First, it seems likely that the increase in positivity can be explained by fewer precautionary tests: fewer people are getting tested “just to be sure”, fewer people are being required by work/travel/etc. to get tested. Therefore fewer negative tests.
Second, it seems likely to me that the “93%, 93%, 91%” numbers are calculated independently from each other. I.e. 93% less likely to contract than unvaccinated, 93% less likely to hospitalize than unvaccinated, and the vaccinated group was 91% less likely to die than the unvaccinated group. So with alpha, all probabilities were reduced ~uniformly. Now consider a variant (delta) where the vaccine is not as effective at reducing symptoms of any level, but is still ~as effective at preventing hospitalizations and deaths. This would decrease the likelihood of the vaccine preventing a positive test or symptoms, while not changing the hospitalization/deaths numbers much. This makes sense in my head, but perhaps there’s something I’m missing?
Finally, a typo that tripped me up a bit:
We should also look at case counts in Israel. On June 18 they had 1.92 cases per million, right before things started rising, on June 14 it was 65.09, for R0 = 1.97. From previous data, we can presume that when Delta was a very small portion of Israeli cases, the control system adjusted things to something like R0 = 1, so we’ll keep that number in mind.
The second “June” should be “July”, as in “July 14”. (Small nitpick, I know, but it took me a minute to work out, so I figured I’d share.)
A few observations.
First, it seems likely that the increase in positivity can be explained by fewer precautionary tests: fewer people are getting tested “just to be sure”, fewer people are being required by work/travel/etc. to get tested. Therefore fewer negative tests.
Second, it seems likely to me that the “93%, 93%, 91%” numbers are calculated independently from each other. I.e. 93% less likely to contract than unvaccinated, 93% less likely to hospitalize than unvaccinated, and the vaccinated group was 91% less likely to die than the unvaccinated group. So with alpha, all probabilities were reduced ~uniformly. Now consider a variant (delta) where the vaccine is not as effective at reducing symptoms of any level, but is still ~as effective at preventing hospitalizations and deaths. This would decrease the likelihood of the vaccine preventing a positive test or symptoms, while not changing the hospitalization/deaths numbers much. This makes sense in my head, but perhaps there’s something I’m missing?
Finally, a typo that tripped me up a bit:
The second “June” should be “July”, as in “July 14”. (Small nitpick, I know, but it took me a minute to work out, so I figured I’d share.)