This is a follow-up on an earlier question regarding mechanistics of vaccines: why current vaccines don't block transmission. In summary, they don't engage mucosal immune cells to produce IgA. Thus, they induce production of IgG by systemic B cells and can lower viral load, but not block viral shedding or uptake through mucous membranes. "Herd immunity" is dependent on halting transmission. So, the AstraZeneca trials were the only ones that assessed sterilizing immunity. Their surrogate for transmission was asymptomatic cases. They found that their LD/SD regimen was about 60% effective in blocking new asymptomatic cases. My first question, then: was this simply due to a reduced viral load (and reduced coughing perhaps) which was expressed as a lower transmission rate? Can this even be equated with measles-type herd immunity? My second question is would this 60% effectiveness at blocking asymptomatics on the basis of reduced viral load be sufficient to achieve herd immunity?

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