By Margarita Nahapetyan
A new study from Italy adds to a mountain of evidence that a mercury-based preservative thimerosal, once used in many vaccines does not hurt children, offering more reassurance to parents.
In the early 90s, thousands of healthy Italian children in a randomized study of whooping cough vaccines were given two different amounts of thimerosal from all their routine shots.Ten years later, 1,403 of those children were examined by the researchers and underwent brain-function tests. As a result, very small differences were found in only two of the 24 measurements - differences that "might be attributable to chance," the researchers wrote in the February issue of the journal Pediatrics, which was released earlier this week. Only one case of autism was found among all, and that was in the group that got the lower level of thimerosal.
"Put together with the evidence of all the other studies, this tells us there is no reason to worry about the effect of thimerosal in vaccines," said the new study's lead author, Dr. Alberto Tozzi of Bambino Gesu Hospital in Rome.
The Autism Spectrum Disorder (ASD) is a complex brain development disorder which is characterized by repetitive behaviors and poor social interaction and communication skills. All these symptoms take place within a child even before he/she becomes three years old. Scientists generally believe that genetics plays a primary role in causing the disorder. An assumption that thimerosal is being linked to the disorder has been discounted in scientific studies many times.
The debate over thimerosal and autism connection has been much stronger in the United States than in Italy, Tozzi said. The preservative, used in some vaccines to prevent the growth of bacteria and fungus, has been eliminated from childhood vaccines in the United States since 2001. U.S. health authorities suggested the removal of thimerosal in order to reduce the overall exposure of children to mercury. Italy and other European countries started removing it already in 1999.
However, parents, some doctors, and even scientists still blame thiomersal as the root cause of autism in spite of being taken out of vaccines and studies showing no relationship between the two. Thiomersal, in its turn, was not taken out of every vaccine but can only be used on children over 2 years of age on an "as needed" ground. Used as an antiseptic and antifungal agent, the preservative is still being used in certain flu shots, diphtheria and tetanus shots, as well as rarely-used anti-venom treatments against pit viper, coral snake, and black widow attacks.
Tozzi said comparing children with no exposure to thimerosal could have improved the study. "However, if thimerosal were a cause of harm, it is likely that this effect would increase with the administered dose," he said.
During the first year of their life, the babies were divided into two groups and given a dosage of either 62.5 or 137.5 micrograms of ethyl mercury in the form of immunization shots. Ethyl mercury is what the preservative in question breaks down to within the body. Before the reduction of thimerosal in the United States, the maximum exposure that was approved for infants was 187.5 micrograms of ethyl mercury.
The children scored average on 11 different tests including memory, motor skills, attention and other brain functions. All 11 tests included twenty four measured outcomes. Only two of 24 different measurements were found not sufficient and the researchers say that these two cases could be attributed to a chance. Both of these below-average ratings were for girls and for a vocabulary test in which they were asked to name common objects, and a finger-tapping test with their dominant hands.
There was no difference in boys on those outcomes or others. The researchers also did not find any difference in tic disorders. And the single autism case found in the lower-intake group was likely a chance finding, Tozzi said.
The Italian study was funded by the U.S. Centers for Disease Control and Prevention and has since been given adequate praises from outside experts for the work that was done on this randomized trial.
"It is yet another well done, peer-reviewed research study that has demonstrated that there is no risk of any neurodevelopmental outcomes associated with thimerosal in vaccines," said epidemiologist Jennifer Pinto-Martin of the University of Pennsylvania.
"This becomes the fourth study to look for subtle signs of mercury toxicity and show the answer was 'no,'" said Dr. Paul Offit, chief of infectious diseases at the Children's Hospital of Philadelphia, the author of a book on autism research and the co-inventor of a rotavirus vaccine.