The idea of “herd immunity” against Covid-19 has reached an almost magical state in the popular imagination. Once we reach that threshold, many Americans believe we will be in the clear and the pandemic will finally go down in history.
But we are unlikely to reach herd immunity with Covid-19; it’s not how this nightmare will end. While the case count is now declining from its winter high, we fear another rise in possible super-broadcast events after the spring break, Easter weekend, Memorial Day and July 4, or even again after the end-of-year vacation. Now is the time to redouble our efforts to eliminate transmission. We need to develop the equivalent of a national immune system to quickly detect and repel new outbreaks that arise, not only for this pandemic, but also for future ones.
Herd immunity is achieved when the percentage of a given population that is immune, from vaccination or previous infection, becomes such that each infected person transmits the disease to an average of less than one new case. The virus, when it finds an inadequate number of people susceptible to infection, begins to disappear.
The threshold for herd immunity depends on the contagiousness of a given disease. For Covid-19, the best estimates suggest that at least 80% of people should be immune.
As of this writing, 130 million doses of vaccine have been given in the United States, leaving 46.4 million Americans completely immunized and 33 million partially vaccinated while awaiting a second dose. In addition, there have been about 30 million reported cases of Covid. CDC and NIH epidemiologists estimate that perhaps an equal number of cases, about 30 million, have not been reported.