In an interview with CNBC, Fauci was asked how many more shots will be needed, and responded "The answer is: we don't know. I mean, that's it…it is likely that we're not done with this when it comes to vaccines."

The comments dovetail with those of Albert Bourla, who said Sunday "It is necessary – a fourth boost – right now," when asked about vaccines.

The Pfizer head said "The protection that you are getting from the third [vaccine] is… not that good against infections, but doesn't last very long but we are just submitting those data to the FDA and then we will see what the experts also will say outside Pfizer."