ATLANTA - There is no room in the childhood immunization schedule for thimerosal-containing vaccines.
The Public Health Service (PHS) and the American Academy of Pediatrics (AAP), in conjunction with vaccine manufacturers, recently announced that thimerosal-containing vaccines should be reformulated without the preservative, which is a mercury derivative, as quickly as possible. This recommendation would affect a number of vaccines in the recommended childhood immunization schedule (see chart).
The PHS and AAP maintain their recommendation that all children should be immunized against the diseases indicated in the childhood immunization schedule. The large risks of not vaccinating children far outweigh the unknown and much smaller risk, if any, of cumulative exposure to thimerosal-containing vaccines over the first six months of life, according to the joint statement.
"Nevertheless, because any potential risk is of concern, the PHS, the AAP, and vaccine manufacturers agree that thimerosal-containing vaccines should be removed as soon as possible," the statement read.
Thimerosal has been used in killed virus vaccines and other biological products since the 1930s. It is the most widely used preservative in vaccines because it is effective in killing bacteria and in preventing bacterial contamination, particularly in opened multidose containers.
The Food and Drug Administration (FDA) estimates it is used in more than 30 licensed vaccines and biologics, but not all of the vaccines recommended routinely for children in the United States contain thimerosal.
The Centers for Disease Control and Prevention (CDC) has reported that manufacturers of vaccines that currently contain the preservative are working to reduce or eliminate thimerosal from their products. The manufactures will then submit supplemental data to the FDA.
FDA spokeswoman Lenore Gelb aid the vaccines will not be resubjected to the rigorous clinical trials required for initial licensure. "Manufacturers will be required to submit supplemental data on vaccine potency and stability without thimerosal," Gelb said.
There is a potential that some children could be exposed to a cumulative level of mercury that exceeds federal guidelines over the first 6 months of life. Two different types of risks that must now be weighed when vaccinating infants:
The joint report said that while there is a margin of safety with existing vaccines containing thimerosal, there are steps that can be taken to increase that margin even more. Clinicians and parents can take advantage of the flexibility within the existing schedule for infants born to hepatitis B surface antigen (HBsAg)-negative women to postpone the first dose of hepatitis B vaccine (HepB). All single-antigen HepB vaccines currently licensed contain thimerosal.
The joint statement recommended postponement of HepB vaccine from birth until 2-6 months of age when the infant is considerably larger, but the AAP specifically recommended waiting until the infant is 6 months of age.
"I think postponing the HepB vaccine for infants until a later date is appropriate because you eliminate potential exposure [of thimerosal] to a very young infant. I hope the companies that make HepB vaccines will have thimerosal-free formulations available in a couple months, so the routine HepB immunization program can be reinstituted," said AAP 2000 Red Book Editor Larry K. Pickering, MD, Children's Hospital of The King's Daughters and director of the Center for Pediatric Research, Norfolk, Va.
Preterm infants born to HBsAg-negative mothers should similarly receive HepB, but ideally not until they reach term gestational age, 6 months of age and a weight of at least 5.5 lb (2.5 kg). Because of the substantial risk of disease, there is no change in the recommendations for infants born to HBsAg-positive mothers or of mothers whose status is not known. The HBsAg status of every pregnant woman should be reviewed at delivery. In populations where HBsAg screening of pregnant women is not routinely performed, vaccination of all infants at birth should be maintained, as is currently recommended.
There is one HepB and Haemophilus influenzae type b combination vaccine (Comvax, Merck) that does not contain thimerosal, but this product is not indicated for use in children younger than 6 weeks of age. The AAP, therefore, recommended that this thimerosal-free vaccine may be given to infants born to HBsAg-negative women beginning at the 2-month visit. Otherwise, HepB should be initiated at 6 months of age.
U.S. Surgeon General and Assistant Secretary for Health David Satcher, MD, PhD, voiced concern over parents losing confidence in vaccines. "Physicians and parents should be reassured that continuing to vaccinate infants, within the flexibility of today's schedule, is the best way to protect infants from devastating childhood diseases. The risk of devastating childhood diseases from failure to vaccinate far outweighs the minimal, if any, risk of exposure to cumulative levels of mercury in vaccines. The choice to vaccinate infants with these vaccines is a sound one."
Pickering, who is also an Infectious Diseases in Children editorial board member, said he's confident that most parents will discuss the issue with their pediatricians or family practitioners and most will make the decision to continue other routine immunizations.
Pickering said he hopes this movement toward making vaccines safer will be a comfort to parents.
"We've had some very positive improvements in the immunization schedule. One is the introduction into the schedule of the less reactogenic acellular pertussis vaccines and the other is doing away with oral poliovirus vaccine in favor of inactivated poliovirus vaccine (IPV). In addition, development of combination vaccines will help reduce the number of injections," he said. "We all care very much for children and want to ensure they are protected from these devastating diseases."
The FDA Modernization Act of 1997 called for the FDA to review and assess the risk of all mercury-containing food and drugs. In concordance with this review, U.S. vaccine manufacturers responded to FDA requests to provide more detailed information about the thimerosal content of preparations which include this compound as a preservative, according to the PHS and AAP.
From this review, the FDA concluded that there is a significant safety margin incorporated into all the acceptable mercury exposure limits, and that 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.
The statement also stressed that infants and children who have received thimerosal-containing vaccines do not need to be tested for mercury exposure.
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