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Tangent: this study estimates the natural inoculum at only a few thousand infectious viruses. https://stm.sciencemag.org/content/12/573/eabe2555 Other estimates using different methods have landed in the same ballpark. https://www.medrxiv.org/content/10.1101/2020.10.21.20216895v1 H/t @dylanhmorris
Couple that with studies that estimate the rate of symptomatic re-infection to be very low. This despite huge variation across people in the magnitude of the immune response. Many people mount wimpy responses, yet it still seems to be protective. https://www.medrxiv.org/content/10.1101/2020.11.18.20234369v1
The highest profile recent evidence came from Pfizer’s first dose. There appears to be protection after ~10 days. Based on their phase 2 data, the mean nAb titers at this point are only ~15. That’s pretty low. https://www.nejm.org/doi/full/10.1056/NEJMoa2034577
Compare that to something super-contagious like measles, where we think we need nAb levels >100 to protect against symptoms, and maybe >1000 to protect against all infections. https://pubmed.ncbi.nlm.nih.gov/2230231/
After the second Pfizer dose, the nAb titers go up 10-20-fold ( https://www.nejm.org/doi/full/10.1056/NEJMoa2027906). That leaves a lot of room for decline while still maintaining protection. Even if every antibody-producing cell died en masse, all at once, the
half-life of IgG alone would keep you immune for months. But of course this mass extinction doesn't happen in this way (part 2 sometime next week). In fact, the Moderna titers through 4 months at least look pretty good: https://www.nejm.org/doi/full/10.1056/NEJMc2032195
Put it all together, and I would be surprised if, in the typical Pfizer/Moderna vaccine response, antibodies fell below the protective threshold in <1 year. For the others where we have less data, I'll speculate more based on the cell biology in part 2 prob. next week.
