After the FDA's surprise refusal to take up a review of Moderna's next-gen flu vaccine, the public disagreement between U.S. regulators and the mRNA specialist took a new turn Wednesday, with the U.S. Department of Health and Human Services (HHS) sharing further rationale behind the decision.

To help answer common vaccine questions, we consulted Dr. Stanley Perlman, a professor at the University of Iowa’s Carver College of Medicine and a member of the U.S. Food and Drug Administration’s (FDA) vaccine advisory committee, and Angela Shen, a visiting scientist with the Vaccine Education Center at Children’s Hospital of Philadelphia.