A vaccine designed to prevent a severe form of diarrhea in babies and children was approved Friday by the U.S. Food and Drug Administration.

Studies showed that the vaccine, RotaTeq, prevented 98% of severe infection and 96% of hospitalizations caused by rotavirus, says Jesse Goodman, director of the FDA's Center for Biologics Evaluation and Research.

Rotavirus infection, also called “winter diarrhea” because it tends to occur between November and April in the USA, causes fever, vomiting and watery diarrhea that can quickly lead to dehydration. It is the most common cause of diarrhea in young children, and by age 2, most children in the USA have had it. Few of the children die, but more than 55,000 each year need to be hospitalized, says the Centers for Disease Control and Prevention.

In developing countries, the CDC says, rotavirus infection kills more than 600,000 children each year.

“There aren't too many things that act like rotavirus,” says pediatrician David Matson, head of infectious diseases at Eastern Virginia Medical School in Norfolk, who led U.S. safety and efficacy studies. Some infections are mild, but in severe cases it can cause such rapid dehydration that a child can go into shock in six hours, he says.

The vaccine is a liquid that can be given by mouth in three doses from age 6 weeks to 32 weeks. The Advisory Committee on Immunization Practices, which makes vaccine recommendations to the CDC, is scheduled to vote this month on whether to advise adding RotaTeq to the list of vaccines that are given to babies routinely.

A previously licensed rotavirus vaccine was withdrawn in 1999 after it was linked to an increased risk for a dangerous bowel obstruction called intussusception.

To detect any sign of intussusception or other unwanted health effects, studies of the new vaccine were among the largest ever done; they involved more than 72,000 babies. No serious side effects were observed in babies who were given the vaccine compared with those who received a placebo, Goodman says, but “given experience with the previous vaccine, we do plan to monitor the safety of this new vaccine very carefully.”

Vaccine maker Merck will conduct post-marketing studies involving 44,000 infants, Goodman says, and the CDC's Vaccine Safety DataLink will provide health data on an additional 80,000 children.

Thanks to vaccines, diseases that killed or maimed millions throughout most of human history have been virtually eradicated. Where strong immunization programs exist, diseases such as polio, measles, mumps and diphtheria are scourges of the past.

This remarkable achievement is periodically threatened by suspicions about vaccines that might prompt parents to resist getting their children inoculated.

The latest furor involves thimerosal, a mercury-based chemical once used routinely as a preservative in childhood vaccines. Some parent activist groups claim that it causes autism, a set of developmental disorders characterized by difficulty in social interactions and behavioral problems.

The anti-thimerosal campaign has taken an unfortunate turn that's heavy on inflammatory rhetoric. Environmental lawyer Robert F. Kennedy Jr. suggests that public health officials conspired with drugmakers to “poison a generation of American children.” The campaign is supported by lawyers who've filed more than 4,800 suits against vaccine-makers, despite these facts:

•Health organizations — including Centers for Disease Control and Prevention (CDCP), Food and Drug Administration (FDA), World Health Organization, Institute of Medicine (IOM) and the American Academy of Pediatrics — agree there is no evidence linking thimerosal and autism.

•If thimerosal causes autism, the prevalence of the disorder should have declined as the chemical was removed from vaccines. No such decline occurred in Canada, Denmark and Sweden, where thimerosal was removed during the mid-1990s.

•Thimerosal was removed from most vaccines in the United States starting in 1999 as a precaution because of concerns about mercury levels. Except for some flu vaccines, no vaccine given to American preschool children now contains it.

Autism's causes are unknown, and there is no proven cure, leaving parents understandably anxious to find one and suspicious of evidence that closes off any avenue of hope.

Government agencies have fed those suspicions by resisting requests to make their records public promptly, inadvertently encouraging precisely the kind of mistrust and conspiracy theories that the agencies want to avoid. They would do well to reassure the public by releasing all relevant data.

But the greater danger is that the anti-thimerosal campaign threatens to scare parents away from protecting their children from infectious diseases without scientific support. Whenever vaccination rates drop, epidemics make a comeback.

Low vaccination rates sparked a measles epidemic in the United States from 1989 through 1991 that involved 55,622 cases and claimed the lives of 123 people, 90% of whom hadn't been vaccinated. As inoculation rates rose, measles declined markedly.

More than 17,000 American preschoolers don't get any of the vaccinations they need each year, a CDCP study found. Needlessly scaring parents and undermining confidence in vaccines proven to save lives won't help a single autistic child. But it could endanger the lives of millions.

The argument over what is causing soaring rates of autism has reached a boiling point with furious parent groups and their famous allies accusing scientists and public health officials of hiding information to cover up their own mistakes.

Scientists are offended and in some cases intimidated by an onslaught of e-mail, Internet slurs and unprecedented criticism.

At issue: whether the mercury-based preservative thimerosal, used in several infant vaccines up until five years ago and still in some vaccines that children get, is responsible for the developmental disorder.

“We're injecting poisons into children,” said environmental attorney Robert F. Kennedy Jr., in a phone interview last week. Last month, Kennedy charged in an article in Rolling Stone and posted online on Salon.com that U.S. health officials purposely covered up the dangers of thimerosal to protect themselves and “Big Pharma” from lawsuits in “a chilling case study of institutional arrogance, power and greed.”

Scientists say those charges are absolutely false. Much of the evidence for a thimerosal link to autism, they say, rests on questionable studies and comments taken out of context. Virtually all medical professional societies, the Centers for Disease Control and Prevention, World Health Organization and Institute of Medicine have stated there is no evidence vaccines cause autism.

“There is a dangerous mistrust of science,” says Paul Offit, an infectious-disease specialist at Children's Hospital of Philadelphia. “I don't think it's new, but it may be worse.”

As the debate moves into popular culture — radio personality Don Imus and TV talk show host Montel Williams have weighed in — Offit and others have been subjected to a storm of e-mail messages and phone calls. Internet message boards have posted attacks on vaccine advocates, in some cases posting their home addresses. The CDC has increased security.

“I got a ton of (electronic) hate mail and got a call to my house,” says Offit, who appeared on MSNBC to refute the thimerosal claims. He says another colleague who spoke out on a radio show received “incredibly scary calls at home, at work. It scared her. She said, ‘That's it, I'm out.' It's a thankless job. You get hammered.”

On May 17, 2004, a committee of the Institute of Medicine (IOM) released a report concluding that the scientific evidence “favors rejection of a causal relationship” between thimerosal-containing vaccines and autism. An earlier review in 2001 found the evidence available at that time was too thin to draw that conclusion, but the 2004 report was based on more than 200 studies and papers. It found that “all well-designed epidemiological studies provide evidence of no association between thimerosal and autism,” and it recommended that instead of spending more money chasing a theoretical link to thimerosal, research dollars should be directed at more promising areas of inquiry.

That was supposed to put the issue to rest. It did not. If anything, it poured gasoline on the embers.

Critics charged the committee was biased and that it failed to give credence to studies they believe suggest that for some children, the exposure to levels of mercury in vaccines is toxic.

Sallie Bernard, co-founder of SafeMinds, an anti-thimerosal group, says the scientists who claim vaccines are safe are “involved in vaccination issues, infectious diseases or public health. They're the ones who have an interest in not finding the connections. They're the ones who have done these studies.” She says federal regulators “should have pulled this stuff from vaccines a long time ago. It's not like mercury is this big mystery substance.”

Thimerosal contains ethylmercury, a different form of mercury from the type spewed out of coal-fired plants and that accumulates in fish. Ethylmercury in minuscule amounts has been used as a preservative in multi-dose vials of vaccines since the 1930s. Health officials recommended in 1999 that it be phased out in infant vaccines as a “precautionary measure.” It is still in some flu shots and diphtheria-tetanus boosters.

U.S. Rep. Dave Weldon, R-Fla., a doctor, says that during the 1990s, a baby who received all the recommended shots could be exposed to mercury levels above those considered safe by the Environmental Protection Agency.

“The amount of mercury we were injecting into kids dwarfs all other exposures,” says Weldon, who is sponsoring legislation to ban thimerosal from children's vaccines. A dozen states have or are considering similar laws.

Autism rates began to climb after two new thimerosal-containing vaccines, Hib (haemophilus influenzae type B) and hepatitis B, were added to the list of recommended shots for babies in the late '80s, Weldon says.

“Autism went from a disease I'd never seen to a disease you hear about everywhere,” he says.

David Kirby, whose book Evidence of Harm (St. Martin's Press, $26.95), details the emergence of the controversy over vaccines and autism, says he and a group of parents and researchers are meeting with legislators to urge attention to the possible thimerosal-autism link.

Kirby cites a study in monkeys that suggests mercury may persist in the brain after it is no longer detectable in blood. Another showed thimerosal harmed mice bred to be susceptible to autoimmune disease. And in a third study, a UPI reporter found a lower-than-expected rate of autism in an Amish community that did not believe in vaccination.

Kirby says there are too many unanswered questions. “It all points to the need for much more research,” Kirby says.

Scientists admit they don't know precisely what causes autism, though they believe it has a genetic component triggered by something in the environment, possibly occurring during the first trimester of pregnancy, which would eliminate infant vaccines as a candidate. Among studies cited by IOM was one that found autism rates in Denmark actually increased after thimerosal-containing vaccines were discontinued in 1992.

Kirby acknowledges other theories. “I am totally willing to accept there are other factors at play. … It may turn out not to be thimerosal at all.”

Time will tell. Scientists and parent groups agree that since thimerosal is now out of most vaccines, autism rates over the next couple of years will provide some clues: If rates drop dramatically, that will lend weight to the theory that thimerosal is involved.

Parents' reaction to the controversy is mixed. Signe Linscott of Falls Church, Va., says that as far as she knows, vaccines had nothing to do with the autism that has affected her older son, Jack, 11.

“Of course we want to find out why this has happened,” she says. “I just wish all the attention and money spent on looking backward could be turned forward, to programs for educating these kids, to get them and their families to a place where real progress can be made.”

Others agree family support and services should be front and center, but they resent feeling dismissed by the medical establishment.

Susie Kelly of Laurel, Del., says her son, Mark, 10, loves Nintendo and videos, and he likes to swim. But he has to sit in the same place every day to put his shoes on, has to exit through the same door he entered, and he doesn't like surprises. On a recent family trip to Mexico, she says, “he got upset when it rained, because it's not supposed to rain in Mexico.”

Kelly, a nurse, is not certain vaccines are to blame, but she is growing suspicious. “You always wonder. I feel like the medical community is just stonewalling it and not listening,” she says.

Parents don't care about politics or blame, says Michele Adamus of Fairfax, Va., whose son Zachary has a developmental delay. “When you have a child who has special needs of any kind, what you want — and the hardest thing to get — is answers.”

Parents may blame vaccines, Adamus says, because, “in lieu of other answers, that is something to hold on to.”

Public health leaders need to do a better job of explaining vaccine safety issues to the public, says Peter Sandman, a risk communicator who provides advice to businesses, non-profits and government, including the CDC.

Sandman says advocates on both sides have fallen prey to a tactic he calls “misleading toward the truth.” It happens when “you believe you know the truth” but don't trust others to grasp it. “It becomes very tempting to leave out the facts that might mislead the public” toward a different conclusion.

When critics say someone is lying, Sandman says, “you have to say, ‘yeah, but are they lying on behalf of the truth or are they lying on behalf of a lie?' ”