Viewpoint: Have You Heard About the Herd? It’s a COVID-19 Fallacy

April 29, 2020
Kevin Kavanagh, MD

Some are advocating for the sole use of herd immunity to stop the COVID-19 epidemic. Some even use this logic to justify the early opening of the economy. But what they fail to realize is that not all virus epidemics burn themselves out. Polio plagued mankind for an eternity or until a vaccine was developed.

The COVID-19 epidemic caught the United States, unprepared and the medical community by surprise. Early in January 2020, the public and healthcare leaders were under the understanding that there was not any person-to-person spread.

However, the Washington Postreported that Peter Navarro wrote a memo on January 30, which warned “…that Chinese reports indicated the virus was likely far more contagious than the flu, and more like the bubonic plague or smallpox.” The contagiousness of infections is described by the R0 (pronounced R naught). This is the number of people which are expected to be infected from each person with the disease. The seasonal flu has a R0 of approximately 1.3or three people with the flu will infect a total of 4 individuals. The bubonic plague has an R0 of approximately 3and smallpox between 5 to 73.

On March 20, 2020 at the presidential briefing, Vice President Mike Pence stated that the COVID-19 virus was three times as infectious as the flu which would place the R0 at approximately 4.

On April 7, 2020 a prepublication manuscript from the Los Alamos National Laboratory calculated a median R0 of 5.7 for the COVID-19 virus.4

Mitigation strategies can decrease a contagion’s R0. To decrease the total number of individuals with active infections, the R0 will have to be less than one. So, on average, each infected person spreads the virus to less than one person. When this happens, the epidemic will eventually burn out.

Herd immunity strategies rely on a significant portion of the population to become immune to stop the spread of the virus. The higher the R0, the larger the percentage of the population who must become immune before the total number of those with active infections decreases and the epidemic burns out. An R0 of 5 to 7 will require 80% to 85% of the population to become immune before the number of infected will start to decrease. Thus, it can be argued that without mitigation, the R0 will be far too high to be able to stop this epidemic with herd immunity. Almost all of the population would have to contract the infection before the epidemic would stop, which probably will not happen. The availability of a vaccine to prevent infections with pathogens having a high R0 is of utmost importance. 

Other concerns are that immunity to coronaviruses which cause the common cold only lasts between 1 to 2 yearsand there is mounting evidence that cats and dogs may become infected, giving concerns of an animal host.

Thus, without a vaccine, this virus will be with us for some time. Herd immunity may slow the expanse of the epidemic, but it is not likely to meaningfully stop it without mitigation strategies. And there is significant concern that those who currently recover could be re-infected in the near future.

Strategies of social distancing, not gathering in large crowds, wearing masks and protecting those at increased risk are required to manage this epidemic.Returning to life as usual with the dependence on herd immunity in a non-vaccinated population to control spread is almost certainly a false narrative.


(1)    Pyrek KM. 100 Years after the Spanish Flu: Lessons Learned and Challenges for the Future.  Infection Control Today.  Oct. 11, 2018.    

(2)    Didelot X, Whittles LK, Hall I.  Model-based analysis of an outbreak of bubonic plague in Cairo in 1801.   J R Soc Interface. 2017 Jun;14(131). pii: 20170160. doi: 10.1098/rsif.2017.0160.     

(3)    Helft L, Willingham E.  What is Herd Immunity?  Nova.  Sept. 5, 2015  

(4)    Sanche S, Lin YT, Xu C, et al.  High Contagiousness and Rapid Spread of Severe Acute Respiratory Syndrome Coronavirus 2.  Emerging Infectious Disease.  Centers for Disease Control and Prevention.  Vol 26. No. 7.  July 2020.   


(5)     Tyrrell D, Myint SH.  Chapter 60 Coronaviruses.  Medical Microbiology. 4th edition.