Italy is showing a continued decline in the rate of deaths from COVID-19, presumably that also means a decline in viral activity.
Some predict the pandemic will meet decline at herd immunity. With a basic reproduction number (
https://en.wikipedia.org/wiki/Basic_reproduction_number) of 3 -- I see numbers between three and four -- that would mean at more than 66% immune, viral activity begins to decrease because there becomes a better and better chance that the average of three who have contact with an infected person are immune. Italy is seeing decline far below this level, enough that I want to say herd immunity isn't the factor, it is more changing behavior.
Herd immunity pretty much has to work to send a virus into decline, if it comes to that. The question on my mind is whether pandemics typically come to that. The Spanish flu is said to have infected 1/4, which would mean viral activity was probably in decline at 1/8 or so of the population (if for simplicity, I assume a symmetric curve), so a reproduction number of 1.14 -- pretty low. Or sustained changes in behavior brought the basic reproduction number down before herd immunity had any significant input.
My question is, given lockdowns and isolations are hard to sustain, especially as the virus declines in a country or state and these measure seem less necessary, and a vaccine only possible later in 2021, is herd immunity still a reasonable prediction for the final end of COVID-19?
