Question: Given the newly accelerated infection rates amongst unvaccinated people, is there any projection about how soon they will come down with the virus?  I’m bracing myself for bad news about our unvaxxed relatives.  Crystal ball, please!

Answer: I don’t have a crystal ball(!), but I will share a few pieces of information related to your question about risk.  First of all, Delta is highly contagious, 2-3x more contagious than the original COVID strain (Figure 1).  The basic reproduction number (R0) gives us an estimate of the epidemic potential of the virus– how quickly the epidemic will grow in the absence of effective public health measures and vaccines.  For Delta, the R0 is estimated to be between 5 and 6; that is, each infected individual would pass the virus on to 5 or 6 others in the absence of effective public health measures and vaccines.  You can imagine the speed at which the virus would move through a susceptible population in the absence of controls.  And if you want to play with the numbers, the Guardian offers a helpful set of visualization tools.  I made a hypothetical scenario based on a population of 1,000 where R0 is 5.11 (similar to Delta), case fatality is 1% (this varies by population, but in the US the case fatality rate is between 1%-2%), susceptible population is 47.6% (in the US as of 8/30, 52.4% of the population was fully vaccinated), and isolation rates are 0% (just for the model) (Figure 2).  In this scenario, it would take approximately 19 days for every susceptible member of the group to become infected.  Of course, if you add in protective measures, the rate of growth slows considerably.  On that front, a pre-print posted earlier this month models just how much masking, testing, and other public health measures (like increased vaccination among older students) can limit the spread of COVID in schools as compared with the absence of protective measures.   

Figure 1. The Delta Variant spreads more easily (from CDC)

Figure 2. Simple model of disease spread given 4 parameters: R0, Case Fatality, Susceptibility, and Isolation (from the Guardian)