BOSTON — Sabrina Rahim doesn't practice any particular faith, but she had no problem signing a letter declaring that because of her deeply held religious beliefs, her 4-year-old son should be exempt from the vaccinations required to enter preschool.

She is among a small but growing number of parents around the country who are claiming religious exemptions to avoid vaccinating their children when the real reason may be skepticism of the shots or concern they can cause other illnesses.

Some of these parents say they are being forced to lie because of the way the vaccination laws are written in their states.

"It's misleading," Rahim admitted, but she said she fears that earlier vaccinations may be to blame for her son's autism. "I find it very troubling, but for my son's safety, I feel this is the only option we have."

An Associated Press examination of states' vaccination records and data from the Centers for Disease Control and Prevention found that many states are seeing increases in religious exemptions claimed for kindergartners.

"Do I think that religious exemptions have become the default? Absolutely," said Dr. Paul Offit, head of infectious diseases at Children's Hospital in Philadelphia and one of the harshest critics of the anti-vaccine movement. He said the resistance to vaccines is "an irrational, fear-based decision."

The number of exemptions is extremely small in percentage terms and represents just a few thousand of the 3.7 million children entering kindergarten in 2005, the most recent figure available.

But public health officials say it takes only a few people to cause an outbreak that can put large numbers of lives at risk.

"When you choose not to get a vaccine, you're not just making a choice for yourself, you're making a choice for the person sitting next to you," said Dr. Lance Rodewald, director of the CDC's Immunization Services Division.

All states have some requirement that youngsters be immunized against such childhood diseases as measles, mumps, chickenpox, diphtheria and whooping cough.

Twenty-eight states, including Hawai'i, allow parents to opt out for medical or religious reasons only. Twenty other states also allow parents to cite personal or philosophical reasons. Hawai'i is not on that list. Mississippi and West Virginia allow exemptions for medical reasons only.

From 2003 to 2007, religious exemptions for kindergartners increased, in some cases doubled or tripled, in 20 of the 28 states that allow only medical or religious exemptions, the AP found.

While some parents — Christian Scientists and certain fundamentalists, for example — have genuine religious objections to medicine, others are simply distrustful of shots. Some fear that the vaccinations themselves may make their children sick and even cause autism.

Though government-funded studies have found no link between vaccines and autism, loosely organized groups of parents and even celebrities such as radio host Don Imus have voiced concerns. Most of their furor has been about a mercury-based preservative, once used in vaccines, that some believe contributes to neurological disorders.

Unvaccinated children can spread diseases to others who are unvaccinated or those for whom vaccinations provided less-than-complete protection.

In 1991, a religious group in Philadelphia that chose not to immunize its children touched off an outbreak of measles that claimed at least eight lives and sickened more than 700 people.

And in 2005, an Indiana girl who had not been immunized picked up the measles virus at an orphanage in Romania and unknowingly brought it back to a church group. Within a month, 31 people had been infected in the nation's worst outbreak of the disease in a decade.

Rachel Magni, a 35-year-old stay-at-home mother in Newton, Mass., said she is afraid vaccines could harm her children and "overwhelm their bodies." Even though she attends a Protestant church that allows vaccinations, Magni pursued a religious exemption so her 4-year-old daughter and 1-year-old son, who have never been vaccinated, could attend preschool.

"I felt that the risk of the vaccine was worse than the risk of the actual disease," she said.

Barbara Loe Fisher, president of the National Vaccine Information Center, said she empathizes with parents tempted to claim the religious exemption that her center discourages: "If a parent has a child who has had a deterioration after vaccination and the doctor says that's just a coincidence, you have to keep vaccinating this child, what is the parent left with?"