“A man has got to know his limitations.” That is one of the memorable phrases uttered by “Dirty Harry” Callahan, the fictional police inspector played by Clint Eastwood. Today, the world is learning its limitations in the ongoing COVID-19 pandemic, and the most important lesson is that a key strategy we banked on to defeat the virus — herd immunity — appears unobtainable.
Herd immunity is reached when enough people in a community acquire immunity to a disease, either through infection or vaccination or both, that it makes sustained transmission impossible and protects even those who are not immune. The strategy of mass vaccination campaigns to achieve herd immunity has worked well for childhood diseases such as diphtheria, which has essentially disappeared, and for other once-common diseases such as smallpox and polio. (The first U.S. case of polio in a decade was recently reported, thought to be brought in from outside the country.)
The percentage of people who must be immune to stop the spread of a disease depends primarily on how contagious the disease is. The commonly cited example is measles, which is extremely contagious and requires immunity in 90% to 95% of the population to halt transmission. In the past few years, measles outbreaks have occurred in the U.S. despite greater than 80% of the local population being vaccinated.
