The idea is straightforward: transmission of a virus depends on a non-immune person bumping into an actively infected person. With more and more vaccination, the likelihood that a non-immune person will come in contact with an infected person is progressively reduced until — poof — the risk of catching the infection is almost gone (though never zero).
In other words, susceptible cows should be culled to lessen the risk of new infections. If the entire herd is immune, then livestock-threatening disease is prevented. Though, of course, the fix — culling — is not an option for human disease, the benefit of an immune herd is self evident.
Fast forward to the 21st century world of vaccines. Pandemics and health care are decidedly more complex, which has led all to wonder: what is the magic number of people we need to vaccinate so we can all forget these disastrous last 14 months?
Here’s the problem: there is no answer. This is not a fund-raiser with a fixed universal goal we all are striving to reach. The above equation evaluates the nation as a homogenized entity, but people live in communities. And things vary community to community. For example, in one locality, people may move around a lot — by car or bus or by walking. Each has an impact on risk of transmission (and therefore the number of immune people needed to protect the rest of the “herd”). In another community, the mobility may be lower.
And what about the weather? For example, San Diego and Minneapolis offer very different climates for viral growth, yet national herd immunity will be rolled into a single number.
Stated more simply, the herd likely is protected at a very different percent of vaccinated people in an Orthodox Jewish community in San Diego where people live near the school and walk to most activities compared to a gated community in a Minneapolis suburb where many prefer to keep to themselves.
So, on our wearying national journey down the yellow brick road to the Oz of an unrestricted life, we need to be realistic. There is not one magic number to signal to the entire country that we have finally made it; no moment of national celebration as the golden spike is hammered in to complete the Transcontinental Railroad. My herd’s number will never be your herd’s number.
This is extremely important to keep in mind in the weeks and months ahead as we continue to vaccinate and wait and vaccinate and wait, chasing a number that is fundamentally misleading. Such information simply will not help the person trying to make an informed decision about whether to go to a restaurant tonight.
As we have seen in the US during the 15-month arc of the pandemic, trust in science and scientists has been the key to progress. Masks work. Vaccines work. Certain medications work. Setting ourselves up for a giant belly flop by forcing public health leaders to produce a magic number will not only make sober experts look like fools, it will further undermine people’s faith in science, thereby slowing our emergence from the long shadow of Covid-19.