb. 21 or who haven't sought an exemption from state health officials will be sent home from public and private schools, preschools, child care centers and Head Start programs. Children can get exemptions for medical reasons with their doctor's approval. Nonmedical exemptions require a certificate from a provider or proof that the parent has watched an online video.

Some parents opt out of all vaccines and some avoid certain shots, said Stacy de Assis Matthews, the immunization school law coordinator at the Oregon Public Health Division.

But there's not one particular vaccine that parents avoid more than others, de Assis Matthews said.

Yet there is a clear divide on vaccination rates for measles between traditional public schools and charters, as the newsroom's analysis shows.

CHARTERS LEAD LIST

The newsroom gathered Oregon Health Authority vaccination data to calculate herd immunity, which is based on the percentage of people vaccinated multiplied by the effectiveness of the vaccine.

The effectiveness of the measles vaccine after two doses of the measles-mumps-rubella shot is 97 percent. According to the authoritative "Vaccines" textbook, herd immunity for measles starts with an immunity rate of 92 percent.

Just over 250 schools in the state fail to reach that threshold. The 10 with the lowest measles herd immunity rates ranged from 37 percent to 69 percent.

The Oregonian/OregonLive analysis found that seven of the lowest 10 were charter schools – two from Portland, and one each from Central Point, Dallas, Eugene, Grants Pass and Silverton.

The three traditional schools among the 10 are in Ashland, Baker City and Eugene.

It's not clear why charter schools have lower measles vaccination rates than traditional public schools.

Kristen Miles, board development specialist at the Oregon School Boards Association, said there's not one pattern of demographics, finances or other measure that characterizes charter schools.

"It's very hard to average them," Miles said.

But she said in general families that gravitate to charter schools tend to look for specialized programs that don't exist in other schools or they want a smaller environment with potentially smaller class sizes.

"Those two things are the biggest draw," Miles said.

Woodland Charter School in Grants Pass has the lowest herd immunity rate for measles in the state. Administrator Phil Centers said he was taken aback to hear the results, but he wasn't necessarily shocked that many parents have opted out of the shots for their children.

He noted that the school's 158 students come from diverse communities, ranging from people who don't want the government meddling in their lives to those who eschew traditional medicine, relying on alternative solutions.

Woodland and several of the charter schools on the list -- Village School in Eugene, Madrone Trail Public Charter School in Central Point and the Portland Village School -- are Waldorf-inspired.

These schools take a holistic approach -- focusing on a student's head, heart and body, Centers said.

"The key motive is to educate the whole child," he said.

The school and others like it attract parents who want their children to experience a personal connection at school, he said. Those parents also want a choice – in the type of school their children attend, for example, and their health care.

"I don't think it's a distrust of the medical community," Centers said. "It's just people saying what's the best thing for my kids, the best medical treatment, the best diet."

'I HAVE ALWAYS BEEN PRETTY SKEPTICAL'

That's the case for Melody Frazier, an Oregon mother who doesn't want to be further identified out of fear of the reaction in the community.

"I have always been pretty skeptical," Frazier said. "I was never vaccinated, and my parents had eight of us."

Yet when she had her son, she started the vaccine schedule. But he didn't react well to the shots, crying nonstop, and suffered chronic ear and eye infections and developed severe colic and eczema, she said.

"I decided to quit," she said. "All these things started to go away."

Still, when he was ready to go to school, she caught him up on his vaccinations. She also had a younger daughter vaccinated.

But her experience with her third child turned her away from vaccines forever.

The girl started shrieking within hours after receiving her shots, she said. The crying went on for months.

Frazier found a pediatrician in Portland, who advised not giving vaccines to children in their first year if the parent has an auto-immune disease. Frazier suffers from celiac disease, when the body mounts a violent immune reaction to gluten.

She stopped having her children vaccinated.

"I was told my kids will die without these but then I'm watching my daughter get better without them," she said.

Many parents who choose not to vaccinate their kids worry about the aluminum in some shots. They also worry about giving a child a vaccine with a live virus, like measles, even though it's weakened to prevent illness.

Others worry about their children becoming autistic, though federal health officials say there's no link.

Parents who opt out also point to their grandparents, noting that they weren't vaccinated.

"Getting measles, getting mumps in childhood is a rite of passage," said Brittany Ruiz, an Oregon mother who hasn't had her children vaccinated. "Every single person that I know that have received vaccines on the CDC schedule are so sick compared to my children."

INJURIES RARE, HEALTH OFFICIALS SAY

Public health officials say vaccines have saved countless lives.

"Injuries, other than sore arms and an occasional fever, are rare," said Cieslak, the state infectious disease specialist.

"We advise parents to get their kids fully vaccinated – according to the carefully thought-out schedule developed by the national Advisory Committee on Immunization Practices and endorsed by the American Academy of Pediatrics."

State health officials have no public information campaigns planned for boosting measles vaccination rates. But Aaron Dunn, immunization section manager at the Oregon Public Health Division, said his unit works closely with coordinated care organizations and local clinics, encouraging them to mount initiatives to bolster childhood vaccinations.

The state also keeps track of vaccination trends, putting the data online for parents and educators to see.

"The thing that makes me sweat is if you dropped a case of measles into one of those schools that has a high exemption rate," Cieslak said.

He said he hopes that doesn't happen.

-- Lynne Terry and Melissa Lewis of The Oregonian/OregonLive