I think of the infectiousness of the original “variant” of COVID-19 virus as spreading relatively slowly like molasses flowing. Despite this slow flow it had little problem encountering new people who lacked immunity as immunity was rare.
The Delta variant taking hold now is much more infectious than the original variant and I liken it to flowing like fresh tree sap (essentially sugar water). There are many fewer “targets” who still lack immunity but the “fast flowing” Delta variant can still encounter them after “bypassing” the immune members of the community. Delta will find many people who lack immunity but, fortunately, they will be mostly younger and healthier. The good news is that these encounters with Delta will confer immunity. I think the Delta increase in infections will build and then dissipate much more quickly than our earlier episodes.
The original Wuhan virus had an R0 of 1.3.
The April 2020 variant that actually spread well had an R0 of 2.5.
The UK, South Africa, and Brazil variants have R0s somewhere in the 4.5 - 5.3 area.
The Delta (India) variant has an R0 closer to 8.
For comparison, the R0 for smallpox is 3.5 - 6, the R0 for polio is 5 - 7, and the R0 for measles is 12 - 18.
“dissipate much more quickly “
Indications from India is that will happen.
OTOH the strong wall of people here with vax or natural immunity will slow it’s spread down, so I expect a longer wave here, but with a lower ‘surge’.
most at risk are rural people who have been less exposed to the virus and have been poorly served by the vaccination effort IMO.
“these encounters with Delta will confer immunity.”
Yep. The hard way or the easy way, we will get to herd immunity
“I think the Delta increase in infections will build and then dissipate much more quickly than our earlier episodes.”
It is definitely a faster burner. Mrsmith posted a link to analyst some time ago, who estimated how much of the population remained without immunity, for Delta to burn through, before reaching Delta’s Herd Immunity Threshold - about 10%, or 30 something million.
That is about half the total number infected in last Winter’s big wave. As you point out shape of the curve will likely be different because Delta moves fast, and also because immunity is now widely different, in different parts of the Country.
Some places are only around 30% vaccinated, while others are over 80%. Naturally acquired immunity is a big wild card, but some places could see a pretty significant surge this month, like Springfield Missouri is having.