You might be thinking of this paper about very extensive data from Israel: https://www.nejm.org/doi/full/10.1056/NEJMoa2101765 https://www.nejm.org/doi/full/10.

You might be thinking of this paper about very extensive data from Israel: https://www.nejm.org/doi/full/10.1056/NEJMoa2101765 https://www.nejm.org/doi/full/10.1056/NEJMoa2101765 One conclusion a friend of mine wrote about this is "One other surprising thing which is not obvious, but is made very clear from the results, is that it really takes real time for this vaccine to get effective. It only starts becoming truly useful after 2 weeks, but even then keeps getting better with time. This effect is one reason why there was a very real lag between when Israel got vaccinated at a very high percentage of population and when the cases started freefalling (2-3 weeks+). "