Rt = R0 * (1 - (vacRate * effRate))
So if Delta R0 is 6, a community is 65% vaccinated, and a vaccine is 80% effective against transmission, then effective Rt would be:
Rt = 6 * ( 1 - (.65 * .8)) = 2.88
Real effective Rt includes impact of mitigation levels and natural immunity. So for instance, in Portland OR, Rt is about 1.4. We didn't get hit as hard as surrounding states in the first few rounds, so we don't have as much natural immunity. In contrast, Seattle got hit harder early on; their effective Rt is a little lower - and Silicon Valley is actually pretty close to 1 right now.
That was my question - how much less do those who are vaccinated and contract the virus infect other people than those who are unvaccinated and contract the virus. That can't be calculated from a single R0 number.
Vaccinated group:
Rt = 6 * ( 1 - (1 * .8)) = 1.2
Unvaccinated group:
Rt = 6 * ( 1 - (0 * .8)) = 6
The question of "how much less" is hard to parse, but the point is that even among the vaccinated group (in this scenario), the virus is still spreading and growing exponentially.
I suppose if Rt was clearly below 1 for the vaccinated group, that might mean something, but if the groups intermingle, that advantage would be lost pretty quickly.
There's no such thing as "R0 within a vaccinated community" - R0 is the starting reproductive number. Effective reproductive number - given various mitigation measures like vaccination, masks, natural immunity, etc - is Rt.
You can try the calculations using any other starting R0, from 5 to 9.5 if you stay consistent with estimates of Delta transmissibility.
The point is that it's possible that a group of vaccinated people would never have an Rt below 1 from vaccination alone, simply because of the limited vaccine effectiveness against transmission.
- Delta variant has ridiculously high R₀ - estimated to be between 5 and 9, where initial COVID-19 variants had it around 2 to 3.
- Viruses spread nonuniformly. Old COVID-19 (R₀ = 2-3) tended to spread through super-spreading events (few people infecting a lot more at a time). R₀ of 5-9 implies Delta variant doesn't need super-spreading events.
- This R₀ value also means it's not possible to eradicate the virus through vaccination or herd immunity - it's going to become endemic (which was always seen as a likely outcome anyway).
- Current vaccines have proven to be effective against the Delta variant, by significantly reducing infections (0.01% to 0.29% chance of breakthrough disease), such infections showing symptoms (~88% percent effective) and severity of those symptoms if they show up, as well as preventing hospitalization (~96% effective against Delta; >95% of all COVID patients in US hospitals are unvaccinated).
- There is evidence that vaccinated people can still transmit Delta, even if themselves they don't show symptoms. As expected, the virus seems to live shorter in the airways of vaccinated people.
- Above and high R₀ mean Delta can spread even through highly-vaccinated communities.
- Individually, for those who can get a shot, it's the best thing to do. Community-wise, we still need other precautions - like mask, widespread testing, hygiene, improvements in ventilation, to protect the vulnerable and slow the spread.
- Vulnerable groups now include children under 12 - they're not eligible for vaccination, and there's a growing (though still small) amount of children suffering from long-COVID and MIS-C.
- The end goal is still to slow the spread, to keep schools open and hospitals running, prevent healthcare personnel burnout, give time to develop better countermeasures and therapies, and let the economy slowly recover (instead of repeating 2020). Slowing the spread down also gives the virus less chances of mutating into a more problematic variant.