Biotech Values

[DewDiligence]

Why so high [90%] a threshold for herd immunity? I understand it may take a while to die out even after herd immunity is reached.

You answered your own question. Immune prevalence at a considerably lower level than 90% would likely be sufficient to stop a de novo outbreak of COVID-19. However, for an existing outbreak, “herd immunity” merely means that the outbreak stops growing, not that it comes to a screeching halt.