Many of you have heard the term “herd immunity”, which was initially introduced about a century ago with the expansion of immunization programs. It is not a new theory or concept. Basically, a herd occurs when a significant portion of a population becomes immune to infectious disease, and the spread from person to person decreases. Herd immunity prevents epidemics. However, the percentage of people in a population needed to create herd immunity varies by disease and, specifically, the infectiousness or transmissibility of the disease. For example, measles is highly infectious, and 95% of the population needs to be immune in order to stop an epidemic. For our current pandemic with COVID 19, it is estimated that 50-67% of the population needs to be immune to prevent epidemic spread. In the U.S. population, this translates to about 198 million people.
The population may develop herd immunity naturally with people who have had the disease and developed natural immunity or by vaccination. In either case, it is important to know about the durability or duration of immunity. If immunity wanes, and the population drops below the herd immunity threshold, epidemics can again occur. Understanding this is important because we still are not sure if natural immunity continues and for how long, and the same is true about immunity induced by vaccination.
Vaccines should allow us to control the current pandemic, but what they will do in the future is yet to be determined. Continued study and surveillance will be needed.