Aluminum adjuvants are used to stimulate an abnormally strong immune response at an injection site so that less antigen (bacteria or virus for the infectious disease) is needed in a vaccine. Aluminum adjuvant has been called “the immunologists’ dirty little secret" because immunologists know that it works to increase the immune response to a vaccine, but not why it works. A 2011 study, Aluminum Vaccine Adjuvants: Are they Safe?, stated that "Aluminum is an experimentally demonstrated neurotoxin and the most commonly used vaccine adjuvant ... aluminum in adjuvant form carries a risk of autoimmunity, long-term brain inflammation and associated neurological complications and may thus have profound and widespread adverse health consequences."
After the 1986 law that provided vaccine manufacturers with immunity from lawsuits, the expansion of the vaccine schedule with new aluminum-containing vaccines such as Hib and Hep B caused children to be exposed to far more aluminum adjuvants in the first six months of life. The amount of aluminum adjuvants in vaccines was further increased in the same 2001-2005 timeframe that the mercury-based preservative called thimerosal was being reduced and phased out of many childhood vaccines. (see Aluminum and Mercury in Vaccines charts).
Aluminum adjuvant has been shown to preferentially generate a Th2-type immune response. In recent years, the use of aluminum adjuvants that induce a strong Th2 response by a mechanism n ot yet fully understood has increasingly come under criticism by vaccine safety advocates, including Dr. Robert Sears, author of “The Vaccine Book”. Given the rapid rise of childhood chronic illness, such as allergies and asthma, that are mediated by Th2 responses, this criticism seems warranted and worthy of investigation. The use of adjuvants that stimulate a Th2 response in a vaccine for a disease that requires a Th1 response leads to a less-effective vaccine that has a higher safety risk. Back in the late 1980s, the need for a Th1 immune response for clearance of pertussis or the tendency of aluminum adjuvants to produce a Th2 immune response (while suppressing the Th1 response) was unknown and not yet on the scientific radar. But in the past 20 years, Th1 versus Th2 immunity has become well-understood and part of the mainstream published scientific literature. (see Pertussis Vaccine Failure Should be a Wake-up Call).
In 2007, an important study investigating links beween Gulf War Syndrome and vaccine ingredients found that aluminum adjuvant caused death of motor neurons in the brain in mice (HERE). Aluminum adjuvant and thimerosal (a mercury-based preservative still used in some vaccines such as the flu vaccine) have synergistic toxicity, meaning that the toxic effective is exponential rather than additive when the two are combined. There has been no formal human study on the safety of aluminum adjuvants in vaccines. However regarding a CDC study that initially showed a 2.5 relative risk of autism for infants receiving higher thimerosal versus lower thimerosal, the lead researcher indicated that the data could have as readily indicated that aluminum adjuvant in the vaccines was the cause of the increased relative risk rather than (or in addition to) the thimerosal.
Are aluminum adjuvants in vaccines a key causal factor in the epidemics of asthma, ADHD, autism, and allergies? Modern scientific research, not bound to the old Pro-vax paradigm and emboldened by the recently-developed understanding of Th1 vs Th2 immunity, is needed to determine the answer.
 B.E. Haley. Mercury toxicity: Genetic susceptibility and synergistic effects. Medical Veritas 2 (2005), 535–542.
 Transcript from "Scientific Review of Vaccine Safety Datalink Information", June 7-8, 2000, Simpsonwood Retreat Center, Norcross, GA