Question: Is there data/evidence on whether low vaccination rates in places like TN cause uptick of breakthrough cases? In other words, do low vaccination rates cause vaccination efficacy to fall?

Answer: Good question.  Here’s a brief answer: 

1. Vaccine efficacy is a measure of risk reduction (see Q&A of 2/19).  In places where the underlying risk of infection is higher (e.g. places with low vaccine uptake), the risk of breakthrough infection is also higher.  In this case, vaccine efficacy hasn’t fallen; rather, differing levels of underlying risk between high and low vaccination settings make breakthrough infections more frequent in low vaccination coverage settings.
2. As the more infectious Delta variant spreads, vaccine efficacy is diminished.  Delta spreads more rapidly in places with low vaccination coverage.  This is another reason why we’d expect more breakthrough infections in places like Tennessee. For more on the risk of breakthrough infection, see Q&A of 6/23.