Katie and Craig Van Tornhout of South Bend, Ind., suffered four miscarriages before Katie became pregnant with Callie Grace, born Christmas Eve 2009. Callie was six weeks premature, but after a week and a half at the hospital, she came home and gained a pound a week. "She just thrived," said Katie, 31. "She giggled. She was just the happiest baby."

One month later on a Sunday, Callie let out a single, light cough. It didn't seem serious, but as a conscientious, first-time mom, Katie visited a doctor the next day, who assured her the baby's lungs were clear. But midweek, Callie briefly stopped breathing. Doctors in the pediatrics intensive care unit at a local hospital ran tests on the baby, trying to identify her disease, but on Friday night Callie quit responding. Despite 45 minutes of CPR by hospital staff, she couldn't be revived.

"When the doctor comes to you with tears in his eyes, you know that it's bad," said Katie. Test results later showed Callie had pertussis, a bacterial infection of the respiratory system that is highly contagious and widely known as whooping cough.

Last year the Van Tornhouts' tragedy wasn't quite as rare as usual. Outbreaks of whooping cough-named for the gasping sound sometimes produced by coughing fits-occurred in Ohio and Michigan, and in California the rate of infection spiked to its highest since 1958. The disease claims about 20 U.S. lives each year, mainly newborns, but last year 10 infants died in California alone. Since a pertussis vaccine is routinely given to children 2 months and older, many observers hastened to blame the outbreaks on a subset of parents who refuse to vaccinate. Most doctors strongly recommend vaccines, but some understand parents' apprehension and say blaming them is misguided.

In response to the outbreak, California passed a law requiring all seventh- to 12th-grade students to have their pertussis immunization up-to-date for the coming school year-or fill out an exemption form. All 50 states mandate immunization for new school children, according to the National Conference of State Legislatures, but most allow parents to sign an exemption if they object to vaccination on grounds that are religious or "philosophical"-meaning at odds with most of the medical community. The percentage of children using these exemptions more than doubled from 1991 to 2004.

In California, the parents of about 1 in 50 kindergartners sign exemption forms for one or more vaccines. But in some communities the sentiment is stronger: In 2009, 175 schools in the state had exemption rates of 20 percent or more, and a few surpassed 70 percent.

The exemption trend worries immunologists, who say that for diseases like pertussis an inoculation rate of about 93 percent is needed to achieve "herd immunity," where transmission is slowed enough to prevent an outbreak.

While nine out of 10 parents follow their doctors' vaccination advice, about half are concerned that some vaccines aren't safe, according to a 2009 survey. Do these parents have reason to worry?

Maybe. Like many drugs, some vaccines don't work well in everybody and come with serious, if rare, side effects. The whooping cough vaccine currently used in the United States on infants and children under 7 is a combination of diphtheria, tetanus, and acellular pertussis vaccines, abbreviated as DTaP. According to the U.S. Centers for Disease Control and Prevention, the side effects of DTaP include seizures in 1 in 14,000 children, three or more hours of non-stop crying in 1 in 1,000 children, and serious long-term injury in about 1 in 1,000,000 doses. The National Vaccine Injury Compensation Program, funded by a nationwide vaccine tax and housed at the U.S. Department of Health and Human Services, has awarded $2 billion to over 2,500 families for non-autism vaccine injuries since 1989. The injuries range from allergic reactions to neurological damage to unexpected infant death. (Most claims are dismissed, however, and whether the vaccines are at fault is often in dispute.)

"For many years the medical community denied that severe vaccine reactions were even possible," said Bob Sears, a pediatrician in Capistrano Beach, Calif. and author of The Vaccine Book: Making the Right Decision for Your Child. "No parent wants their child to be a disease statistic, but they also don't want their child to be a vaccine reaction statistic, either."

Sears said the medical establishment today is more forthcoming about the reaction risks, though he'd like to see more independent research on vaccine safety. However, he recommends vaccines to parents because he believes the benefits outweigh the risks.

So does Gainesville, Fla., pediatrician Tom Benton, a longtime member of the Christian Medical & Dental Associations (CMDA), which officially endorses vaccination. During Benton's residency program at a Houston children's hospital in the 1980s, he saw children suffering the effects of meningitis (induced by the Hib bacterium) on a weekly basis: "Their mental health would be damaged. Their hearing would be damaged. And some of them didn't make it."

Since the 1987 introduction of the Hib vaccine, such infections have virtually disappeared. Benton acknowledges that there are risks to taking vaccines-but he doesn't think they warrant forgoing shots altogether: "I accept that not every parent agrees with that. But it doesn't sway me."

(Benton also noted some Christians object to certain vaccines that were originally developed using tissue from aborted fetuses. CMDA lists them on its website, cmda.org.)

It's true that some diseases are rare in the United States today, but they're often kept at bay by the largely immunized population. Remaining unvaccinated increases risk for all: A 2009 Pediatrics study found that the children of parents who decline vaccinations were 23 times more likely to catch pertussis. Outbreaks sometimes target groups of unvaccinated people as well, as in 2005 when an Indiana teenager returned from a Romanian mission trip carrying measles and passed the virus to three dozen church members. Most were young, homeschooled, and hadn't been vaccinated for the disease.

Although no deaths occurred, outbreaks prompt some disease experts to push for government intervention. A 2007 article in the Journal of Law, Medicine & Ethics argued state governments should enforce vaccination among homeschoolers just as they do public- and private-school students. North Carolina has already taken that approach, requiring parents to keep immunization records for their children at home. West Virginia and Mississippi don't allow public- or private-school kids to opt out of vaccines unless they have a medical reason to do so.

"I would be against vaccine mandates even if occasionally it did result in an outbreak," said Jane Orient, an internist and executive director of the 5,000-member Association of American Physicians and Surgeons based in Tucson, Ariz. "It's a basic freedom issue." Orient views parental resistance to vaccines as positive because it helps guard against "sloppy testing" for safety.

In spite of the whooping cough outbreak in California, Sears takes parents' concern in stride. He guessed roughly one-fifth of his patients choose not to vaccinate, and another 10 percent use a delayed schedule, spacing the shots out over a child's first years. "Most doctors actually kick such patients out of their office. I don't think that's a very open-minded policy."

The real blame for whooping cough outbreaks, Sears said, lies less with apprehensive parents and more with the fallibility of the DTaP vaccine, which provides actual immunity in about 85 percent of cases. In addition, whooping cough regularly increases and decreases in the United States about once every five years. The last peak was in 2005, so the 2010 outbreaks are "actually no surprise," said Sears. (Some also speculate that new strains of pertussis are resisting the vaccine.) Still, the vast majority of whooping cough cases he has seen, said Sears, have been among unvaccinated patients.

Whooping cough is generally only dangerous for infants, but newborns like Callie Grace are too young to receive a pertussis vaccine themselves. To provide a cocoon of protection around a baby, doctors recommend family members get vaccinated. The whooping cough shot for teens and adults, called "Tdap," isn't foolproof-and its effectiveness wears off over time-but it reduces the chance of transmission.

Katie Van Tornhout said she and her husband weren't aware of the adult pertussis vaccine when Callie was born. Today, they aren't taking any chances: With another baby in the house (Chesney Rose, born in December), they've put a "do not enter" sign on the front door for anyone who hasn't had a Tdap booster: "People think we're crazy, but if it's going to protect somebody's life, then I'm all about it."

Lost linkage

Mystery over the apparent increase in children with autism over the last several decades has led many to speculate that the developmental disability can be triggered by vaccination. Although controlled studies looking for a link between vaccines and autism have failed to find one, a survey last year showed 1 in 4 parents still suspects a connection.

The vaccine-autism hypothesis faced two major setbacks in 2010: In February, the British journal The Lancet retracted an influential 1998 paper linking autism and childhood vaccinations and said the lead author, Dr. Andrew Wakefield, had falsified his data. In May British medical authorities barred Wakefield from practicing medicine in the UK. Wakefield responded saying his work had been "grossly distorted."