Heartland Charter School in Kern County has several dream field trips on the calendar this spring, including tours of In-N-Out Burger, an Amtrak train ride along the Central Coast and a matinee performance of “Harry Potter and the Cursed Child” at the Hollywood Pantages.

The outings may not seem unusual, but Heartland’s student body differs from other California schools in one major way.

Just 5% of Heartland’s 810 kindergarten students received all their childhood vaccines last year, and 9% were vaccinated against measles, according to a Times analysis of data that California schools report to the state. The vaccination rate for kindergarten students across the state last year was 93.7%.

Heartland is among the largest of California’s independent study charter schools, which allow parents to enroll their children in the public school system but avoid the state’s strict vaccine requirements by educating them at home or online.

Such programs — sometimes called homeschool charters, online charters or virtual charters — boomed during the COVID-19 pandemic and offer more flexibility than a traditional school.

They also serve as a legal refuge for California parents who don’t want to vaccinate their children or leave the public school system. Some public health departments in the Golden State attribute declining vaccination rates to such programs, which can enroll hundreds or even thousands of children.

The publicly funded schools are among the few remaining soft spots in California’s stringent childhood vaccination laws, which lawmakers tightened after a measles outbreak that began at Disneyland in 2014 sickened more than 300 people.

In 2015, lawmakers passed Senate Bill 277, which banned personal belief exemptions for childhood vaccinations. In 2019, they tightened scrutiny of medical exemptions for unvaccinated children. The laws still allow parents to skip immunizations for children who are enrolled in independent study programs and do not “receive classroom-based instruction.”

But the state’s vaccination laws don’t specify what “classroom-based instruction” means, including whether students must be vaccinated if they attend some in-person classes offered by their school or by a third-party vendor, or if they attend school-sanctioned activities such as field trips, soccer practice or prom.

“There is a tremendous amount of gray area,” said Jeff Rice, the founder and director of Assn. of Personalized Learning Schools & Services, or APLUS+, a trade group for charter schools with students who pursue a mix of in-person, at-home and online learning.

Under California’s education code, a school is “nonclassroom-based” if 80% of learning occurs off campus.

When California tightened its vaccination laws, Rice said that he pressed for clarity in immunization requirements for students who don’t attend traditional in-person schools five days a week. Rather than define what “nonclassroom- based instruction” meant, he said, the state left that decision to the school boards and county education offices that regulate charter schools.

Among the 100 schools that are APLUS+ members, Rice said, two-thirds of students take classes in person at least one day a week.

“Vaccinations are an issue for a small percentage of parents who have very strong and passionate feelings about it,” Rice said. Schools with low vaccination rates, he said, “are a reflection of the values of that individual community.”

According to a state Department of Education statement, the Department of Public Health oversees the California law that “outlines the rules for mandatory immunizations.” A spokesperson for Public Health said the department “does not have regulatory authority over this issue,” and added that “decisions on student participation in school field trips or athletics are decided at the local level.”

The U.S. is in the midst of the largest measles outbreak in six years, with 800 cases and three deaths reported in 25 states, including nine cases in California.

Dr. Shannon Udovic-Constant, a pediatrician in San Francisco and the president of the California Medical Assn., said measles is “incredibly contagious,” spreading when someone coughs or sneezes and lingering in the air for up to two hours. She said 90% of unvaccinated people who are exposed will contract measles.

To be unvaccinated, she said, “is a risk, and it’s a risk you can’t see.”

The vast majority of unvaccinated students are enrolled in individualized education plans or independent study programs, which under state law means they don’t have to be vaccinated. The number of students who reported medical exemptions granted by doctors is very low.

Most of the state’s largest online charter schools had low vaccination rates, but not all. River Springs Charter in Riverside County, which reported a mix of online and in-person instruction, said that 77% of its 1,036 kindergarten students were up to date on all their vaccines last year, state data show.

Feather River Charter School in Sutter County, part of the Sequoia Grove Charter Alliance in Northern California, reported to state regulators that the program is 100% “nonclassroom-based.” Last year, 18% of the school’s 321 kindergarten students were up to date on all their vaccines and 21% were vaccinated against measles. Two other schools in the alliance also reported overall vaccination rates below 20% last year.

The alliance’s website includes a calendar featuring a “Tween/Teen Games Meet Up” in Elk Grove, regular library visits and a masquerade-themed prom night Friday. A video posted on Feather River’s Facebook shows a large group of kids attending a recent field trip to Shasta Caverns.

At Visions in Education in Sacramento County, 40% of the school’s 580 kindergarten students were up to date on all their shots last year and 44% were vaccinated against measles, according to state data. The school requires students in seventh grade and above to get their Tdap booster, which provides elevated immunity against tetanus, diphtheria, and pertussis, or whooping cough. On its Instagram account, the school has marketed a middle school soccer club and an ice-skating field trip.

Representatives for Heartland and the Sequoia Grove alliance did not respond to requests for comment.

“As a longtime part of California’s public school community, our commitment to accountability includes following the state and federal laws,” Visions in Education Supt. Steve Olmos said in an emailed statement.

Olmos did not address questions on whether students have to be vaccinated to participate in field trips or group sports, but said the school has a “comprehensive system in place to ask families for their students’ vaccine history at several points during their enrollment.”

Former state Sen. Richard Pan, a Sacramento Democrat who wrote California’s vaccine laws, said regularly gathering in person “certainly violates the spirit of the law.” Still, he said the low vaccination rates at online charter schools didn’t surprise him, because he knew when he wrote SB 277 that not every parent would vaccinate their kids.

“Having an online school or an independent study program where they’re not in school with all the other kids was a deliberate option that we provided to those families,” Pan said. But, he said, getting a cohort of unvaccinated children together puts them in danger of contracting communicable disease.

“They shouldn’t be doing that on a regular or frequent basis,” he said.

Lance Christensen, vice president of education policy and government affairs at the California Policy Center, a conservative think tank, disputed the idea that some schools and parents are using the online programs to avoid vaccination requirements while still operating in similar ways to traditional in-person classrooms.

“There is no such thing as loopholes in the law,” Christensen said. “They are using whatever legal means they have to do whatever they want to do. Whether I agree with it or not, I don’t care…. I’m not everybody’s dad.”

Christensen, who unsuccessfully ran for superintendent of public instruction in 2022, said he vaccinated his five children and believes in the importance of some childhood immunizations.

Like many families during the pandemic, he also enrolled his children in virtual charter schools when their Sacramento-area schools remained closed. Many families, he said, choose these schools for a variety of reasons, whether vaccine-related or because they think they offer better education than traditional in-person public schools.

Tom Reusser, Sutter County Schools’ superintendent, said such virtual schools were largely to blame for the county’s childhood immunization rate, which, at 73%, is the state’s lowest. Most of the traditional, in-person public schools in his district have reported vaccination rates largely about 95%, he said.

“Pull the charters out, and we’re doing just fine,” Reusser said.

Public health officials in Sutter County also attributed their decline in vaccination rates to a “small number of charter schools and independent study students.” The “majority” of the students enrolled in those schools don’t live in the county, they said.

Homeschool and online charters can enroll students from both their home counties and surrounding counties. Feather River, for example, serves students in Sutter, Butte, Yuba, Placer, Sacramento, Yolo and Colusa counties, according to the school’s website. Kern County schools such as Heartland can also enroll students from San Bernardino, Los Angeles, Ventura, Santa Barbara, San Luis Obispo, Kings, Tulare and Inyo counties, a potential attendance area of hundreds of miles.

At Heartland, parents are asked to keep their children home if anyone in the household is sick, but vaccination requirements aren’t mentioned. In a Q&A posted on its website, Feather River, the school in Sutter County, notes that because the school is an “independent study program with no classroom-based instruction,” immunizations are not required.

“While you will be asked to submit an immunization form at the time of enrollment, it does not need to be complete and will not affect your enrollment status,” the website reads.