The term herd immunity is primarily used in infectious-disease modeling, and it refers to the point at which enough people have immunity such that disease transmission can’t be sustained. But the concept is most useful in closed populations, such as a ship, a nursing home or a herd of cows, and it has little relevance in the real world, which is far more interconnected. There is no case in which herd immunity has been achieved through natural infections aside from select, contained circumstances, yet it has become an outsize talking point in public discourse for the coronavirus.
The vaccine-induced herd immunity threshold for covid-19 is likely north of 80 percent when accounting for new, more transmissible variants. But that threshold is continuously changing depending on variables such as population density, human movement, long-term vaccine efficacy, prevalent variants and the extent of non-pharmaceutical interventions, including masking and distancing. Ascribing a single number to success might create a false goal post for the pandemic.
Ultimately, achieving herd immunity is a matter of numbers and can become a distraction from what people actually care about. For many people, it’s more important to see loved ones without worry or to live without having to wear masks in public spaces. Rather than herd immunity, we must focus our efforts on building community protections through vaccinations and public health interventions to help stamp out transmission chains and prevent ongoing infections.
Even if herd immunity is achieved through vaccinations, it would likely take decades to eradicate the virus, as was the case with polio. Far more likely is that immunity would fail over time, as was the case with pertussis, or that vaccine hesitancy would cause a resurgence in the disease, such as with measles. Already 1 in 5 Americans has said they would never get a covid-19 vaccine, and vaccine-generated immunity is unlikely to be permanent.
Nor are vaccines perfect. Too many parts of the world remain dangerously susceptible, meaning the virus has ample opportunities to replicate and mutate. In addition, we have limited understanding of the role of animal reservoirs. These factors alone argue against herd immunity as an attainable goal. Even if communities achieve herd immunity for a while, this protection would wane as immunity weakens over time or as non-immune people are born or move into that community.
Nevertheless, if we keep vaccinating, the future could look different from the past. Neither covid-19 elimination nor eradication is needed for Americans to live in a world that is no longer dominated by the virus. To achieve this, we must understand how the virus continues to spread at the community level in the United States and address this in locally led public health efforts. We must also ensure that catastrophic epidemics such as the one we are seeing in South Asia do not recur. We must vaccinate as much of the world as possible as quickly as possible; the recent TRIPS waiver supported by the U.S. government is one step toward global covid-19 control, but globally, we still require technology transfers, provision of essential supplies and serious scaling up of vaccines in countries that have not yet seen adequate access.
As of today, dozens of countries have seen no vaccines. Not only is this morally unacceptable, but it also leaves everyone vulnerable to recurrent epidemics, as was seen Manaus, Brazil, which suffered a tsunami of disease even after an estimated 76 percent of the population had been exposed. In the absence of a vaccine, waning immunity and new variants are a double whammy.
Among the many important lessons that covid-19 has taught us is that we are intimately connected with those across seas and borders. This is sometimes easy to forget in the United States, where there have been far fewer serious infectious-disease outbreaks than in many other parts of the globe. This virus has humbled us, and we must continue to respect it even while we commit our best efforts to control it. But we don’t need to pray to the false god of herd immunity to do so.