Two senior leaders in the US FDA’s vaccine review office are stepping down, even as the agency works toward high-profile decisions around Covid-19 vaccine approvals, authorizations for younger children and booster shots. The retirements of Dr. Marion Gruber, director of the Office of Vaccines Research and Review at FDA's Center for Biologics Evaluation and Research, and Dr. Philip Krause, deputy director of the office, were announced in an internal agency email sent on Tuesday and shared with CNN by the FDA.