The main determinants of Covid’s spread (other than vaccines, which are extremely effective) remain mysterious. Some activities that seem dangerous, like in-person school or crowded outdoor gatherings, may not always be. As unsatisfying as it is, we do not know why cases have recently plunged. The decline is consistent with the fact that Covid surges often last for about two months before receding, but that’s merely a description of the data, not a causal explanation.

“We still are really in the cave ages in terms of understanding how viruses emerge, how they spread, how they start and stop, why they do what they do,” Michael Osterholm, an epidemiologist at the University of Minnesota, has told me.

In coming weeks and months, it is possible that the virus will surge again, maybe because of a new variant or because vaccine immunity will wane. It is also possible that the population has built up enough immunity — from both vaccines and previous infections — that Delta will have been the last major wave.

We don’t know, and we do not have to pretend otherwise. We do not have to treat Covid as a facile referendum on virtue.