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Volume 2 Published - 14:00 UTC    08:00 EST    08-March-2001      
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Back To Vidyya Questions And Answers:

Thimerosal

Q 1. What is Thimerosal?

A. Thimerosal is a very effective preservative that contains mercury and has been used in some vaccines and other products since the 1930s. Thimerosal is the most widely used preservative in vaccines. The FDA estimates that it is used in more than 30 licensed vaccines and biologics currently marketed in the U.S.

Q 2. Why is Thimerosal used in vaccines?

A. Thimerosal is used as an extra safeguard against contamination. It may be used during the manufacturing process or added to the final container to prevent contamination after multi-dose vials are opened.

Thimerosal is very effective in helping to prevent a vaccine from spoiling and inactivating bacteria used to formulate several vaccines, and in preventing bacterial contamination of the final product. Some but not all of the vaccines recommended routinely for children in the United States contain thimerosal. Disease outbreaks have occurred following contamination of multi-dose vaccine vials in the United States and from other countries. For example, in April, 1995, three infants died in India from toxic shock syndrome after they received contaminated measles vaccine at one health center. While use of thimerosal as a preservative does not eliminate the possibility of bacterial contamination, it can greatly reduce its possibility.

Q 3. If thimerosal has been used in vaccines for many years, why is it a concern now? What recommendations have the Public Health Service and American Academy of Pediatrics taken with respect to thimerosal in vaccines?

A. Although mercury is found in the environment, in food and in household products, exposure to mercury is of concern and, when possible, should be avoided. In recent years, various federal agencies have been addressing the health risks of mercury. One type of mercury, called methyl mercury, is found in seafood and has received particular attention because high doses have been associated with health effects, particularly infants whose mothers were exposed to high doses during pregnancy. Several federal agencies including the Environmental Protection Agency, Agency for Toxic Substance and Disease Registry, and the Food and Drug Administration have developed guidelines for intake of methyl mercury.

Thimerosal contains a related mercury compound called ethyl mercury. Other than the occurrence of local hypersensitivity reactions, no harmful effects have been reported from thimerosal at doses found in vaccines. However, a recent review conducted by the Food and Drug Administration suggested that some infants, depending on which vaccines they receive and the timing of those vaccines, may be exposed to levels of ethyl mercury that could build up to exceed one of the federal guidelines established for the intake of methyl mercury. It is important to note that the mercury exposure from vaccines is within the safety margin incorporated into the guidelines.

The Public Health Service agencies, the American Academy of Pediatrics, and vaccine manufacturers agree that thimerosal should be reduced or eliminated in vaccines to make already safe vaccines even safer. The Food and Drug Administration has committed to expediting the review of new vaccines that do not contain thimerosal.

Q 4. Can all vaccines be made Thimerosal-free, or within accepted guidelines? If so, how quickly?

A. In August 1999 the FDA licensed a thimerosal preservative-free hepatitis B vaccine, and other thimerosal-free vaccines are currently under review. In addition, clinicians can use the inherent flexibility in the current immunization schedule to fully vaccinate children and meet even the most conservative guidelines for cumulative mercury exposure. Given the availability of vaccines that do not contain thimerosal as a preservative, and the absence of any recognized harm from exposure to thimerosal in vaccines, hepatitis B, DTaP, and Hib vaccines that contain thimerosal as a preservative can continue to be used in the routine infant schedule beginning at age 2 months along with monovalent or combination vaccines that do not contain thimerosal as a preservative.

Reported failures by some health care providers to vaccinate newborns at high risk for perinatal hepatitis B virus (HBV) transmission at birth suggest that some institutions may have misinterpreted or improperly implemented the recommendations to postpone hepatitis B vaccination only for newborns who are not at high risk. Chronic hepatitis B virus infection develops in approximately 90% of infants infected at birth; among chronically infected infants, the risk for premature death from hepatitis B-related liver cancer or cirrhosis is approximately 25%. With the availability of hepatitis B vaccine that does not contain thimerosal as a preservative, all hospitals and pediatric care providers should ensure that newborn infants receive hepatitis B vaccine as recommended. If the supply of single-antigen hepatitis B vaccines that do not contain thimerosal is limited, the priority for its use should be to vaccinate newborn infants.

Q 5. Why isnít the federal government eliminating thimerosal from vaccines if there is concern?

A. Making vaccines safer and more effective is a constant goal for the federal government. There is a significant safety margin incorporated into all federal mercury exposure guidelines. Furthermore, other than local hypersensitivity reactions, there is no evidence of any harm caused by the level of exposure that children may have encountered under the existing immunization schedule. Today, we are discussing a minimal risk from thimerosal found in vaccines versus the large and devastating risk of childhood diseases like bacterial meningitis and whooping cough if parents and physicians fail to vaccinate children appropriately. Missed vaccinations put children at real risk of disease.

Removing thimerosal from all vaccines will take time. Although several vaccines are available without thimerosal as a preservative, each product that currently uses thimerosal must be reformulated and undergo testing to ensure that safety and efficacy have not been altered.

Q 6. How much mercury would a 6-month-old child get in the last six months from vaccines? How dangerous is that?

A. Regardless of which vaccines a child has received, the decision choice to vaccinate is a sound one. The mercury exposure from vaccines is well within the safety margins included in any guideline established by federal agencies and there is no evidence that children have been harmed by the amount of mercury found in vaccines. However, Public Health Service agencies are working toward further increasing the margin of safety. It is important that we limit childrensí exposure to mercury, but parents should feel confident in the safety of vaccines and continue to vaccinate their children according to the recommended schedule.

Q 7. If there are vaccines that are mercury-free, why shouldn't parents just ask for those?

A. Although vaccines that do not contain thimerosal as a preservative are available, they may not always be readily available from your health care provider. Some vaccines, such as influenza, are not yet available without thimerosal. The American Academy of Pediatrics, the Advisory Committee on Immunization Practices for CDC and the Surgeon General all recommend that parents do not let their child miss a vaccination when safe and effective vaccines are available. Today, we are discussing a minimal risk from exposure to thimerosal found in some vaccines versus a larger and devastating risk of childhood diseases like bacterial meningitis and whooping cough if parents and physicians fail to vaccinate children appropriately.

The American Academy of Pediatrics, the Advisory Committee on Immunization Practices for CDC and the U.S. Surgeon General want parents to be fully informed about childhood vaccines and if you have questions or concerns, we encourage you to speak to your childís health care provider.

Q 8. Iíve heard that children may be getting toxic levels of mercury from vaccines. Is that true?

A. No. Everyone is exposed to mercury, it is in the environment and found in some foods (particularly seafood) and in some household products. As part of an ongoing assessment of mercury in the environment and in products, many agencies have developed guidelines for acceptable levels of mercury, levels defined to be many times below any amount known to cause harm. Some children, depending on which vaccines they receive and the timing of those vaccines, are exposed to cumulative levels of mercury close to the safety ranges of guidelines. Itís important to understand that these highest acceptable levels include a "safety cushion" to take into account all the variables that people face in their exposures to mercury. No children are getting toxic levels of mercury from vaccines.

Q 9. I understand some people are sensitive to thimerosal and must avoid it. Do they have problems with thimerosal-containing vaccines?

A. Some individuals experience local skin reactions such as redness and swelling that may suggest a delayed type hypersensitivity reaction following injection with products containing thimerosal. While one study found that most patients do not develop reactions to thimerosal given as a component of vaccines even when they have had patch or intradermal tests for thimerosal that indicated hypersensitivity, a prior history of hypersensitivity to thimerosal in a vaccine is considered a contraindication to further vaccination with thimerosal-containing vaccines.

Q10. What is mercury?

A. Mercury is a metal, a chemical element found everywhere. As such, it is neither created, nor destroyed -- the same amount of mercury has existed since the earth was formed.

Two major forms of mercury exist in nature, an inorganic form (the mercury used in thermometers) and the organic form. Humans and wildlife are exposed to both, but the metallic mercury is quickly released from the body. The organic form tends to accumulate in humans, and particularly in large predator fish. Humans are usually exposed to organic mercury from eating fish which have accumulated it in their muscle tissue. Very high levels of mercury are toxic. Because mercury is everywhere, it is not possible to prevent all exposure to mercury. Federal agencies, including the Agency for Toxic Substances and Disease Registries and the Food Administration and Environmental Protection Agency have established guidelines for levels of mercury exposure considered safe. In addition, uses and releases of mercury have been reduced very substantially in recent decades in the U.S. and most other industrialized countries.

Q11. Who is most vulnerable to mercury?

A. Two groups are most vulnerable to methyl mercury: the fetus and pregnant women. Premature babies are more vulnerable because they tend to be very small and their brain is not as developed as a full term baby. Children may be at higher risk of mercury exposure than are adults because they eat more per pound of body weight and because they may be inherently more sensitive than adults since their nervous systems are still developing. The guidelines for mercury exposure are based on amount of mercury per weight. This helps estimate reference level of exposure according to the person's weight.

Q12: What happens if your exposure exceeds the recommended levels?

A. The nervous system is very sensitive to all forms of mercury. Methyl mercury and metal vapors are more harmful than other forms, because more mercury in these forms reaches the brain. Exposure to high levels of metallic, inorganic, or organic mercury can permanently damage the brain, kidneys, and developing fetus. Effects on brain functioning may result in irritability, shyness, tremors, changes in vision or hearing, and memory problems.

Short-term exposure to high levels of metallic mercury vapors may cause effects including lung damage, nausea, vomiting, diarrhea, increases in blood pressure or heart rate, skin rashes, and eye irritation.

It is important to remember that there is a significant safety margin incorporated into all acceptable mercury exposure limits. There are no data or evidence of any harm caused by the level of exposure that some children may have encountered in following the existing immunization schedule.

Q13: How can mercury affect children?

A. Very young children are more sensitive to mercury than adults. Mercury in the mother's body passes to the fetus and can pass to a nursing infant through breast milk. However, the benefits of breast feeding may be greater than the possible adverse effects of mercury in breast milk.

If a pregnant woman ingests mercury at high levels, harmful effects that may be passed from the mother to the developing fetus include brain damage, mental retardation, and lack of coordination, blindness, seizures, and an inability to speak. Children poisoned by mercury may develop problems of their nervous and digestive systems and kidney damage.

Q 14. Why are chemicals and other substances added to vaccines?

A. Many foods and medicines have chemicals added to them to prevent the growth of germs and reduce spoilage. Chemicals are added to vaccines for similar reasons, to inactivate a virus or bacteria and to stabilize it, helping to preserve the vaccine and prevent it from losing its potency over time.

Some additives are used in the production of vaccines. Vaccines may include suspending fluid (e.g., sterile water, saline, or fluids containing protein); preservatives and stabilizers (e.g., albumin, phenols, and glycine); and adjuvants or enhancers that help the vaccine improve its immunogenicity (ability to protect against disease).

Q15. How can I find out what chemical additives are in specific vaccines?

A. Ask your health care provider or pharmacist for a copy of the vaccine package insert. The package insert lists ingredients in the vaccine and discusses any known adverse reactions.

Q 16. What could happen if parents ignored recommendations to vaccinate children appropriately?

A. Children would be at very real risk from illnesses that can be prevented with safe and effective vaccinations. High rates of vaccination led to declines of 95% to 100% in the occurrence of vaccine preventable diseases in the United States. Despite this, the pathogens responsible for most vaccine preventable diseases still circulate and rates of disease would increase if vaccine coverage dropped. For example, if vaccination coverage among infants dropped from 95% to 70%, an additional 2,500 cases of pertussis would be expected to occur. Moreover, the risk of death from pertussis is greatest in young children. A second severe vaccine preventable disease among young children is Haemophilus influenzae type b (Hib). Before vaccine was introduced, this pathogen was the leading cause of meningitis and other severe invasive infections among children; now cases of invasive Hib disease have virtually disappeared. If vaccination for Hib declined to 70%, 2,000 excess cases would occur with 1,200 cases of meningitis, resulting in about 100 deaths and 180 children who would suffer mental retardation and hearing loss.

Q17: How can parents learn more about childrenís immunizations?

A. To learn more about children's immunizations, vaccinations, or baby shots from a CDC information specialist, please call CDCís National Immunization Information Hotline: 1-800-232-2522, for English, 1-800-232-0233, for Spanish.


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Editor: Susan K. Boyer, RN
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