Is the COVID-19 Herd Immunity Threshold as Low as 15 Percent?
Since the first case of community transmission in the U.S. of the COVID-19 coronavirus was reported seven months ago, more than 7 million Americans, that is, around 2.2 percent of our population, have been diagnosed with the disease according to the Centers for Disease Control and Prevention (CDC). However, researchers estimate that undetected COVID-19 cases are likely to be 10 times greater than diagnosed cases. The machine learning epidemic model run by Youyang Gu and his colleagues estimate that around 16 percent of Americans have already been infected with the COVID-19 coronavirus.
That could be good news if a brief preprint study published yesterday by a team of Scottish researchers that calculates a COVID-19 herd immunity threshold of 15 percent pans out. The Scottish researchers achieve their result by modifying the epidemiological model in a June study in Nature that found that non-pharmaceutical interventions, specifically lockdowns, had averted millions of COVID-19 deaths in Europe.
That Nature study estimated the initial reproduction number (Rt) for the virus to 3.8, that is, each infected person was likely to transmit the virus to 3.8 other people. The classical formula for calculating a herd immunity threshold is 1 minus 1/Rt, which, in this case, would suggest a threshold of nearly 75 percent of the population that would have to be infected or vaccinated. The article also assumed that everyone was equally likely to become infected (homogeneity) and that about 1.1 people out of every 100 infected would die (infection fatality rate or IFR) of the disease.
In their analysis, the Scottish researchers relax the assumpt
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