The Biden administration has hailed the need for COVID-19 vaccine booster shots as the Delta variant cripples some states’ health-care systems, but top government scientists are urging it to hit the breaks. Dr. Janet Woodcock, the acting head of the Food and Drug Administration, and Dr. Rochelle Walensky, the director of the Centers for Disease Control and Prevention, had a meeting with the White House’s COVID-19 coordinator Jeff Zients on Thursday, The New York Times reports. In it, the two told Zients they needed more time to collect and review the science behind booster shots, including whether they should only be given to those with the Pfizer vaccine and how many of those individuals should take it. As a result, they said, the White House should scale back its push on boosters, which it had hoped to offer later this month.

The White House said it was waiting on full FDA approval before moving forward. “We always said we would follow the science, and this is all part of a process that is now underway,” spokesman Chris Meagher said. “When that approval and recommendation are made, we will be ready to implement the plan our nation’s top doctors developed so that we are staying ahead of this virus.”