The COVID-19 Pandemic: Herd immunity—what is it and how do we achieve it?Feb 22, 2021
The underlying concept is straightforward; an epidemic will end when a population does not include enough susceptible individuals to support its perpetuation. Why “herd” immunity and not “population” immunity? The concept dates to a Kansas veterinarian, George Potter in the early 20th century. As quoted in The Lancet about epidemics of spontaneous abortion in cattle and sheep, Potter wrote: “Abortion disease may be likened to a fire, which, if new fuel is not constantly added, soon dies down. Herd immunity is developed, therefore by retaining the immune cows, raising the calves, and avoiding the introduction of foreign cattle.”
In its basic formulation, there is a deceptive simplicity in the mathematical formula for herd immunity. The critical threshold for achieving herd immunity (Vc) is estimated as 1-1/R0. R0 or “R naught” is the number of individuals newly infected by an infectious individual at the start of an epidemic. If R0 is below one, then the epidemic cannot propagate. For example, if the R0 is 3 (about the value for SARS-CoV-2), then transmission by each infectious individual must be to less than one individual so that the critical threshold is above 67% (2/3). In this basic formulation, there is the assumption that the intervention (vaccination and acquisition of immunity through infection) is completely efficacious. There is also an assumption of uniform mixing in the population; that is, each person has the same probability of being in contact with every other person.
Reality negates these assumptions. Current vaccines for SARS-CoV-2 are not 100% efficacious and our populations are heterogeneous, and some are isolated. Both of these factors raise the value of Vc that needs to be reached. Looking to maintaining herd immunity over the next few years, we also do not know how long immunity will last, whether from having been infected or from vaccination. Now two months into providing vaccination in the United States, we recognize some barriers: the complexities of vaccine delivery and insufficient supply at present, vaccine hesitancy that spreads unequally, and the potential of increasing frequency of higher-transmissibility variants.
The New York Times provides a user-friendly model for the timing of reaching herd immunity that allows for the variation of vaccine supply, presence of non-pharmaceutical interventions, and prevalence of variant strains; the source article is published in the American Journal of Preventive Medicine. In some higher vaccine delivery scenarios, the threshold range for herd immunity is reached across the summer.
What about Colorado? I am increasingly asked about the fall, as we look to the reopening of the CU Anschutz Medical Campus and the Colorado School of Public Health. Modeling suggests that herd immunity can be reached at the state level as the fall begins. Undoubtedly, many groups within the state will not be adequately protected and I anticipate that distancing and mask mandates will still be in place. Fortunately, Colorado’s epidemic curve continues to fall with hospitalizations for COVID-19 dropping below 400, a level exceeded since October 20, 2020. Vaccination, particularly of older Coloradans, has likely played a role in the decline.
The future of the pandemic in Colorado depends not only on its course here, but on what happens in the other states and globally. For example, cases are currently rising in the mountain resort communities and there are outbreaks among employees in the ski industry. These communities have out-of-state and international visitors. Our mixing is not just among Coloradans and we will be better protected as herd immunity is achieved widely. When will that happen? Chris Murray, Director of the Institute for Health Metrics and Evaluation (IHME), has opined with despair on the likelihood of achieving herd immunity, specifically in advance of a surge next winter. He cites barriers: vaccine hesitancy, vaccine efficacy, variant strains, and waning immunity from infection and vaccines. His summary with regard to achieving herd immunity: “But if it is possible, it nevertheless seems unlikely.” In a quote of uncertain provenance, “It is difficult to make predictions, particularly about the future.”
I also urge you to look at the front page of Sunday’s New York Times, half occupied by nearly 500,000 dots representing the almost half a million deaths from COVID-19 since the pandemic began. Let’s get to herd immunity soon, while maintaining the transmission control measures that have been working. We don’t want to see a front page filled by a million dots.
Hang in and await herd immunity,
Jonathan Samet, MD, MS
Dean, Colorado School of Public Health