As Trump tried to reassure Americans about the new vaccine, Harris said she wouldn't take it if Donald Trump, our president at the time said to take it.
Biden also suggested that the vaccine wouldn't be as rigorously tested under Trump as it should be...ie - he promoted hesitancy at a critical time.

The amount of left wing search engine scrubbing is impressive on this....but both of them absolutely contributed to hesitancy that they later attempted to condemn and punish.