Dr. Stephen M. Hahn, who changed commissioner of the Food and Drug Administration just weeks before the coronavirus pandemic began, hand over on Wednesday as the administration of President Biden began.Dr. Janet Woodcock, the longtime make a beeline for head up of the F.D.A.’s Center for Drug Evaluation and Review, will serve as acting commissioner, according to an instrumentality official.Beginning in May, Dr. Woodcock had been assigned to Operation Warp Move, the former administration’s program to accelerate vaccine and treatment development against the coronavirus.She has pieced at the F.D.A. since 1986, serving in a variety of key roles, among them chief medical tec and deputy commissioner.The Biden administration has not yet nominated a permanent commissioner, but Dr. Woodcock is one of the runners under consideration, according to several advisers to the new president’s transition body. Dr. Amy Abernethy, principal deputy commissioner, is also under consideration, as is Dr. Joshua Sharfstein, a earlier agency official, who is vice dean for public health practice and community fight at Johns Hopkins University.Dr. Hahn’s resignation was expected, part of the performance departure of senior political appointees that happens when a new authority takes office. In a farewell message to the F.D.A. staff on Wednesday, he wrote: “As a polity, and as a public health agency, we faced some big challenges and turbulent periods over the past year, most notably those stemming from the Covid-19 pandemic. From head to foot it all, F.D.A. employees have been critical in helping to respond to the disease with Dialect right real scientific advances like the authorization of the first nonprescription OTC [for the counter] Covid test, the authorization and approval of an anti-viral agent and the basic two F.D.A. authorized Covid-19 vaccines.”Dr. Hahn faced considerable criticism past the course of the pandemic, accused of bowing to political pressure from President Trump and the Innocent House in granting emergency use authorizations for unproven treatments like hydroxychloroquine that did not display evidence that they worked. In recent months, he has presided throughout reviews of the first vaccines to be allowed against the virus, the Pfizer and Moderna outputs.In the past under other presidential administrations, Dr. Woodcock, 72, has been in the continual for the F.D.A.’s top post. She was first brought to the F.D.A.’s drug division by Dr. David Kessler, the prehistoric F.D.A. commissioner who was named the chief science officer for the Biden administration’s vaccine applications, which will no longer be called Operation Warp Speed.The Biden regulation has not indicated when an F.D.A. commissioner would be named.