Thanks for the link! Lots of interesting details in there. I'll attempt a TL;DR of points most relevant for non-Americans (but please check the article for context, particularly on numbers, and link to sources):

- 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.

One clarification - between 0.01% and 0.29% (actually 0.54% now) of fully vaccinated people are affected by breakthrough infection, while 88% effective means in a situation where an unvaccinated person would become infected, the vaccinated person would have an 88% chance of not being infected. So the difference between the two is exposure, which is a function of prevalence.

What is the delta variant R₀ in fully vaccinated communities though? Certainly it's not in the range of 5 to 9. If it's something like 2, then sure, a mask mandate makes sense for everyone. But if it's much lower than 1, then is it fair to force the vaccinated to wear masks? Is this sort of information hidden from the public because officials are too cowardly to force vaccine passports? There are so many information gaps like these that leave room for doubters.

You can figure it with math. Given a starting R0, a vaccination level, and an efficacy, you can figure effective Rt:

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.

You are assuming that R0 for vaccinated that contract the virus is the same as those who are unvaccinated that contract the virus, which does not appear to be the case.

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.

If you restrict it to the vaccinated population and the unvaccinated population, then you can consider the vaccinated population fully populated, and the unvaccinated population entirely unvaccinated.

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.

I think you are still missing my point. You are using a circular definition of R0. Where did you get that number 6? Is that for a fully unvaccinated population? Are partially vaccinated population? If so, what percentage? Your calculation makes absolutely no sense, it's circular. You can't calculate R0 using R0.

No, R0=6 is a given. I'm not calculating R0. R0 for Delta is estimated between 5 and 9.5. I picked 6, just for illustration. Given a starting R0 and a vaccination rate and a vaccine effectiveness, that is how you can calculate effective Rt. There's nothing circular about that.

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.

If one population spreads a disease more than another, it makes absolutely no sense that R0 is a constant across all populations, by definition. Perhaps you should look up the meaning of R0.