In the original description of herd immunity, the protection to the population at large occurred only if people contracted the infections naturally. The reason for this is that naturally-acquired immunity lasts for a lifetime. The vaccine proponents quickly latched onto this concept and applied it to vaccine-induced immunity. But, there was one major problem – vaccine-induced immunity lasted for only a relatively short period, from 2 to 10 years at most, and then this applies only to humoral immunity. This is why they began, silently, to suggest boosters for most vaccines, even the common childhood infections such as chickenpox, measles, mumps, and rubella.