Originally Posted by Kapusta

Let's clear something up: the CDC states (and pretty much all experts agree) that the vaccine DOES reduce the chances of being infected by the delta variant. Nationwide data continues to back this up. Why else would it be spreading so much faster through unvaccinated populations than the vaccinated population?

You are misinterpreting what those numbers above tell us. You are not alone in this, many people do, and it is perpetuating a lot of bad information. That example does NOT disprove the assertion that vaccinated people are less likely to get infected than non-vaccinated. It actually says very little about it. Here is why:

You can't just look at the proportion of vaccinated vs un-vaccinated that got infected. You need to compare that to how they are represented in the whole population (in this case, the population of the gatherings). That is what tells you how likely members of each group are to be infected. Here is an example:

Lets say you have a group of 100 people where 95% are vaccinated (*see below for reasoning)

So you have a group of 95 vaccinated, 5 un-vaccinated.

Lets say that a vaccine is 90% effective. This means that in a given scenario, a vaccinated person is 1/10 as likely to be infected as an un-vaccinated person.

OK, so lets expose this group to a spreader event. Lets say that the conditions are such that an vaccinated person has a 50% of being infected. You would expect 2.5 un-vaccinated people to be infected. The vaccinated people have 1/10 the chance of being infected as the un-vaccinated. 1/10th of 50% is 5%. 5% of 95 people is 4.75 vaccinated people infected.

SO, you end up with 2.5 vaccinated and 4.75 vaccinated people infected.

In other words, even though a vaccinated person is much less likely to get infected in this situation, they still make up a greater part of the total number of infection. THIS is why you can't use one particular scenario like this to draw a conclusion that the vaccines are not effective in reducing the likely hood of infection.

Yes, I used hypothetical numbers here. Similar results could happen with a lower vaccine effectiveness and lower vaccination rates. But it explains why the CDC is (wisely) not going to look at that and see it as a counterexample of the vaccines effectiveness, when nations data shows that vaccinated are far less likely to be infected.

Also, when looking at particular instances, there is also a certain amount of dumb luck. Maybe the spreader individuals were mostly running in particular circles of these gatherings that were disproportionately vaccinated.

What that scenario DOES show is just how much more contagious Delta is. And THIS is why the CDC is now recommending masking up, and taking extra precautions even if you are vaccinated. But it is not just this case study, it is the nationwide trends.

And this was the point I was making above: riding with a group of vaccinated people is less likely to expose you than riding with an un-vaccinated group, simply because people in the vaccinated group are less likely to be infected.

*This could reasonably happen in a part of the country and particular community that takes vaccination seriously. Vaccine status is closely tied to the social and cultural circles people are in. For example, of my 25 or so closest local friends and family, nearly 100% are vaccinated. Only the small children are not. Heck, my whole neighborhood is probably 95%. Yet we live in a county with a similar vaccination rate to Barnstable County, Mass. In other words, it is very possible in a county with a 69% vaccination rate that the rate at a given gatherings could be much, much higher, even 95%.