Image of a longhorn cow laying on the grass, looking straight at the camera. There is another smaller cow behind it, and there is a fence behind the two of them.
“Let me tell ya about herd immunity, pardner…”

What do you mean by “Herd Immunity”?

Herd Immunity or Community Immunity has been touted as the thing that will end the pandemic. But, with several populations reaching the herd immunity threshold, why are we still in this mess?

René F. Najera, MPH, DrPH
6 min readOct 22, 2021


According to the Gospel of Luke, Jesus was asked about loving one’s neighbor with a simple question, “Who is my neighbor?” Jesus answered with the parable of the Good Samaritan. It’s a good parable, even when you strip away all the religious context, it is a story that can teach us a lot about how we deal with people who are not like us and are in trouble. It answers the question by basically telling us that our neighbors are anyone we come across, from any walk of life, who need our help.

The parable speaks of a man who is on his way to Jericho, gets attacked and robbed, and is left for dead. A couple of people go by and see the man — one of them being a priest — and ignore or avoid the man. Another man, a Samaritan, stops and renders aid to the man, taking the man to an inn and paying for the man to stay there while he recovers. This is how we now arrive at the concept of a “Good Samaritan,” someone willing to render aid without being asked and without obligation to do so.

Vincent D’Onofrio, as Marvel Comics’ villain Wilson Fisk, delivers the best telling of the Good Samaritan story, by the way:

At the beginning of the pandemic, we epidemiologists were bombarded with questions about herd immunity. What is the threshold of people who must be immune to the novel coronavirus causing COVID-19 for herd immunity to kick in? How do we get to it? With letting everyone catch it and manage the effects? With vaccines? A combination?

Yet, no one really asked the one important question in all of this: Who is your community? Who makes up your “herd”?

The formula for the threshold on herd immunity is simple enough: Take the basic reproductive number of a pathogen, divide…



René F. Najera, MPH, DrPH

DrPH in Epidemiology. Associate/JHBSPH. Adjunct/GMU. Epidemiologist. Father. Husband. (He/Him/His/El)