The idea of “herd immunity” against Covid-19 has achieved almost magical status in the popular imagination. Once we reach that threshold, many Americans believe, we’ll be in the clear, and the pandemic will finally fade into history.
But we are unlikely ever to reach herd immunity with Covid-19—it’s not how this nightmare will end. Although case counts are now declining from their winter peak, we fear another spike from potential super-spreader events following spring break, Easter weekend, Memorial Day and the Fourth of July, or even again after the end-of-year holidays. The time to double down on our efforts to stamp out transmission is now. We must develop what amounts to a national immune system to quickly detect and repel the new outbreaks ahead, not just for this pandemic but for future ones as well.
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, finding inadequate numbers of susceptible people to infect, then starts to die out.
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 would need to be immune.
As of this writing, 130 million doses of vaccine have been given in the U.S., leaving 46.4 million Americans fully immunized and 33 million partially immunized as they await a second dose. In addition, there have been about 30 million reported cases of Covid. Epidemiologists at the CDC and NIH estimate that perhaps an equal number of cases, some 30 million, have gone unreported.
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