Q 1. What is Thimerosal?
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
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
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
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
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
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
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
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
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
Q15. How can I find out what chemical additives are in specific
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
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
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.