[tunesmith]

Yes, I brought up that equation merely to illustrate how vaccination alone can impact Rt downward - for a while it was unclear to me exactly how "partial vaccination" (before herd immunity) was a benefit; that helped me see that it really just mean it slows down the doubling rate until you get to Rt=1. But overall, Rt is impacted by all forms of mitigation including natural immunity.

The way I actually use that equation in my personal dashboards is to estimate what Rt "would" be, at today's mitigation levels, if no one had gotten vaccinated. So for instance, Portland's Rt is currently 0.92 (according to one model). By plugging in Portland's vaccination rate and efficacy estimates, you can estimate that Portland's Rt would be around 2 today if not for the vaccinations, if all other mitigation were the same.

On the one hand, 2 is a lot better than those estimates of 5 to 9.5, which means that we're impacted a lot by current mitigation practices (masks, distancing) and natural immunity. On the other hand, 2 is huge! Given estimates on infectious periods, that means that currently our cases halve every 90 days or so, but 2 would mean cases would be doubling every week or so. Gargantuan difference. So just an illustration that vaccination has a big impact and matters a lot.