I've heard people claim that vaccines aren't tested before being given to infants. I looked into it and found lots of references to extensive human trials, but the best reference I've found about testing in children is this from a site run by the College of Physicians of Philadelphia:
If the vaccine is intended for children, researchers will first test adults, and then gradually step down the age of the test subjects until they reach their target.
But how do they do tests in very young children or infants? It seems a bit weird for any parent to want to sign their newborn up for drug trials. Can anyone provide an authoritative reference on this?