Thimerosal is a mercury-based preservative that is used for multi-dose vials of vaccines. It prevents bacteria from contaminating these vials, which require repeated introduction of needles to remove multiple doses of the vaccine.
Manufacturers stopped adding this preservative to certain childhood vaccines (although most, including MMR, have never contained thimerosal) as a precaution when some people theorized that it may be linked with autism. This link has never been proven. In fact, after removing thimerosal from vaccines, there has actually been an increase in autism diagnoses. Still, vaccines that are currently intended for children six years and under now contain either trace amounts or no thimerosal at all.
Multi-dose vials of vaccines are much less expensive than single-dose vials. At the time of this writing, many physicians and health departments are offering only the multi-dose option and the nasal spray for H1N1 vaccination. The single-dose versions of seasonal and H1N1 flu vaccines do not contain preservatives (again as a precaution in case there is any potential risk) and may be offered to children ages 6 months to 3 years. However, if a preservative-free option is not available, it is still recommended that children in this age group receive the multi-dose vaccine to protect them against the flu since young children are at high risk for complications of seasonal or H1N1 flu infection. The risks from the infection itself are thought to be much higher than any risk from the vaccine, with or without thimerosal.
The nasal spray (mist) flu vaccine does not contain thimerosal. It is an option for people ages 2 years to 49 years who are not pregnant and do not have chronic medical conditions such as asthma.