Debunking the thimerosal myth

April 20, 2003

The vaccine community is still facing the backlash from the decision to

remove thimerosal from childhood vaccines.

by Bechtel

Staff Writer

April 2003

PHILADELPHIA - The thimerosal controversy continues to be the most pressing

vaccine safety issue facing immunization programs today.

There is no doubt that vaccines are effective in reducing diseases. Because

of vaccines, smallpox has been eradicated from the wild, polio eliminated

from most of the world, and diseases such as diphtheria, tetanus, pertussis

and measles, once prevalent killers of children in the United States, are

now rare.

One of the most difficult challenges faced by vaccines these days is whether

exposure to the preservative thimerosal, because it contains a form of

mercury known as ethylmercury, can cause neurologic damage or autism.

But now vaccines have become a victim of their own success. Along with such

success of immunization come doubts, about the risk of the diseases vaccines

seek to prevent, but also about the safety of vaccines in general.

Before vaccines are introduced to U.S. children, they are put through

rigorous clinical trials - testing for both efficacy and safety. The FDA

will only license a vaccine after it is proven safe in its targeted

population.

Yet vaccines remain controversial to some parents. A series of challenges

through the media have brought to light serious allegations about the safety

of U.S. children.

One of the most difficult challenges faced by vaccines these days is whether

exposure to the preservative thimerosal, because it contains a form of

mercury known as ethylmercury, can cause neurologic damage or autism.

Thimerosal was regularly used in vaccines until a 1999 recommendation from

the AAP and the CDC called for its removal. According to reports at that

time, the total mercury a child could be exposed to in the first year of

life through thimerosal-containing vaccines exceeded safe levels of mercury

exposure by the Environmental Protection Agency (EPA).

But the EPA guidelines were intentionally liberal in their estimation of

what constitutes dangerous levels of mercury exposure. Furthermore, leading

vaccine advocates say, the recommendation to remove thimerosal was based on

flawed assumptions, meaning that thimerosal was never dangerous in the first

place.

Still, the removal of the preservative from vaccines was sensationalized in

the media as a conclusion that thimerosal was in fact dangerous. What

started out as a cautionary decision thought to benefit the American public

blew up in the face of the public health community.

Vaccine manufacturers are now facing the ramifications of that decision in

the form of lawsuits that threaten their ability to stay in business. As

well, parents are now frightened by a backlash of misinformation campaigns

and some are even scared to have their children immunized.

The removal of thimerosal

Prior to the introduction of thimerosal, systemic infections from agents

like Staphylococcus aureus were common in vaccine recipients. But the

introduction of thimerosal, a bacteriostatic agent intended to kill those

same bacteria, changed all that, according to Offit, MD, director of

infectious diseases at the Children's Hospital of Philadelphia, and a

professor of pediatrics at the University of Pennsylvania.

" Once we figured out a way to make bacteriostatic agents like ethylmercury,

it really revolutionized our ability to make vaccines in multidose vials, "

Offit explained.

Thimerosal was regularly used in vaccines throughout the 20th century to

great success. Secondary bacterial infections in vaccine recipients dropped

dramatically and vaccination coverage rates soared.

In 1997, when the Food and Drug Modernization Act was passed into law,

federal agencies were required to list all foods and drugs that contained

mercury. The law was part of an overall effort to reduce unnecessary mercury

exposure to U.S. children.

" The impetus was certainly noble, " said Offit. " It was simply a way to make

sure we understood just what contained mercury and how much mercury it

contained. "

As a result of that analysis, it was found that the

diphtheria-tetanus-acellular pertussis, hepatitis B and Haemophilus

influenzae type b vaccines all contained trace amounts of thimerosal. When

the amount of mercury was added up, however, public health officials

realized that children could be exposed to levels of mercury deemed unsafe

by the EPA.

The AAP and CDC issued a statement calling for the removal of thimerosal

from all childhood vaccines based on the " precautionary principle, " or that

if it is possible to remove a potentially harmful element from a product

without doing harm to the integrity of that product, then that element

should be removed.

The decision seemed simple enough. Thimerosal could reasonably be removed

from vaccines, and vaccines could instead be manufactured in single-dose

vials.

But, according to Offit, the cautionary principle does not apply in this

case. " By precipitously removing thimerosal there were a number of

consequences that were negative, " he said.

EPA guidelines

The EPA guidelines were based in large part on a study done in Iraq in 1971.

The Iraqi government unknowingly imported 90,000 metric tons of grain that

had been fumigated with methylmercury, a close chemical cousin of

ethylmercury.

The grain was made into bread and distributed to farmers in Iraq. The

result, however, was over 6,000 hospitalizations and 450 deaths due to

excessive mercury exposure.

There were also babies born with neurologic damage to mothers who had eaten

the tainted bread. Researchers took the opportunity to analyze hair samples

from exposed mothers - an indicator of chronic exposure - to find the lowest

level of mercury present in mothers who had children with mild neurologic

damage.

Researchers recommended that children not be exposed to levels of mercury

that exceed their liberal estimate so as to reduce the possibility of

mercury poisoning.

Once they had that information, the researchers took an extra precautionary

step and divided that number by 10. They recommended that children not be

exposed to levels of mercury that exceed their liberal estimate so as to

reduce the possibility of mercury poisoning.

The EPA, in establishing its guidelines for U.S. children, took that low

number as their maximum allowable exposure limit. So, then, children who may

have been exposed to mercury through vaccination were still exposed to

levels of mercury 10 times below levels found to be harmful.

But the EPA estimates were based on two incorrect assumptions, according to

Offit. First, the EPA assumed that methylmercury was the same as

ethylmercury, which is used to make thimerosal.

Methylmercury, though, has a half-life of around 50 days, while for

ethylmercury it is approximately seven days. As well, while methylmercury

actively enters the central nervous system, ethylmercury does not.

" There is a critical difference between these two molecules, " said Offit.

Secondly, the EPA assumed that in utero exposure, as was the case in the

Iraq study, would have the same harmful effects as exposure after birth, as

would be the case in vaccinated children. But that is not the case,

according to Offit.

" We know from teratogenic agents and viruses that they are far more likely

to cause damage in utero than to an already developed central nervous system

ex utero, " he said.

Bad results

The removal of thimerosal was played up in the media as a conclusion that

thimerosal was dangerous. As a result, lawsuits have been filed across the

country seeking hundreds of millions of dollars in damages and for medical

monitoring.

The resulting litigation crisis has led several prominent vaccine

manufacturers to speculate whether they can even stay in business. The

thimerosal scare also led some hospitals to initially stop giving the

hepatitis B vaccine to newborn children, a decision that resulted in chronic

infections and deaths as a result of hepatitis B.

The controversy also scared many parents, many of whom are now afraid to

have their children immunized, leaving them at risk for potentially serious

diseases. While the removal of thimerosal was well intentioned, it had

immediate fallout, and wound up doing more harm than good. " What we did, at

best, was substitute a theoretical risk for a real risk, and did harm, " said

Offit.

April 20, 2003