The best plausible explanation I’ve seen is that Delta’s serial interval might be much shorter, which would mean R is lower than you’d think if you assumed Delta had the same serial interval as older strains. (Roughly speaking, in the time it would take Alpha to infect R individuals, Delta has time to infect R and for each of those individuals to infect another R, leading to R + R^2 infections over the same period.) That makes it easier for behavior changes and increasing population immunity to lower R below 1.
The important thing about this hypothesis is that it multiplies the effect of all other modifications, like schools or heat, so it’s at least part of the answer whatever the proximal cause is (which I still think is possibly just this + behavioral changes, but do feel a little underwhelmed without some other factor).
The best plausible explanation I’ve seen is that Delta’s serial interval might be much shorter, which would mean R is lower than you’d think if you assumed Delta had the same serial interval as older strains. (Roughly speaking, in the time it would take Alpha to infect R individuals, Delta has time to infect R and for each of those individuals to infect another R, leading to R + R^2 infections over the same period.) That makes it easier for behavior changes and increasing population immunity to lower R below 1.
The important thing about this hypothesis is that it multiplies the effect of all other modifications, like schools or heat, so it’s at least part of the answer whatever the proximal cause is (which I still think is possibly just this + behavioral changes, but do feel a little underwhelmed without some other factor).