Before doctors are allowed to use them, all vaccines are thoroughly tested for many years to make sure they are safe and that they work. The Food and Drug Administration approves vaccines after they have been tested successfully. After a vaccine is approved, the Vaccine Adverse Events Reporting System (VAERS) helps track any health effect that happens hours, days, weeks, or even months later after someone gets a vaccine. Anyone can report a possible vaccine side effect to VAERS so that it can be studied. All of these activities help make sure vaccines are safe.
Autism is a complex condition that science still has much to learn about. It is still not clear exactly what causes someone to develop autism. Autism definitely has a strong genetic link, and it may have some environmental factors too. More research is being done every year to try to find out what causes autism and how to prevent it. However, study after study has found NO link between vaccines and autism.
A study published in 1998 started the idea that the Measles, Mumps, and Rubella (MMR) vaccine might cause autism. The study has since been found to have many flaws, but the idea that the MMR vaccine causes autism has not gone away. Since then, 25 studies have found no connection between the MMR vaccine and autism.
Thimerosal is a type of preservative that contains mercury. Thimerosal is no longer used as a preservative in any childhood vaccine except for a few kinds of flu vaccine. By the year 2001 it had been removed from most vaccines, even though numerous studies have found no link between thimerosal and autism. In fact, after thimerosal was removed from vaccines, the number of children found to have autism every year still continues to rise.
The Vaccine Education Center at The Children's Hospital of Philadelphia has a large variety of materials about vaccines and vaccine safety.
The Immunization Action Coalition has resources on vaccine safety for the public and health professionals.