"Thimerosal has been used for decades, but the surge in vaccinations caused fear that possible accumulations of ethyl mercury, the kind in thimerosal, might exceed safe levels -- at least, when based on the stringent risk guidelines applied to its better-understood chemical cousin, methyl mercury, which is associated with eating fish," lead researcher Dr. Michael Pichichero, professor of microbiology/immunology, pediatrics and medicine at the University of Rochester, said in a prepared statement.

"One of the unanswered questions when this first popped up as a controversy was, when you got thimerosal as an injection, how long would it stay in your blood," co-author Dr. John Treanor, a professor of medicine at the University of Rochester Medical Center, said in an interview.

The new research, he added, showed that "the levels of thimerosal don't go very high and they go down right away. By the time it's time for the next dose of vaccine, the levels are right back to where they were at the beginning."

For their study, Pichichero's team tracked 216 infants from R. Gutierrez Children's Hospital in Buenos Aires, Argentina, where thimerosal is still routinely used in vaccines. Use of thimerosal in childhood vaccines was discontinued in the United States after a joint decision in 1999 by U.S. health officials, pediatricians and vaccine manufacturers.

The infants in the study were put into three age groups and their blood-mercury levels were tested both before and after vaccinations were given to newborns, and at their two- and six-month checkups.

Pichichero's group found that for all three age groups, the half-life of ethyl mercury in the blood -- the time it takes for the body to get rid of half the mercury, and then another half, and so on -- was 3.7 days. That's significantly less than the half-life of methyl mercury, the kind found in fish, at 44 days.

"Until recently, that longer half-life was assumed to be the rule for both types of mercury. Now it's obvious that ethyl mercury's short half-life prevents toxic build-up from occurring. It's just gone too fast," Pichichero said.

"If you thought thimerosal was responsible for autism, you would be looking at mercury levels that were far below anything anyone's previously thought as being toxic," Treanor added.

"Though it's reassuring to affirm that these immunizations have always been safe, our findings really have greater implications for world health," Pichichero said. "Replacing the thimerosal in vaccines globally would put these vaccines beyond what the world community could afford for its children." "

The study findings were to be released Monday in the February issue of the journal Pediatrics. But they were released early by the American Academy of Pediatrics, which is requesting that the ABC network cancel the premiere episode of a new show Thursday dealing with the thimerosal-autism controversy.

The new findings also follow a recent report from the California Department of Health that rates of autism continue to climb there even after thimerosal was removed from childhood vaccines.

And they follow a series of studies, including a large-scale U.S. Institute of Medicine review in 2004, that failed to uncover a link between childhood vaccines and autism. The first report of a possible connection appeared in British study in the late 1990s and has since been discredited.

Current estimates by the U.S. National Institutes of Health say that one American child in 150 has been diagnosed with autism, although experts wonder if that increase owes to better diagnoses and a broader definition of the disorder.

Still, at least one vaccine critic worries that inoculations are making children prone to autism, a developmental disorder characterized by impaired social interaction, communication problems, and unusual, repetitive, or severely limited activities and interests. And if it's not thimerosal, then it must be some other vaccine-related interaction, said Barbara Loe Fisher, co-founder and president of the National Vaccine Information Center.

"There are many biological mechanisms involved in vaccine-induced brain and immune system changes that could quite well lead to autism," she said.

"Mercury doesn't belong in any product," Fisher added. "Mercury doesn't belong in vaccines whether it's proven or not proven that mercury is a problem in vaccines."

In ABC's new TV series Eli Stone, the premiere Thursday focuses on a lawyer arguing that a vaccine caused a child's autism. While the show includes statements that science has refuted a link between autism and vaccines, the program reinforces the connection as the jury awards the mother $5.2 million, according to the AAP.

"If parents watch this program and choose to deny their children immunizations, ABC will share in the responsibility for the suffering and deaths that occur as a result. The consequences of a decline in immunization rates could be devastating to the health of our nation's children," AAP President Dr. Renee R. Jenkins said in a prepared statement.