Thimerosal-Free Vaccines -- A Progress Report

[Clinician Reviews 10(8):95-96, 2000.

Abstract

How much mercury will our infants continue to absorb from vaccines that contain the mercury-containing preservative thimerosal? A recent joint statement from an alliance of four major health organizations reports the progress drug manufacturers have made in eliminating or reducing thimerosal content in the vaccination regimen.

Introduction

The commitment to achieving thimerosal-free routine childhood vaccination has been reaffirmed in a joint statement from the American Academy of Family Physicians, the American Academy of Pediatrics, the Advisory Committee on Immunization Practices, and the US Public Health Service.^[1] Thanks to a timely response by major vaccine manufacturers to the alliance’s original statement in July 1999, it is estimated that the maximum infant exposure to ethylmercury as a result of routine immunization has been reduced by 60% (from 187.5 µg to 75 µg).

Progress Reported

According to the new statement, Merck Vaccine Division began to produce a thimerosal-free hepatitis B vaccine in August 1999. A similar formulation from SmithKline Beecham contains no thimerosal and achieves a 96% reduction in mercury content from its previous vaccine (now <0.5 µg, an amount considered "clinically insignificant").

Three of the four US-licensed vaccines for Haemophilus influenzae type b (Hib) do not contain the preservative. The fourth, manufactured by Wyeth Lederle, is available in both single-dose (thimerosal-free) and multidose (thimerosal-containing) formulations; as of July 2000, the company is scheduled to produce only the single-dose, thimerosal-free Hib formulation for use in the United States. The combination hepatitis B/Hib vaccine produced by Merck has always been free of the preservative. Currently, SmithKline Beecham produces the only available thimerosal-free vaccine against DTaP (diphtheria and tetanus toxoids with acellular pertussis). Authorities project that by April 2001, two new vaccine products each for hepatitis B, Hib, and DTaP should be available.

Vaccines for measles, mumps, rubella, varicella, inactivated polio, and pneumococcal conjugate have never contained thimerosal. Certain vaccines that are not included in the routine childhood immunization schedule (eg, meningococcal, diphtheria-tetanus, and influenza vaccines) will continue to contain the preservative–but should be used as indicated until new products are approved.

Continuing Research

To determine possible links between thimerosal use and neurologic, developmental, or renal disorders, the Centers for Disease Control and Prevention is using large automated databases that link vaccination and International Classification of Disease codes stored in medical records by managed care organizations. Preliminary data from two of these organizations suggest a weak correlation between vaccine-related thimerosal exposure and incidence of language and speech delays, attention-deficit/hyperactivity disorder (ADHD), unspecified developmental delays, and tics. However, further investigation using data from a third managed care organization showed no association between thimerosal-containing vaccines and speech delay or ADHD.

Neither investigation associated thimerosal-containing vaccine use with infantile cerebral palsy, epilepsy, extrapyramidal disease, unspecified renal conditions, autism, migraines, childhood psychosis, mixed emotional conditions, or stammering, or with sleep or eating disorders.

Findings from research to determine infant blood levels of mercury from vaccines that contain thimerosal have been contradictory. Preliminary data from one collaborative investigation involving the National Institutes of Health, the University of Rochester in New York, and the Bethesda Naval Hospital in Maryland indicated that "a very small number of" term infants who had received thimerosal-containing vaccines had mercury blood levels less than 2.0 µg/L. Yet another recent study published in the Journal of Pediatrics revealed mercury levels exceeding 2.9 µg/L in nine of 15 premature, very low—birth weight infants who were immunized against hepatitis B before age 1 week.^[2]

General Recommendations

Withholding vaccination because no thimerosal-free product is yet available would pose a far greater risk to a child than would potential exposure to the preservative, the alliance authors stress. Until an adequate supply of each thimerosal-free vaccine becomes available, the joint statement advises, use of vaccines that contain thimerosal as a preservative is acceptable.

References

1. Thimerosal in vaccines: joint statement of the American Academy of Family Physicians (AAFP), the American Academy of Pediatrics (AAP), the Advisory Committee on Immunization Practices (ACIP), and the United States Public Health Service (PHS). Available at: www.aafp.org/policy/camp/20.html. Accessed July 14, 2000.
2. Stajich GV, Lopez GP, Harry SW, Sexson WR. Iatrogenic exposure to mercury after hepatitis B vaccination in preterm infants. J Pediatr. 2000;136:679-681.