WASHINGTON — As President Joe Biden approaches his 100th day in office, he has yet to fill a number of key positions throughout the federal government, which former officials and advocacy groups warn could hobble agencies responsible for the influx of migrants at the border to the safety of Covid-19 vaccines.

Across the federal government, there are more than 400 positions requiring Senate confirmation for which Biden has yet to put forward nominees, including the heads of the Food and Drug Administration, Customs and Border Protection, and the Office of Management and Budget.

The jobs are being filled temporarily by officials limited by their acting capacity: They are unlikely to appoint the permanent staff below them or to make any structural or cultural changes. Even once nominations are made, it can take months for nominees to make it through the confirmation process and into their permanent roles — meaning a number of agencies and departments may have no permanent leaders through the summer.

"It's the equivalent of having a substitute teacher. They could be an amazing educator, but everyone knows the substitute teacher doesn't get respect from the classroom, and they don't see themselves as owning the long-term education of the class," said Max Stier, president of the Partnership for Public Service, a nonprofit good government group. "As a result, they are unlikely to take on the big hairy problems or things that require a long time to resolve, and people on the outside are unlikely to receive them as having much authority, certainly not long-term authority."