What’s the Herd Immunity Threshold for the COVID-19 Coronavirus?

“Herd immunity is the resistance to the spread of a contagious disease that results if a sufficiently high proportion of a population is immune to the illness. Some people are still susceptible, but they are surrounded by immune individuals, who serve as a barrier preventing the microbes from reaching them. You can achieve this through either mass infection or mass vaccination.”

“Most the evidence so far suggests that people who recover from a COVID-19 coronavirus infection do, at least for a time, develop immunity to the microbe. If that’s true, what is the disease-induced herd immunity threshold for the COVID-19 coronavirus? Various epidemiologists offer different answers, depending upon their estimates for the disease’s R0 and other variables, but most have converged on a threshold at around 60 to 70 percent.

More recently, some researchers have suggested that this threshold may be too high. In a new preprint, three mathematicians from Sweden and the United Kingdom, using an R0 of 2.5, calculate a reduction in the herd immunity threshold from 60 percent to 43 percent by incorporating some assumptions with respect to populations’ social activity levels and age structures.A couple of new reports speculatively lower the possible herd immunity threshold for the coronavirus to just 10 to 20 percent of the population. This conjecture depends chiefly on assumptions about just how susceptible and connected members of the herd are.” 

“There are no solid estimates for the percentage of the U.S. population that has already been infected by the coronavirus, but Youyang Gu and his team at COVID19-Projections estimate that right now the number is between 2.2 to 4.7 percent. That would mean that somewhere between 7.3 and 15.5 million Americans have been infected. A similar result emerges from a very rough calculation that multiplies the number of confirmed cases at 1.4 million by a 10-fold factor of undiagnosed cases and infections. (The 10-fold factor is derived from data recently reported by Indiana University researchers.)

The upshot: The U.S. as a whole is not yet close to achieving even the speculatively low estimate of the herd immunity threshold.”


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