The CDC finds with Delta the vaccinated can be just as contagious as the unvaccinated, but this was based on questionable information and it's likely wrong
UPDATE: Aug 6, 2021
I'd previously questioned the CDC making a major change to their recommendation on masking based on a single unrepresentative outbreak in a gay party town. It should be noted though that wasn't the only alarming information that's come to light, there are two separate studies from Wisconsin and Singapore that also found there to be similar viral loads in breakthrough Delta infections in the vaccinated as in the unvaxxed.
It's data to be considered but the preponderance of the evidence still shows the vaccines to not just protect well against serious infection but impede transmission of the virus and slow or turn around outbreaks. The latter point remains critical because it has profound consequences for the case for mandatory vaccination (see below).
In this good Twitter thread Nate Silver goes over why these studies showing similar viral loads are weak, and he points to a much larger study from the UK that did in fact find that the vaccinated have lower viral loads.
Much of the real-world information we have is supportive too. It shows that more vaccinated places tend to have lower case counts. In the UK, though they faced another outbreak of infections since reopening, we've just seen a sharp drop off in new cases. In the US more vaccinated places are seeing fewer cases than the less vaccinated.
Now, we should always be cautious in making too much of comparisons of data like this. It could be other things that explain it, like regional trends, etc. But I think we can confidently say that the vaccines are having a significant, indeterminate effect on slowing the spread of the virus. Outbreaks definitely are happening among the vaccinated, maybe to the disappointment of some, but the impact has been far less severe than it would have been, and given that the vaccines I think impede transmission of the virus, to a significant degree, it makes the case that one reason people should get it is for those around them, not just themselves.
It's been widely reported that an internal CDC report found that with the Delta variant, breakthrough infections in the vaccinated can be just as contagious as infections in the unvaccinated. While the vaccines are holding up in protecting the infected against serious illness, this would have significant consequences for the case for mandatory vaccination given that one of the stronger arguments for even young and healthy people to get the shot is to protect others. It appears though to be an example of the CDC inexplicably undermining their own case for vaccination based on very unrepresentative data.
Some of the new information that led the CDC to reach this conclusion was an outbreak among the highly vaccinated population that visited Provincetown, Massachusetts, a well-known gay party town, on the July 4th weekend. The event is described as having been a jam-packed party with 60,000 people crammed into "a narrow spit of land." Attendees describe dancing and making out with each other; they've justified their behavior by claiming they were told that because they were vaccinated, they could do pretty much anything.
This is clearly a very unrepresentative case and it's been called out as such by gay liberals familiar with the place. It's not surprising that they found a shockingly high R0 (Reproduction number) given the extremely covid-unsafe behavior that occurred at this event. There's been good reason to think that the vaccines work to impede transmission of previous covid variants and though we've seen widespread breakthrough infections with Delta, in widely vaccinated places like the UK cases have dropped off