The idea of ”herd immunity” against Covid-19 has reached an almost magical status in the popular imagination. Once we hit that threshold, many Americans think, we will be clear and the pandemic will finally disappear into history.
But it’s unlikely we’ll ever achieve herd immunity with Covid-19 – it’s not how this nightmare will end. While the number of cases is now declining from their winter peak, we fear a new spike in potential super spreader events after Spring Break, Easter Weekend, Remembrance Day and the Fourth of July, or even again after the end of the year holidays. The time to redouble our efforts to stamp out the broadcast is now. We need to develop what amounts to a national immune system to quickly detect and fend off the coming outbreaks, 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 less than one new case on average. The virus, which finds insufficient numbers of susceptible people to infect, then begins to die out.
The herd immunity threshold depends on the infectivity of a particular disease. For Covid-19, the best estimates suggest that at least 80% of people should be immune.
At the time of writing, 130 million doses of vaccine have been given in the US, leaving 46.4 million Americans fully immunized and 33 million partially immunized while waiting for a second dose. In addition, about 30 million cases of Covid have been reported. Epidemiologists from the CDC and NIH estimate that perhaps an equal number of cases, about 30 million, have not been reported.