Published reports have indicated that the White House has officially embraced — as policy — the “strategy” of herd immunity to combat the SARS CoV-2 pandemic. While most public health experts have roundly condemned this approach, it may still be unclear as to what exactly this term “herd immunity” means. Researchers from Yale University have done an excellent job outlining what “herd immunity” is all about.
What is “herd immunity”?
The precise scientific term is “indirect protection” or “community immunity.” This is the protection against infection of a susceptible group of individuals when a sufficient proportion of the larger population is immune from the infection. When there are enough immune individuals, then sustained transmission is interrupted.
How many people need to get infected or vaccinated to achieve “herd immunity”?
This is not arbitrary. There is actually a formula, and it is based on the R0, or “R-naught,” which is the average number of persons that get infected by one infected person. The formula for the threshold of “herd immunity” is:
Studies estimate that the R0 for SARS CoV-2 is between 2 and 3. Doing the math then, the estimated threshold of individuals that will need to get infected to achieve “herd immunity” is 50–67% of the population. Studies have estimated that less than 10% of the U.S. population has been infected with SARS CoV-2.
Do the math
In order to achieve “herd immunity” in the United States, between 165,000,000 and 221,100,000 people need to get infected by SARS CoV-2, according to the formula above. Assuming a case fatality rate of 0.5%, if we let the infection rip through society to achieve “herd immunity,” between 825,000 and 1,105,500 people will die. As of this writing, 219,499 have died. So, for those embracing “herd immunity,” it seems that they are OK with orders of magnitude more people dying.
Caveats to “herd immunity”
All of the above assumes that immunity to SARS CoV-2 is long-lasting. We don’t know that. All of the above also assumes that people interact with each other in a random fashion. That does not happen in America, given the vastness of our country and the variable population density. So, it will take a lot longer time — with much more death and destruction — to achieve “herd immunity.”
What about Sweden?
Ah yes, Sweden. The authors addressed the case of Sweden and its approach to the pandemic:
There are only rare instances of seemingly sustained herd immunity being achieved through infection. The most recent and well-documented example relates to Zika in Salvador, Brazil. Early in the COVID-19 pandemic, as other countries in Europe were locking down in late February and early March of 2020, Sweden made a decision against lockdown. Initially, some local authorities and journalists described this as the herd immunity strategy: Sweden would do its best to protect the most vulnerable, but otherwise aim to see sufficient numbers of citizens become infected with the goal of achieving true infection-based herd immunity. By late March 2020, Sweden abandoned this strategy in favor of active interventions; most universities and high schools were closed to students, travel restrictions were put in place, work from home was encouraged, and bans on groups of more than 50 individuals were enacted.
And you know how much of the population of Stockholm got infected with SARS CoV-2? It was estimated to be 8% in April 2020.
I am not arguing that we should completely lock down the country, and I recognize that this is an incredibly difficult situation. What would you choose: dying from Covid-19 or dying from starvation? My recent discussion about lockdowns further emphasizes this point.
That said, it bears repeating that this “strategy” — apparently embraced by the White House — of letting SARS CoV-2 freely propagate through the population to achieve “herd immunity” is a path lined with death, destruction, and misery. Do not let anyone convince you otherwise.