Vaccines have spared millions of people from disease, and side effects from vaccines are very rare or very mild. The U.S. has taken steps in recent decades to insure vaccine safety. There have been a few missteps in vaccine history.

• 1942: A yellow fever vaccine administered by the U.S. Army was contaminated with hepatitis B, causing 100 deaths.
• 1947: In New York, a smallpox outbreak killed two people. A vaccine was linked to six deaths.
• 1955: After 200,000 people were injected with a bad batch of vaccine containing live polio virus, 70,000 became ill, 200 were permanently paralyzed, and 10 died.
• 1976: Of 45 million people immunized against swine flu, including President Gerald Ford, above, more than 500 contracted a paralysis called Guillain-Barré syndrome; 32 died.

With more than 100 potential vaccines in development around the world, according to the World Health Organization, an infectious disease specialist warned against relaxing scientific standards in a recent New York Times opinion piece: “Unproven vaccines have real risks.”

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