Dose of precaution

Nick Harris jokes that his mom has printed out the entire Internet in her mission to understand the neurological disorder that causes the Hamilton teen to, in his words, ''float through my day."

''It's hard to grab on," he said. ''My attention will drift."

The teen's daily struggles are so intense that Harris, 14, withdrew from school last winter to be privately tutored.

He has been diagnosed with Asperger syndrome, a form of autism that Harris's mother, Suzanne Messina, believes was triggered by thimerosal, a mercury-based preservative once commonly used in childhood vaccines and now the center of a national debate.

Messina is so passionate about the issue that she has started a statewide coalition to ban mercury from vaccines in Massachusetts. The preservative has been removed from most shots given to children since 2001. However, state Representative Bradford Hill of Ipswich recently filed legislation calling for a total prohibition, inspired, he said, by Messina's campaign.

Similar legislation is pending in a number of other states, including Delaware, Illinois, and New York.

Parents are also asking Congress to get involved in the thimerosal battle. Several hundred gathered at a rally last week on Capitol Hill. The day before, government health officials assembled leading scientists to, once again, publicly tell parents that they have found no link between vaccines and autism.

Nearly every facet of the thimerosal issue is under scrutiny and debate, including whether the nation's number of autism cases has skyrocketed -- as some contend -- or whether doctors are simply diagnosing the disorder more frequently. Children often are diagnosed around the age of 3 after they've received the bulk of their vaccines, but much about the developmental disorder -- including its cause -- continues to mystify scientists. According to the Centers for Disease Control and Prevention, the reported cases of autism among American children rose from about 2 in 10,000 in the late 1980s to 1 in 166 today.

Fueling the latest controversy are a new book, called ''Evidence of Harm," by David Kirby, and a June article in Rolling Stone magazine by Robert Kennedy Jr. Both suggest that thimerosal may be linked to the dramatic increase in the number of cases of autism, Asperger and related disorders diagnosed in children -- an assertion roundly rejected by most scientists.

Kennedy and Kirby trace the rise in autism to the late 1980s, when new shots containing the preservative were added to the already-crowded schedule of vaccines children received. Kennedy also accused federal regulators, including officials from the CDC and the Food and Drug Administration, of relying on flawed science and of covering up health risks -- charges that officials have strongly denied. Also weighing in on the debate is New England Patriots back-up quarterback, Doug Flutie, who recently said he believes his 13-year-old son's autism can be traced to the immunizations the teen received as a child. Thimerosal has been used in vaccines since the 1930s to prevent bacterial contamination.

Now the mercury controversy is heating up among some well-known environmental advocates on the North Shore, including Jan Schlichtmann, a Beverly attorney made famous by the book and movie ''A Civil Action," and Lori Ehrlich, a Marblehead mom who spearheaded a grass-roots campaign in the 1990s to clean up pollutants from the state's aging power plants.

The two recently cohosted an Internet talk show on the topic.

''As a father of two sons, 9 and 8, and a daughter just 13 months, what goes into vaccines is of great interest to me," Schlichtmann said in a separate interview.

''I want to get the word out and get a public discussion on the issue," he added. ''I believe the evidence . . . definitely is pointing towards a connection."

The thimerosal debate has smoldered since the mid-1990s, when questions started surfacing and some research pointed to a possible link.

Then, in 1999, federal regulators said some infants who received all the recommended vaccines could have been exposed to mercury levels in their first six months that exceeded recommended safe guidelines. That prompted regulators, joined by a national pediatricians' group, to urge pharmaceutical companies to reduce or remove the preservative from vaccines ''as soon as possible." No recall was ordered, but vaccine manufacturers started to phase out the preservative over the next couple of years.

Today, thimerosal is used only in trace amounts, or not at all in US vaccines prescribed for children -- except for some flu shots and a tetanus-diphtheria vaccine given to children age 7 and over.

Yet studies have reached conflicting conclusions about the preservative. Some say there is a potential for harm. Richard Deth, a pharmacology professor at Northeastern University, told Congress last October that his work on human brain cells shows that some children may have a genetic risk factor that makes their bodies more sensitive to thimerosal. This mercury, he said, appears to affect the areas of the brain that control attention, awareness, and the ability to form complex thoughts. But many others -- including scientists with the Institute of Medicine, a panel that advises the government -- have ruled out a link. Ongoing battles over the science, combined with recent headlines, are prompting fresh questions from uneasy parents to area pediatricians.

''We say it's largely been taken out, and that there has never been a link between autism and thimerosal to begin with. But even if there were, there are such trace amounts now, that there's no worry at all," said Dr. Brian Orr, a Gloucester pediatrician who keeps a file with studies to share with anxious parents.

After parents review the information, most go ahead with the shots, he said.

In Massachusetts, the state buys all recommended childhood vaccines and distributes them to pediatricians, ensuring a central inventory and quality control system, said Dr. Alfred DeMaria, chief of communicable disease control for the Department of Public Health. Since late 2001, DeMaria said, the state has purchased vaccines that are mercury-free or contain only a trace of the preservative -- except for flu shots, because mercury-free flu vaccine supplies are scarce.

DeMaria said he has no doubt the number of autism cases nationwide increased since the late 1980s, and that the increase is not simply tied to more doctors diagnosing the disorder. But he does not believe the increase is linked to thimerosal, saying he has not seen ''credible evidence" linking the two. However, DeMaria also said he is closely watching whether the nationwide numbers of reported autism cases start to decline, a sign some say would indicate the disorder was linked to mercury vaccines.

''I want to keep an open mind about this," DeMaria added. ''If you are going to dismiss it, if you say [thimerosal] doesn't cause any problem in children, then why did you take it out? I think it was good to take it out."

In the meantime, DeMaria and other health officials are worried that the latest controversy may create a backlash against immunizations, raising the potential for serious, vaccine-preventable diseases that are largely unknown today to make a comeback.

''People don't know what polio is," said Dr. Edward Bailey, chief of pediatrics at North Shore Medical Center. ''Diseases they have never heard of . . . severe, crippling, life-threatening illnesses have disappeared entirely due to public health efforts."

As scientists continue their search for a definitive answer to the thimerosal debate, pediatricians and parents remain on the front lines. Nurse Beth Duffy, a Saugus mother of three, including a 1-month old, understands the uneasiness on all sides. She said the uncertainty has prompted her to change her children's vaccine schedules, to avoid giving them the recommended multiple doses of medicine and preservatives all at once. Instead, she spreads the shots out over several extra months. Duffy said she has encountered resistance from pediatricians.

''Sometimes doctors offices aren't forthcoming," she said. ''I've asked about [thimerosal]. . . and sometimes you get the feeling they are pooh-poohing you."

Still, she said, she believes vaccines are a must.

''Diseases can cause major problems. They can cause our kids to be really, really sick," Duffy said. ''But vaccines also do need to be safe."

Kay Lazar can be reached at klazar@globe.com.