Before clinicians can administer a vaccine in the United States, the FDA must approve and license it. Investigators conduct extensive research leading up to this process, typically testing a vaccine in thousands of patients over 6 to 7 years or longer. Even with large sample sizes and rigorous study designs, rare adverse effects may be missed. For example, RotaShield, the first vaccine for rotavirus, was withdrawn from the market in 1999, despite being tested in more than 10,000 patients. Postmarketing surveillance demonstrated that a rare, yet serious, risk of intussusception was linked to the vaccine, and the FDA determined that the vaccine’s risks outweighed its benefits
