From the CDC's MMWR comes a story out of Marin, CA.
In one school district in Marin, all teachers were vaccinated except for two. One of them caught COVID. The school's policy was for teachers to wear masks at all times, but this teacher took it off occasionally to read to the class.
Of the 24 students in his classroom, 12 of them ended up infected - most of them the closest students to him while he was reading.
In other classrooms, several infections were seen. They were tested, and the samples sequenced to ensure that they were all the same strain (they were) thus suggesting that they all came from the same index patient (the unvaccinated teacher.) A total of 27 people were infected. All the children were unvaccinated because they were too young to be vaccinated. Of the adults who became infected, three were vaccinated.
This tells us a few things:
1) The Delta variant is extremely infectious. In a school setting with sloppy (but implemented) COVID protocols, one infection led to 26 other infections.
2) Vaccination is effective but does not work 100%. Most of the staff (and parents) avoided infection, and almost all of them were vaccinated. But three staff members/parents who were vaccinated contracted the infection.
3) NPI's are critical, and it's not OK to be sloppy or causal about them. Masking prevents infected people (whether they know they are infected or not) from spreading the disease.
https://www.cdc.gov/mmwr/volumes/70/wr/mm7035e2.htm?s_cid=mm7035e2_w