CNN  — 

Two senior leaders in the US Food and Drug Administration’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.

In the email, CBER Director Dr. Peter Marks said Gruber will retire on October 31, and Krause is leaving in November. Marks thanked Gruber for her leadership throughout efforts to authorize and approve Covid-19 vaccines, and Krause for serving in a “key role in our interactions to address critical vaccine-related issues with our public health counterparts around the world.”

Marks said in the email the search for the next director of the Office of Vaccines Research and Review would begin imminently and that he would serve as the acting director.

The letter made no mention about why Gruber and Krause are leaving, but the departures sparked questions about whether it would affect FDA’s work during the pandemic.

“We are confident in the expertise and ability of our staff to continue our critical public health work, including evaluating COVID-19 vaccines,” FDA spokesperson Stephanie Caccomo said in a response to CNN.

When asked about the departures during a briefing on Tuesday, White House Covid-19 Response Team Coordinator Jeff Zients did not directly answer whether he was concerned about the departures could affect the level of trust in the FDA’s process. He said the White House was “grateful for the tireless work of the senior team and the whole staff at FDA.”

But President Joe Biden has not yet named a nominee for the agency’s top job. It’s currently led by acting Commissioner Dr. Janet Woodcock, a longtime senior leader at the FDA.

Woodcock told FDA staff the agency was operating in “difficult times.”

“Following today’s announcement that Marion and Phil have decided to retire, I wanted to reach out to reiterate my full support and complete confidence in your ability to continue delivering on the agency’s mandate to deliver safe, effective and high-quality vaccines to the American people,” Woodcock said in a separate email to staff that was sent to CNN.

“These are difficult times, requiring extraordinary effort, and I want to take the opportunity to once again thank you for your perseverance and dedication,” she added.

“The issues are complex and the days are long, but please know the work you all have done to date and will continue to do in the days, weeks and months ahead, will hopefully one day allow us to fully put COVID-19 behind us and better prepare us for future challenges,” Woodcock added.

She said she was confident in the team Marks would lead. “We have put together a plan that will allow us to continue prioritizing science, while meeting timelines that are important to ensuring the end of this devastating pandemic,” she said.

And there’s more work to come. In the next few months, the agency will assess and make decisions on additional Covid-19 vaccine approvals and vaccines for children younger than 12.

US health officials also recently announced a plan to begin offering booster doses to people who received mRNA vaccines