Data from the Centers for Disease Control and Prevention (CDC) has demonstrated what anonymous administration officials described to Politico as a “worrying drop” in coronavirus vaccine efficacy over time, leading to the Biden administration’s conclusion to push vaccine booster shots.

“This is what moved the needle,” a senior Biden administration official said to Politico of the CDC’s report.

According to the outlet, the data looked at the effectiveness of the coronavirus vaccines at different points in time and found a “decline in the initial round of protection against Covid-19 [Chinese coronavirus] infection that’s coincided with a resurgence in cases driven by the more contagious Delta variant.”