KYW Newsradio’s Medical Reports are sponsored by Independence Blue Cross.
By Dr. Brian McDonough, Medical Editor
PHILADELPHIA (KYW Newsradio) — The term “herd immunity” has been used when talking about how a vaccine can help control an infection. But what does it mean?
There are a couple of facts to know: there is no such thing as herd immunity, only herd protection, and the term is meant to be used in relation to a vaccine.
Epidemiologists can estimate the proportion of a population that needs to be immune before the herd impact occurs. This threshold depends on a term called "R0,” which is the number of cases on average, spawned by one infected individual in an unexposed population.
The formula for calculating the herd immunity threshold is 1–1/ R0, meaning that the more people who become infected by each individual who has the virus, the higher the proportion of the population that needs to be immune to reach herd immunity.
Using this calculation shows that natural herd immunity would cost the lives of three to four million people.