Several childhood vaccines used to contain mercury as part of a preservative called thimerosal.  While all routine childhood vaccines are currently available without thimerosal, many myths continue to abound regarding the effects of thimerosal.  There is no proven link between thimerosal and any behavior or neurologic diagnosis such as autism, Attention-Deficit Hyperactivity Disorder (ADHD), or speech delay.

Thimerosal is a preservative that is 49.6% ethylmercury, an organic form of mercury.  It is found in many applications, including vaccines and nasal sprays.  Thimerosal has been used in multi-dose vaccine vials since the 1930s; failure to use an effective preservative in vaccines resulted in skin infections.  Mercury is known to be toxic in high doses to many parts of the body, particularly the brain and kidneys, but the amount of mercury contained in one vaccine dose is minute and was not felt to be a problem until the 1990s. Prior to 1993, the only routine vaccine that contained thimerosal was DTP (Diphtheria-Tetanus-Pertussis).  When HepB (Hepatitis B) and Hib (Haemophilus influenzae type B) vaccines were added to routine vaccinations, the amount of thimerosal given to infants increased significantly.  (Live virus vaccines such as MMR never contained thimerosal.)

In 1999, the American Academy of Pediatrics found that the amount of mercury contained in vaccines given to infants under six months old exceeded one (FDA) of four federal guidelines for mercury exposure.  The FDA guidelines, like the other guidelines, are theoretical calculations of safe mercury exposure amounts and are set at least ten times below what is calculated to be the minimum toxic exposure for mercury.  At most, infants were receiving two to three times the amount of the FDA limits, and it is worth knowing that the amounts were below other federal guidelines such as the EPA and the WHO guidelines.  However, the American Academy of Pediatrics decided it was best to recommend the removal of thimerosal from all vaccines so that there would be absolutely no riskóknown or theoreticalóto infants.

Mercury is not an issue for newborns today because no routine childhood vaccine (except for influenza) contains thimerosal.  Occasionally the vaccine may be labeled as having "trace" thimerosal, but this means the amount of thimerosal is below the level of detection.  For comparison, many vaccines used to contain 12.5 to 25 micrograms (mcg) of thimerosal, whereas a "trace" amount is less than 0.01 mcg of thimerosal.

The question remains as to whether children born between 1993 and 1999 were put at any additional risk for developmental delay and behavior disorders such as autism.  While studies continue to be done, those studies that have been completed have not found any link between mercury and autism, speech delay, or ADHD.  Several reviews published in 2001 and 2004, including one by the Institute of Medicine, have also not found any link between mercury, thimerosal, and developmental delay.

There has been some recently publicity regarding a study published by Geier and Geier that purports to show a link between thimerosal and autism, but the study is highly flawed and draws upon numbers that reported possible side effects from vaccines but have not necessarily been substantiated by research.  For more information see the response written by the American Academy of Pediatrics.

The only vaccines today that routinely contain thimerosal are tetanus booster, influenza, and meningococcus.  Only influenza vaccine is given to children as young as six months, and the amount of thimerosal in influenza vaccine is well below the FDA limits.  While thimerosal-free influenza vaccine is being manufactured, it is produced in limited quantities and is difficult to obtain, and we do not have any in stock.  Both the Centers for Disease Control and the American Academy of Pediatrics recommends giving influenza vaccine regardless of whether it contains thimerosal or not, because the amount is exceedingly small and poses no risk to your child.