With the standard coronavirus, we estimated R0 to be about 2.5, which means we needed about 60% of the population to be immune to reach herd immunity. With the Delta variant, we estimate R0 to be between 5 and 8, which means we need between 80% and 88% of the population to be immune.
Right now, we have approximately 45% of Utahns fully vaccinated, according to the Utah Department of Health. Again, it’s reasonable to assume that somewhere between 30% and 60% of the unvaccinated population has gotten COVID-19. That means roughly 60% to 75% of the population has immunity — enough to reach herd immunity levels with the old version of the virus but not the new one.
Essentially, we’ll likely see Delta variant case growth until a larger percentage of Utahns either get vaccinated or infected.
Also, it would be helpful if public health and media would distinguish between “break through cases” and “break through disease”. A vaccine merely boosts your immune system to respond quickly – if the disease is widespread (R-0 = 5 to 8!) then you can easily get infected but your immune system will respond quickly so you have no or mild symptoms.