The White House’s chaotic, contradictory messaging on Covid-19 vaccine booster shots has given Americans whiplash. But more concerning, experts say, is that it risks undermining President Biden’s campaign pledge that he would listen to the scientists and adhere to official approval processes. The administration’s latest move — a decision to expand booster eligibility that came in the middle of the night Friday — puts the spotlight on Rochelle Walensky, the director of the CDC, who overruled her own advisory panel of scientists to make the call.