Jessica Elliott is not antivaccination.

But when her son was born eight years ago, the San Francisco mother questioned the wisdom of “putting a disease” into such a young body. She finally decided that the vaccination schedule recommended by pediatricians and public health experts wasn’t a good fit for her family, so she delayed some of his shots and skipped others entirely. She did the same when her daughter was born four years later.

Parents like Elliott, public health experts believe, are responsible for the bulk of children who report to school with incomplete vaccination records — in California, on average about 8 percent of kindergarteners start school missing at least one vaccine.

Some parents question vaccines entirely. But a large, possibly growing number of parents are simply hesitant about vaccines, not dead set against them, public health experts say. Reaching those parents and overcoming their misplaced, if understandable, fears should be a public health priority.

Diseases still kill millions

“We understand that their children are very precious to them, and they don’t want them to feel pain or suffering,” said Dr. Yvonne Maldonado, chief of pediatric infectious diseases at Lucile Packard Children’s Hospital. “But these (vaccine-preventable) diseases are still killing 3 million-plus children a year in the developing world. We don’t want to see them coming back to the United States.”

Parents like Elliott are caught in the middle of a decadelong feud — re-energized in recent weeks due to a measles outbreak that’s growing daily — between anti-vaccine crusaders and supporters of childhood immunizations.

The outbreak, which started at the two Disney theme parks in Anaheim late last year, has amped up demands for more stringent immunization requirements in California. And parents who question vaccines are being repeatedly called out as ignorant or selfish.

At some schools with low vaccination rates, furious parents are demanding that unvaccinated kids stay home. Many infectious disease experts say California is too lenient on parents who choose to under-vaccinate their children or skip immunizations entirely.

California is one of only 17 states that allow personal belief exemptions — forms parents can sign that allow them to enroll their children in school without vaccinations. In states where those exemptions aren’t allowed, childhood immunization rates approach 97 or 98 percent; in California, about 92 percent of children are fully vaccinated by kindergarten, although those rates vary widely from county to county.

The rates can be misleading since they don’t differentiate between parents who refuse all vaccinations, parents who delay or opt out of only certain vaccinations, and parents who have simply fallen behind on the schedule, maybe due to financial or access-to-care problems.

At Greenwood School, a private prekindergarten-through-eighth-grade school in Mill Valley where 61 percent of kids are not up to date on their immunizations, “selective vaccination” is common, said Betsy Anderson, director of admissions.

“If a parent chooses to delay MMR (measles, mumps, rubella vaccine), which is a common one for them to choose ... it shows up” as an exemption, Anderson said. “That parent has every intention of vaccinating their child. They just chose to wait a little bit.”

Talk to parents

Many doctors say parents should be able to opt out of vaccinations only for medical reasons — if their child is immune-compromised, for example. Some pediatricians have refused to see children whose parents won’t vaccinate them.

But most doctors say that’s not the right approach. Instead, they say it’s important that doctors not reject families that shy away from vaccines and instead have frank conversations with them and do their best to work with them.

“I don’t make the final decision on vaccination, the parents do,” said Dr. Randy Bergen, a pediatrician and infectious disease expert with Kaiser in Walnut Creek. With some parents, he said, “ultimately it is a negotiation.”

The current measles outbreak is raising concerns that so many families are under-vaccinating that some diseases once thought to be eliminated could resurface in the United States.

Measles already is showing signs of resurging. From 2000 to 2010 there were, on average, fewer than 100 cases of measles nationwide annually, according to the U.S. Centers for Disease Control and Prevention. Last year there were 644 cases, and this year there were more than 90 cases in January alone, almost all of them in California.

As of Friday, the Bay Area had 13 cases this year, including two reported Friday of siblings in Marin County — which is regularly spotlighted as a hub of anti-vaccination parents activity. The Marin County children were not vaccinated.

At schools and in doctors’ offices around the Bay Area, anger and frustration are growing with parents who continue to doubt vaccines. A father in Marin County whose son had cancer and is still immune-compromised — which means he can’t be vaccinated — demanded last week that unvaccinated children be forced to stay home.

Frustration with doubters

That message has been echoed at other schools.

Anne Janks, whose child goes to an Oakland school where 9 percent of children are not fully vaccinated, fears for the health of a student at the school known to have a compromised immune system.

“This isn’t about your personal belief. Santa Claus is your personal belief,” Janks said. “My family vaccinated because it’s the right thing to do. We believe in science.”

In Oakland, Jennifer Simon had to quarantine her 6-month-old infant, Livia, who is too young to be vaccinated, after the baby was exposed to measles at her pediatrician’s office by an unvaccinated child with the illness.

“I know how contagious measles is so we were really worried. This is why people need to get vaccinated: to protect those who cannot be vaccinated,” said Simon, who had to keep her daughter isolated for 28 days. The quarantine period ended Friday and her daughter is not sick.

“People think of vaccinations like a personal parenting decision. It’s not,” she said. “It’s a public health decision.”

The current outbreak may actually help persuade more parents to vaccinate, public health officials said. Many pediatricians say they’ve heard from parents who are doubtful about vaccines in general ask for the specific measles vaccine.

Outbreak sways some

It’s not surprising that some parents previously on the fence about vaccines are being swayed now, doctors say. Many parents, and even some doctors, have never seen measles — or other diseases like polio that are mostly gone now — and don’t understand how dangerous they can be.

“I remember when the polio vaccine became available, the line was around the block at school. Everybody wanted to get vaccinated,” said Dr. John Swartzberg, an infectious disease specialist at UC Berkeley. “Today, our success with vaccines has created this ironic problem — we just don’t see these diseases in the United States. And if no one sees it, what’s to worry about? All I see is you’re sticking a needle in my child.”

For parents of young children, their only personal experience with vaccine-preventable illnesses may be chickenpox and the flu. They likely don’t remember chickenpox as an especially serious illness, and the influenza vaccine, which this year is only about 25 percent effective, “is not our best, obviously,” said Dr. Jeffrey Silvers, an infectious disease specialist at Sutter Health’s Eden Medical Center in Castro Valley.

“A measles vaccine is an extremely safe, well-tested vaccine that’s extremely effective,” Silvers said.

Claims debunked

When few parents have ever seen or heard of a case of measles or other vaccine-preventable disease, the vaccine can start to look like the more dangerous threat, public health experts say. And vaccines do have side effects — they can cause fever, a mild rash, fatigue and nausea. In rare cases — about 1 in 3,000 — the measles and some other vaccines can cause seizures.

The CDC reports that even more rare — thought to be about 1 in a million vaccine doses — are serious complications including brain damage or death. But those events are so unusual that doctors aren’t convinced they’re directly tied to vaccines.

Some vaccine critics still believe long-debunked research that showed a link between vaccination and autism. Others believe that vaccines, when given too early and in too high doses, can trigger asthma, allergies and other chronic conditions.

But there’s no scientific evidence to their claims, including the notion that vaccines are too harsh for an infant or child’s immune system, doctors and public health experts say. Indeed, to even suggest that there’s a “debate” over vaccine safety is problematic, because the science is so supportive of immunizations, said Swartzberg at UC Berkeley.

“There’s no debate here. It’s like debating whether the sun is going to rise tomorrow,” he said.

While delaying vaccines or opting out of some shots may seem like a reasonable choice for families, it’s a dangerous practice, health care providers say.

Shot timing important

The childhood immunization schedule — which includes 17 injections in just the first two years of life — is designed to give kids the best protection at the right ages, said Maldonado of Packard Children’s Hospital. Parents who tinker with the schedule are leaving their children vulnerable.

Some children will undoubtedly fall behind, doctors say. Altering the schedule also could threaten the “herd immunity” — the widespread vaccination required to keep an infectious disease from becoming endemic and to protect those who can’t be immunized.

Not all parents who hesitate to vaccinate their children necessarily believe that childhood immunizations cause autism or other chronic health problems. Many simply have trouble accepting as fact that dosing a small child, multiple times in his first few years of life, with vaccines is necessary and entirely safe. Elliott, for example, delayed some vaccines for her son and daughter out of concern about overwhelming their developing immune systems.

“When he was born, we decided we’ll totally do vaccines, but let’s give his body time to strengthen his own immunity,” said Elliott, whose two children are now vaccinated for everything but chickenpox and hepatitis B.

Elliott acknowledges the social responsibility of protecting others, especially those who are unable to get vaccinated. She admits to succumbing to a bit of peer pressure in meeting that obligation.

“In a vacuum, I would have done no vaccines,” she said. “All of the voices that said 'get the vaccines’ got a little louder.”

Erin Allday and Victoria Colliver are San Francisco Chronicle staff writers. E-mail: eallday@sfchronicle.com, vcolliver@sfchronicle.com