- One set of advocates are concerned about the damage that thimerosal, a mercury-based preservative, may have had on their infants. A direct causal relationship between thimerosal and autism has never been proved to the satisfaction of the medical mainstream, but there are many parents, doctors and journalists who are convinced that the link is real. Thimerosal is no longer included in any required vaccines, and even the flu vaccine can be requested in a thimerosal-free form.
- Another set of advocates (and I'm sure there's some overlap between the two groups!) are concerned about the damage that the MMR (mumps/measles/rubella) vaccine may have had on their 18-month-old toddlers. Again, a direct causal relationship between the MMR and autism has never been proved to the satisfaction of the medical mainstream, but many are convinced that the link is real (based in part on the fact that many children start to develop autistic symptoms around or at the same time as the vaccine is administered). It's important to note that the MMR does not contain thimerosal, but does contain live viruses.
- Yet another group of advocates (again with overlap) argue that, causal proof aside, multiple vaccinations for infants makes no medical sense. In fact, they argue, the practice is potentially dangerous and is completely unnecessary. This same group points out that even the MMR need not be given as a single, combined shot (which may be more dangerous). Again, it's important to note that parents CAN request that the MMR be administered in separate shots over time and NOT as a single multi-vaccine.