Earlier quoted context omitted.
Because we know that natural immunity only lasts 6 months and vaccination immunity lasts like 8 months (for Pfizer at least. Moderna probably a little bit longer, unsure how much longer though). Either way, you need a round to re-up your immune system. A lot of these Delta cases are "breakthroughs", in vaccinated populations... or reinfections (ie: in already infected populations) For example: South Dakota was someth…
It's my understanding we only measure antibodies, which hang around after an infection for a while. We still have "the pattern" to create more once our immune system learns how to beat something.
https://pubmed.ncbi.nlm.nih.gov/33909850/
https://www.thelancet.com/article/S0140-6736(21)00183-5/full...
B.1.617.2 (aka Delta) also has shown properties showing that it evades natural immunity after 180-days, similar to P.1 / Gamma.
The "antibody measurement" studies are trying to narrow down the exact time that our immunity wanes. But it is a well-proven fact that natural immunity lasts less than a year, given what we've seen in Araraquara, Brazil and Manaus, Brazil.
It seems to be ~180 days, subject to more tests. So that's the number I'm going to assume until I see a better study.