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I belong to a local parenting "listserv," where moms and dads share information on everything from breastfeeding and circumcision to tantrums and teething. Over the past few weeks, there's been a persistent topic of both curiosity and debate roiling through our group's e-mails: vaccine slowdown.

Vaccine slowdown, or spreading out the number of shots given to babies and toddlers over several doctor visits, has become a growing phenomenon and a white-hot issue among parents. This idea was sparked by the continuing concern that the shots kids get during childhood cause autism—the devastating spectrum of developmental disorders that can trigger severe problems with a child's speech, behavior, socialization and motor skills.

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Before you decide what's best for your children, it's important to know all the facts.

The idea that the very vaccines that are designed to protect our kids against serious, life-threatening diseases like polio, measles and tetanus might actually harm them is not new. And neither is the fear that autism is linked to immunization. However, time and again major studies have found no connection between vaccinations and autism. The American Academy of Pediatrics (AAP) and other mainstream medical organizations have reported repeatedly that there is no proven association.

"My best friend's brother is autistic, so I am very empathetic to a family's struggle with autism," says Sharon Denise Allison-Ottey, a physician and executive director of the COSHAR Foundation, a national organization dedicated to improving health awareness. The foundation hosts "Immunization Sundays" in African-American churches to raise consciousness about the importance of vaccines. "Certainly any parent wants to protect their children," says Allison-Ottey, "and the best way to do that is by vaccinating children from preventable diseases that are life-threatening."

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So in the face of science, why does this fear of immunization persist? First, it's a numbers game. Autism diagnoses have skyrocketed since the 1990s. The Centers for Disease Control and Prevention has estimated that one in 150 children—most often boys—have some form of it. It is not clear if the mounting rates are due to a change in the definition of the disease, an improved ability to diagnose it or an actual increase in illness, but, according to the Autism Society of America, it is now the nation's fastest-growing developmental disability.

At the same time that autism and related disorders have exploded, so has the number of shots given to children. Back in the 1940s, only one vaccine—smallpox—was administered routinely. (Of course, in those days, children contracted all kinds of diseases, including polio, and many died.) Now, by the age of 12, most kids will have received 30 to 35 doses of vaccines to protect against 34 diseases.

Maybe it's just bad timing, but symptoms of autism and other developmental disorders often show up before age 2—right around the time very young children get the bulk of their vaccinations. Horror stories abound—especially on the Web—of children who begin to show developmental delays right after they've gotten immunized. Is this a terrible coincidence, or did those shots cause the problem? No one knows for sure.

"If a child has a genetic predisposition to autism, signs of the disease would be revealing themselves at the same time that child would be getting another series of vaccines," says. Allison-Ottey. "But at this point, there is no way to tell which came first."

How exactly could a shot or series of shots cause autism? In the past, the worry was that thimerosal, a mercury-based preservative used in vaccines, might be the culprit. As a precautionary measure, in 1999, the AAP asked manufacturers to remove thimerosal from all pediatric vaccines, and it has not been used in shots given to kids since 2001. Nonetheless, autism cases have continued to rise.

More recently, the theory has been tweaked: Looking at the number of shots given at each pediatric visit, many parents began to wonder whether a child's immune system could withstand so many immunizations. Many are especially wary of the MMR, a combination vaccine given to children at 1 year and 5 years to protect against measles, mumps and rubella. Could all those shots—at once—weaken the immune system, leaving the child vulnerable to autism and who knows what else? Again, no scientific data and no mainstream medical organization have found a connection. In Japan, the combined MMR vaccine was banned in 1993, and children receive individual injections against each disease. Yet autism has continued to increase there as well.

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Still, according to the National Network for Immunization Information, almost half of the parents surveyed by the organization expressed concern about autism as a reason for not vaccinating their children. And many parents are choosing not to vaccinate. In April, the CDC reported that more than a quarter of American children are not meeting the U.S. government's recommendations for childhood vaccinations. In early May, unvaccinated adults and children triggered a large outbreak of measles in at least seven states, leading to more cases than in any recent year. Measles is highly contagious and can cause severe illness, including pneumonia.

These recent statistics and stories are extremely worrisome for experts. "Vaccines are the biggest public health success since clean water," says Jennifer Shu, an Atlanta-based pediatrician and spokeswoman for the AAP. "But you don't hear about it. I'm very concerned that as parents and pediatricians are going in the other direction, we will see more and more of these preventable diseases come back."

This is clearly a complicated issue and very frightening. So rather than not vaccinating children at all and leaving them vulnerable to dangerous, contagious diseases, many parents are taking the compromise position of asking their pediatricians to limit the number of shots a child receives in one visit.

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Robert W. Sears, a pediatrician, author of several books and part of the "Sears pediatric dynasty" that includes his father, mother and two brothers, is probably the most reputable, well-known proponent of the alternative vaccine schedule. In "The Vaccine Book," Dr. Bob, as he likes to be called, offers a plan for spreading out vaccines among 14 visits, rather than the 10 generally recommended by the AAP between birth and 3 years of age. His schedule also suggests breaking up the combined MMR into three separate shots.

For example, in Dr. Bob's scenario, at 6 months, your baby would receive only two shots—DTaP (against diphtheria, tetanus and pertussis) and rotavirus—rather than the six that the AAP recommends at that age. To make up for the missed shots, Dr. Bob's alternative schedule calls for two extra trips to the doctor before age 1.

"The main difference in my suggested alternative vaccine schedule is it spreads the infant vaccines out over the first few years of life, instead of bunching them all up in the first 18 months," he writes on his Web site. "It gives fewer vaccines at a time, gives the most important vaccines first, and slightly delays the less important vaccines. But ultimately the end result is the same—a fully vaccinated child."

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More complicated—and controversial—is splitting up the MMR vaccine into separate components. Rather than giving children the combined shot at 1 year and a booster at 5, Dr. Bob suggests administering the mumps vaccine at 12 months, rubella at 2 years and measles at 3 years. In his book and on his site, Dr. Bob admits that the alternative schedule is a hassle and may involve a fight with your doctor. However, he writes: "Sure, vaccinating this way means more visits to the doctor's office, more gas money, more insurance co-pays and more time off work to take your baby in. BUT, some parents may decide it's worth the extra time, money and trouble."

On the flip side, both of the medical experts interviewed for this article disagree vigorously with Dr. Bob.

"None of the proposed alternative slowdowns I've seen is based on any science," says Shu. "There is no evidence that spreading out vaccines does anything, except give parents a sense of control that they are doing something, anything to prevent autism."

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Additionally, she worries that the alternative schedule could make matters worse. "The current schedule protects kids when they are most vulnerable to childhood illness that can harm or kill them," says Shu. "A slowdown puts them at risk for getting these diseases when they are not covered. Plus, when you give certain vaccines together, it boosts the body's immune response. There is a benefit to grouping them together and no evidence at all that it overloads the system."

Allison-Ottey's concerns are more practical. "If your child has a fear of the doctor, is it right to take them back again and again for more painful shots?" she asks. "And remember that each time you visit the pediatrician, your child is at risk for catching something from the sick children in the waiting room."

Finally, Shu offers this big-picture assessment. "What bothers me maybe the most is that there is so much focus on vaccines, especially in the media, that we are neglecting other possible causes of autism," she says. "We know that autism has a genetic link, but we need to understand more. Is it something in the environment combined with genetics that causes this? What else? There are other real possibilities like pesticides, prenatal infections, parental age, childhood viruses and infections that receive so little attention. If we shifted the focus, we could look into some of those and get some real answers."

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Whatever you decide, be sure to get as much information as you can from reliable sources. The AAP's home pagehas a link to facts about vaccines. Sears spells out his alternative vaccine schedule in detail in his book, and he provides immunization news and updates on his Web site. If you choose to go that route, be prepared to have an informed, perhaps forceful discussion with your pediatrician. Says Allison-Ottey: "An open conversation with your physician is very important. You have to trust your physician, but you should also trust yourself. You have the right to be active in your child's care and your child's health."

Linda Villarosa is a health columnist for The Root. "Passing for Black" is her first novel. For more go to her Web site.