A sign marks the entrance of the Kenneth Copeland Ministries Eagle Mountain International Church, in Newark, Texas. The Texas megachurch is linked to at least 21 cases of measles and has been trying to contain the outbreak by hosting vaccination clinics, officials said. LM Otero / AP

An outbreak of measles among unimmunized members of a Texas megachurch is fueling new health worries about pockets of vaccine-wary parents -- just as more than 50 million public school kids head back to class across the nation.

More possible measles cases are being reviewed in Tarrant County, Texas, where at least 21 people have been sickened this month at the Eagle Mountain International Church, whose ministers have been critical of vaccination. Local officials say several more cases of infections with fever and rash have been reported, but not confirmed.

“It’s concerning. It’s something we jump on,” said Russell Jones, a Texas state epidemiologist who’s been tracking the situation. “It could get into the schools.”

Public health officials say that the northeast Texas outbreak is just the latest in a small but growing number of places -- think San Diego, Calif., Boone and Hamilton counties, Ind., and, most recently, Brooklyn, N.Y. -- where vaccine resistance has sickened children and put the wider community at risk for potentially deadly infectious diseases.

“The rate of change has sort of accelerated,” said Dr. Saad Omer, an infectious disease epidemiologist at Emory University in Atlanta who studies clusters of vaccine exemptions in public schools.

“Tarrant County was not on my radar,” he added.

He’s more familiar with places such as Marin County, Calif., where rates of new kindergarteners excused from mandatory shots nearly doubled from 4.2 percent in 2005 to 7.8 percent at the start of last year, county figures show. There, at the small New Village School in Sausalito, 74 percent of entering kindergarteners said no to vaccinations.

“Like-minded people end up creating pockets of non-vaccinated or under-vaccinated communities,” explained Dr. Ari Brown, an Austin, Texas, pediatrician and spokeswoman for the American Academy of Pediatrics. “They kind of seek each other out because they have similar philosophies.”

But vaccine critics say that distrust of the drugs -- and of the public health community that pushes them -- is behind rising rates of vaccine rejection across the country.

“These are pharmaceutical products that carry a risk of death or injury that can vary according to the individuals,” said Barbara Loe Fisher, co-founder and president of the National Vaccine Information Center in Vienna, Va.

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Critics object to vaccines because of fears about safety, worries about the effects of injecting young children with dozens of doses of vaccines by age 6 and worries about possible links with other diseases, including autism, a theory that has been repeatedly debunked.

Public health officials, however, say there’s no question that the risk of vaccination is far outweighed by the benefit of immunization. Risks of vaccine side effects or failure are possible but rare, and they’re much lower than the risks associated with disease, said Omer.

In the decade before the measles vaccine was introduced in 1963, between 3 million and 4 million people in the U.S. were infected each year, 48,000 were hospitalized and 400 to 500 died. Another 1,000 developed chronic disabilities, CDC says.

Domestic cases of the disease were considered eradicated in 2000, but outbreaks continue because of imported infections brought back by travelers from areas where measles remains common.

The Texas megachurch outbreak was sparked by a non-vaccinated visitor who was infected in Indonesia and then returned to expose unvaccinated church members, staffers -- and children in a day care center. In the wider community, more than 98 percent of kids are immunized and less than 1 percent are exempt, county officials say. But the congregation of unvaccinated people allowed the disease to catch hold.

“When you hang out with other people who don’t vaccinate, you’re hanging out in the wrong herd,” said Brown, a reference to herd immunity, the necessary level of protection that keeps a disease from spreading within a community.

Church leaders, including Kenneth Copeland and his daughter, Terri Pearsons, senior pastor at Eagle Mountain, have advocated faith-healing and questioned vaccines in the past. After the outbreak began in mid-August, Pearsons has urged church members to get vaccinated and held immunization clinics.

When more than 10 percent of a certain community opts out of vaccinations, it leaves others at risk, particularly those too vulnerable for immunization, or those for whom a vaccine doesn’t work or wears off.

For some diseases, such as mumps, herd immunity can drop to as low as about 75 percent and still offer wider protection. But other diseases, such as the highly contagious measles or pertussis, known as whooping cough, require collective immunity of up to 94 percent to keep everyone safe, according to the CDC.

To be clear, the vast majority of U.S. parents do vaccinate their kids, latest federal health statistics show. Nationwide, only about 1.8 percent of the more than 4.2 million kindergarteners who entered public school last fall were exempted from laws that require that they get shots to protect against what once were the deadliest diseases of childhood: measles, mumps, rubella, polio. (Homeschooled children aren’t included in the count.)

But exemption rates also vary widely among states, from 0.6 percent in Alabama to 4 percent in Alaska and Colorado and 6.4 percent in Oregon, which leads the nation, CDC figures show.

Rates are highest in the 20 states that allow for exemptions based not only on medical and religious grounds, but also on “personal beliefs.” And within those states they can vary even more. In Oregon, at least four counties have double-digit exemption rates ranging from 12.6 percent in Curry County to 15.2 percent in Wallowa County.

In states where it has been easy to obtain exemptions -- with a simple signature, for instance -- rates of exemption have risen sharply, according to a 2012 study by Omer and colleagues. Between 2005 and 2011, the non-medical exemption rate in 13 “easy” states rose to 3.3 percent, an average annual increase of 13 percent, he found.

Some states are trying to change that trend, making it more difficult to obtain exemptions. In Washington state, where kindergarten exemption rates peaked at 7.6 percent in 2008, public health officials changed the school paperwork two years ago, separating the vaccine record from the exemption form, said Dr. Ed Marcuse, a professor of pediatrics and epidemiology at the University of Washington.

“At the same time we did that, we changed the law. In order to be exempt, you had to be counseled about vaccine-preventable diseases,” he said. The child's physician has to sign a form proving that the counseling took place.

The point was not to "simply put in hassles,” Marcuse said, pointing out that some states require notarization of exemption forms. “We wanted very much to be certain that parents made as thoughtful and well-informed a decision as possible."

The effort appears to be paying off. Kindergarten exemption rates fell from 6.2 percent in 2010 to 4.6 percent in 2012 after the initiative, state records show. Now, Oregon and California have passed similar laws bolstering requirements for exemption.

The goal, Marcuse said, is to enact policies that balance the inherent conflict in the vaccine debate between private choice and public good.

“It’s not so simple as ‘Let’s wipe out these pockets of resistance,'” said Marcuse. “Let’s help people make decisions that are both in their personal interest and in the interest of the community.”

JoNel Aleccia is a senior health writer for NBC News. Reach her on Twitter at @JoNel_Aleccia or send her an email.