When the letters and e-mails started to pour in, Dr. Paul Offit braced himself.

The pediatrician and vaccine inventor is a prominent defender of childhood vaccines, tackling those who have argued that immunizations can cause autism.

His book, "Autism's False Prophets," takes on British researcher Dr. Andrew Wakefield, whose now-debunked 1998 study in the prestigious medical journal The Lancet linked the measles, mumps and rubella vaccine to autism. It also criticizes organized groups that advise parents to avoid vaccinating their children for fear the vaccines may cause autism.

The issue is at the center of a vociferous and often vicious debate, despite the preponderance of scientific opinion in favor of vaccination.

Offit has endured hate-filled letters, death threats and even a phone call that menaced his children. However, his book was greeted with an outpouring of support from parents of children with autism who had previously remained silent.

"It's actually been exactly the opposite of what I would have guessed," Offit said in an interview.

One mother of an 8-year-old autistic boy wrote: "It really angers me when I hear others vilify you."

Another example: "I am a very unpopular mother at my children's school as I do advocate that children need to get their shots," writes the mother of a 10-year-old boy with autism.

"I would rather deal with autism (even though some days I go bananas) than bury a child to a disease that could have been prevented."

DESPERATE PARENTS

Autism is a brain disorder characterized by problems with social interaction, repetitive behavior and other symptoms.

People with a mild version called Asperger's syndrome usually function relatively well in society, although they have problems relating to others. People with the most extreme symptoms may be unable to speak and may also suffer severe mental illness and retardation.

Surveys by the U.S. Centers for Disease Control and Prevention indicate that one in every 150 children falls into the so-called autism spectrum. No one knows what causes autism, there is no good treatment and parents are understandably often despondent.

Last week a special U.S. federal vaccine court ruled against three families who claimed vaccines caused autism in their children. Offit hopes the ruling, on top of dozens of scientific reports, may reassure parents whose fears about vaccines have caused a plunge in vaccination rates in developed countries.

As a result, childhood illnesses like measles are making a comeback. More than 1,300 measles cases were reported in England and Wales in 2008, and 197,000 people died globally from measles in 2007.

In January, an unvaccinated 7-year-old in the U.S. state of Minnesota died of meningitis caused by Haemophilus influenzae, an infection prevented by a routine childhood vaccine.

These numbers frustrate public health officials, who cite study after study showing no link between vaccination and autism.

Some vaccine doubters believe that doctors and federal health agencies such as the CDC have colluded with vaccine makers to cover up vaccine dangers.

Many have accused Offit of harming children. One caller even threatened Offit's own children.

"The guy said, 'We all want what is best for our children, and you want what is best for your children, Will and Emily, who go to the Kenwood school.' And then he hung up. That scared me," Offit said.

Offit has in particular been doubted because he helped invent the Rotateq vaccine now marketed by Merck and Co to prevent rotavirus.

SHOCKING DEATH

"We were never paid by Merck. Our funding always came from NIH (the U.S. National Institutes of Health)," he said.

His determination to invent a vaccine began in 1979 when, as doctor in training at the University of Pittsburgh, he helped a team struggling to save a 9-month-old baby with severe diarrhea and vomiting from rotavirus.

"I have never seen a child so dehydrated," Offit said. "We tried to get an IV line into her. She never moved," he said.

"We all just stood there in shock that someone had died of viral diarrhea. The mother was outside. Then you have to open the door and tell her to come in and see her dead girl, this previously healthy, 9-month-old girl."

Offit and other experts say that parents today rarely see their children die of diseases that a few decades ago routinely carried off young children, and thus sometimes cannot appreciate the value of vaccines.

They note that every few years the rationale against vaccination changes. First opponents said the measles vaccine somehow caused the mysterious condition; then they argued that it was the mercury-based preservative in some vaccines, and later put forward the idea that some children are somehow predisposed to be hyper-sensitive to vaccines.

Some fear vaccines somehow damage or weaken the immune system. Yet in the vaccines against 14 different diseases now given to U.S. children, there are fewer immunological agents than in two shots given in 1980, Offit said.

"You have living on the surface of your body trillions of bacteria. You are hammered when you are born, and you can handle it. Vaccines are nothing. Vaccines are a drop in the ocean," he said.