School nurses have until next week to tell Missouri whether students have all the vaccinations required to attend school.

And it's likely thousands of children won't have them.

In the past several years, more and more Missouri schoolchildren have skipped immunizations for religious or medical reasons. Now health experts fear even more parents will resist efforts to vaccinate children against swine flu.

Though only a small fraction of parents have excluded their children from mandatory school vaccinations, last year about 7,600 schoolchildren got exemptions for each of the required shots. Five years before, that number was about 4,900. That's a 55 percent increase in exemptions at a time when enrollment has remained steady.

In less than a fourth of the cases, the exemptions are for doctor-approved reasons, such as allergies or a natural immunity to a disease. Far more often, parents simply claim a religious exemption, which the state or school does not verify.

School officials often suspect the parent is simply wary and doesn't necessarily have a particular religious belief about vaccines; officials are left to promote the medical benefits and quell fears.

Schools are playing a greater role in immunizing children — many will serve as distribution centers this fall for vaccines for the H1N1 virus, otherwise known as swine flu. The Centers for Disease Control and Prevention says it is having to reassure concerned parents, worried that the testing and production was rushed, that the vaccine is safe.

Koreen Bowers of Jefferson County is not getting regular flu or H1N1 shots for her three daughters. She figured she would rather deal with the risk of getting sick from the flu rather than getting sick from the vaccine.

According to an Associated Press poll, as many as 38 percent of parents said they were unlikely to give permission for their kids to be vaccinated for H1N1 at school, often over concerns about side effects.

Bowers put her daughters on a delayed and selective schedule for other vaccinations after her oldest daughter, now 6, became jittery and unable to sleep after receiving vaccinations as an infant. She got religious exemptions for the oldest daughter, now in first grade, and the middle daughter, now in kindergarten.

The girls attend a private Lutheran school, and Bowers has since learned some other school parents have done the same. "I just think people need to be open to different alternatives and do a little bit more research," she said. "It doesn't mean they don't care for their kid; it doesn't mean you're antivaccine. It means you're doing what you think is best."

Jeannie Ruth, the bureau chief of immunizations for the state of Missouri, says the increasing numbers of parents who opt out of vaccines is a concern, but not statistically significant. "But we're not going to ignore it, and we're going to continue to educate people so they understand the safety of vaccines," she said.

Linda Neumann, president of the Missouri Association of School Nurses and a nurse at Hixon Middle School in the Webster Groves School District, said this year seven of Hixon's 650 students have religious exemptions on file. Most are Christian Scientists, who may choose not to get their children vaccinated. Usually fewer than 10 students every year have exemptions, she said.

"If it's for their organized religion, we have to respect that," Neumann said. "It's the parents that come in and say, 'I hear there's a religious exempt card I can sign.' Those are the parents we invite to sit down, and we just talk about immunizations. Most of the time they get (their children) immunized."

About 9,700 Illinois schoolchildren, a fraction of a percent of all students, got religious and medical exemptions for the most common vaccines last school year, and about 5,300 of them got exemptions 10 years before that. That represents an 83 percent increase at a time of stable school enrollment.

Anxiety about immunizations is on the rise, even as the cases of vaccine-preventable diseases has decreased. The Journal of the American Medical Association has tracked an increase in nonmedical vaccine exemptions between 1991 to 2004.

All states permit medical exemptions, 48 allow religious exemptions, and 18 states allow exemptions based on philosophical or personal beliefs, according to the National Vaccine Information Center. Missouri and Illinois allow only religious and medical exemptions.

The reasons for anxiety about vaccines vary. Some parents don't like the idea of the government telling them how to oversee their children's health. Others may fear adverse reactions, ranging from swelling at the injection site to death.

In recent years, the most explosive issue has involved allegations from vaccine safety groups that some vaccines are tied to autism, but federal officials cite multiple studies that can't prove a link.

Karla Allsberry of Troy, Mo., chose not to give her now 10-year-old son any more immunizations or boosters because of severe reactions he had as a baby and toddler. She said he became ill with intestinal problems and rashes, and his arm swelled.

He has a medical exemption on file, even though her son is now home-schooled, Allsberry says. Her younger son, 4, attends preschool two mornings a week and is exempted for religious reasons. Because of his brother's reactions, Allsberry has not had him vaccinated, and the boy is very healthy, she said.

"I'm not an antivaccine person, I just advocate for safer vaccines," says Allsberry, who would like to see more third-party oversight on vaccine ingredients like thimerosal. "I don't think anybody — a doctor, the governor — should tell a parent that they should or should not get their child vaccinated. I think it's the parent's decision to decide that." And parents should learn for themselves what goes into vaccines, she said.

Studies say unvaccinated children are at risk for vaccine-preventable diseases, and interventions such as school immunization requirements have led to record or near-record lows in the levels of vaccine-preventable diseases. Experts also say the vaccines have been proven safe.

"The most common side effects are just redness at the injection site and some swelling. Other side effects are very rare," said Arleen Porcell-Pharr, spokeswoman for the CDC. "The decision of not getting your child vaccinated is not without risk either."

Ruth, with the Missouri Department of Health and Senior Services, says the department has sponsored several more ad campaigns in the past year than previously, mainly because it is getting more immunization grant funds from the Centers for Disease Control and Prevention. The campaign includes public service announcements in English and Spanish on radio stations and magazines such as Rural Missourian, which reaches audiences that might not have access to information on vaccines or adequate medical services.

"We really need to have healthy Missourians," she said. "Vaccines are one of the best public health practices ever. When you look at how disease rates have gone down by 98 percent because of vaccines, that just makes you feel good at the end of the day."