Public health policies, public perception potentially affected as terminology changes.
In a shocking reversal that’s akin to redefining reality, the World Health Organization has changed its definition of herd immunity. Herd immunity occurs when enough people acquire immunity to an infectious disease such that it can no longer spread widely in the community.
When the number susceptible is low enough to prevent epidemic growth, herd immunity is said to have been reached. Prior to the introduction of vaccines, all herd immunity was achieved via exposure to and recovery from an infectious disease.
Eventually, as vaccination became widespread, the concept of herd immunity evolved to include not only the naturally acquired immunity that comes from prior illness, but also the temporary acquired immunity that can occur after vaccination.
Redefining Herd Immunity
In June 2020, WHO’s definition of herd immunity, posted on one of their COVID-19 Q&A pages, was in line with the widely accepted concept that has been the standard for infectious diseases for decades. Here’s what it originally said, courtesy of the Internet Archive’s Wayback machine:
“Herd immunity is the indirect protection from an infectious disease that happens when a population is immune either through vaccination or immunity developed through previous infection.”