The basis for the CDC recommendation for masking up indoors in high transmission areas was a study of a spreading event in Provincetown, MA over the 4th of July:
Critically, the study found that vaccinated individuals carried as much virus in their noses as unvaccinated individuals, and that vaccinated people could spread the virus to each other. […]
Scientists said the Provincetown outbreak and other recent data on breakthrough infections make clear that the vaccines do work, as hoped, against severe illness and death, but do not offer blanket protection against any chance of infection. Only a handful of people in the outbreak were hospitalized, but four of them were fully vaccinated. […]
The study’s authors note that Massachusetts has a high vaccination rate and the virus was still able to spread.
“Findings from this investigation suggest that even jurisdictions without substantial or high COVID-19 transmission might consider expanding prevention strategies, including masking in indoor public settings regardless of vaccination status,” they write. […]
The study makes clear that vaccines offer significant protection, but do not prevent infection entirely even among the fully vaccinated. On July 3, the Massachusetts Department of Public Health reported a 14-day average of zero covid-19 cases per 100,000 in Barnstable County — but by July 17, that number had increased to 177 cases per 100,000.
“This report demonstrates that vaccination against SARS-CoV-2 is not perfect, particularly in a setting with a highly contagious variant, in a large group in close contact, even if most are vaccinated against the virus,” said Gregg Gonsalves, associate professor at the Yale School of Public Health. “The good news here: If you’re vaccinated, refrain from large group gatherings and mask up, chances are good you’ll be okay. This is not 2020. But we’re not out of the woods.”
I excerpted quite a bit because the details are discouraging but not a reason to panic. The key point as recorded in the Morbidity and Mortality Weekly Report that’s the basis for this story is that none of the 4 hospitalized vaccinated persons died, and only 4 of the 346 vaccinated persons were hospitalized. I think it changes some of my assumptions about what I need to do, as a vaccinated person, to protect myself and others — specifically, I’m masking up in public indoors again, even though I’m in a low transmission/high vaccination rate area. (And, “mask up” for me is a KN-95 or KF-94 mask.)