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The idea of “collective immunity” against Covid-19 has reached an almost magical status in the popular imagination. Once we reach that threshold, many Americans believe, we will be in the clear, and the pandemic will eventually fade into history.
But we’re unlikely to ever achieve collective immunity with Covid-19 – that’s not how this nightmare will end. While the number of cases is now declining from their winter peak, we fear another potential spike of potential super-spreader events after spring break, Easter weekend, Memorial Day, and July 4, or even again after the end of year holidays. Now is the time to redouble our efforts to eliminate transmission. We must develop what amounts to a national immune system to quickly detect and repel new outbreaks to come, not only for this pandemic, but also for future ones.
Collective immunity is achieved when the percentage of a given population that is immune to vaccination or a previous infection becomes such that each infected person transmits the disease to an average of less than one new case. The virus, finding an insufficient number of people likely to be infected, then begins to die off.
The collective immunity threshold depends on the contagiousness of a given disease. For Covid-19, the best estimates suggest that at least 80% of people should be immunized.
As of this writing, 130 million doses of the vaccine have been administered in the United States, leaving 46.4 million Americans fully immune and 33 million partially immune pending a second dose. In addition, around 30 million cases of Covid have been reported. CDC and NIH epidemiologists estimate that perhaps an equal number of cases, some 30 million, have gone unreported.
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