What It Takes to Get to Herd Immunity
The simple (and not-so-simple) math behind one of the biggest questions surrounding Covid-19
How many need to catch Covid-19 before herd immunity is achieved? It’s complicated.
Photographer: Bruce Bennett/Getty Images North AmericaThe term “herd immunity” appears to have first been used in its modern sense in a December 1916 article in the Journal of the American Veterinary Medical Association with the curious title, “The Present Status of the Abortion Question.”2
The abortion in question was an infection causing cattle to give birth prematurely to stillborn or ailing calves. U.S. Department of Agriculture researchers Adolph Eichhorn and George Potter observed “there is a constant tendency for the disease to die in an infected herd,” which they attributed to acquired immunity.1 To take advantage of this “herd immunity,” they advised, cows that contracted the disease should be returned to the herd after an isolation period because in most cases they were able to give birth successfully the next time around, and “the animals which have required a resistance are more valuable, in an infected herd, than newly introduced, susceptible animals.”