In traditional products liability actions, section 402A of the Restatement (Second) of Torts governs a manufacturer's duty to warn. Section 402A dictates that a manufacturer is strictly liable for a product sold in a "defective condition unreasonably dangerous to the user or consumer." Products sold absent adequate warnings of risks or dangers are deemed defective. Under Pennsylvania law, however, courts consider certain prescription drugs, including vaccines, "unavoidably unsafe products." As such, they fall outside the realm of section 402A strict liability. Because section 402A does not apply to vaccines, the Supreme Court of Pennsylvania, in lncollingo v. Ewing, determined that vaccine manufacturers owe a "duty to exercise reasonable care" to inform users of the product's dangers.

In Mazur v. Merck & Co., Inc., the United States Court of Appeals for the Third Circuit evaluated whether, under Pennsylvania law, a vaccine manufacturer had met its duty to exercise reasonable care by informing vaccinees of the dangers of the MMR II vaccine. First, the court examined whether the learned intermediary rule extended to a registered nurse who supervised student vaccinations. Next, the court addressed whether a vaccine manufacturer satisfied its duty to adequately warn vaccinees by contractually obligating the Centers for Disease Control (CDC) to provide such warnings.