Herd immunity is the indirect protection from a contagious infectious disease that happens when a population is immune either through vaccination or immunity developed through previous infection.
When enough of the population is resistant to a germ, its spread stops naturally because not enough people are able to transmit it. Thus, the “herd” is immune, even though many individuals within it still are not.
Vaccines create herd immunity too, either when given widely or sometimes when administered in a “ring” around a new case of rare infection. That’s how diseases like smallpox were eradicated and why polio is close to being erased. Various vaccine efforts are under way for this coronavirus, but they may not be ready for more than a year.
The coronavirus is new, so it doesn’t appear that anyone is immune to it: that’s what lets it spread and why it can have such severe effects in some people.
For herd immunity to take hold, people must become resistant after they are infected.
The current germ’s rate of spread is higher than that of the ordinary flu, but similar to that of novel emergent influenzas that have occasionally swept the globe before.
RISKS TO HERD IMMUNITY
Mass vaccination has been highly successful in inducing herd immunity for many diseases, protecting those that are unable to build up immunity, such as people with immune deficiencies or whose immune systems are being suppressed for medical reasons.
When herd immunity is well established, however, some people choose to behave as ‘free riders’, essentially benefitting from everyone else getting vaccinated, while abstaining from vaccination either because they choose not to or are actively anti-vaccination.
When a population has too many of these free riders, the overall immunity level is compromised and herd immunity can be lost, putting everyone at risk.