People worry about the preservative thimerosal because it contains trace amounts of mercury. While mercury can be poisonous, the amount of thimerosal in vaccines is really tiny—and studies have shown that there are no adverse effects associated with vaccines that contain it.
Note that there will be some preservative-free swine flu vaccine available. Supplies are limited, but if you're concerned, check with your doctor about how you might get some. —Claire McCarthy, MD, assistant professor of pediatrics at Harvard Medical School