Herd Immunity to Most and “Herd Mentality” to Mr. Trump

When being interviewed by George Stephanopoulos as to the ravages of the covid-19 pandemic on September 20th , 2020,, Donald Trump contributed the malapropism “herd mentality” to the pandemic lexicon! “It would go away without the vaccine…but it’s going to go away a lot faster with it. You’ll develop like a herd mentality.” Of course, he intended to invoke herd immunity, a controversial and complex concept. Looking for some way out of the colossal federal mismanagement of the epidemic, he invoked this path to a solution. He neglected to mention that 222,000 Americans have died from the disease (as of October 20, 2020) and left to a natural path to achieve a guestimate of what herd immunity requires, infecting 70 percent of Americans, the total deaths would be estimated at approximately seven times that number, not counting the profound morbidity of many survivors and the costs to the affected families, persons on the financial edge, the economy, our healthcare system, and our way of life.

In the case of covid-19, a severe respiratory viral infection (but with a wide array of multiorgan involvement) the most informed estimate given as the requirement to reach herd immunity is 70 percent of the population infected. In the United States, this goal means 0.7 times 330 million persons or approximately 230 million persons. This estimate is an educated guess based on the sense of what fraction of the population would have to be immune so as to reduce susceptible persons sufficiently to dramatically limit the spread of the infection. In the case of measles, for example, a very highly contagious infectious disease, the estimate is closer to 95 percent of the population.

So where are we and where do we have to go. Well, if we accept a 70 percent target, than that represents 230 million Americans who would have to develop immunity to the virus, SARS-CoV-2, either through natural infection or immunization with a vaccine. In the case of natural infection, the reported total as of mid-October 2020 is 8 million Americans who have been reported as infected based on testing. The CDC studied a sample of the U.S. population and examined the frequency of anti-SARS-CoV- 2 antibodies and found 9 percent of the population had been exposed and developed antibodies. This finding represented nearly four times the number reported as infected by testing or a total of 30 million Americans ostensibly immune. This number is what happened in approximately 9 months of natural spread, 30 million or 9 percent of the population of the United States infected. So, if matters progress in the next 9 months (November 2020 to July 2021) as they did in the prior 9 months (mid-January to mid- October 2020), natural spread would have us at approximately 60 million infected or a bit below 20 percent of the population. Still a wide distance from the 70 percent needed for herd immunity.

So what about the additional effect of a vaccine? If 60 million had acquired a natural infection by June 2021, to get to 70 percent of the population immune to the virus, presumptive herd immunity, one would have to successfully immunize 230-60 or approximately 170 million Americans. But, there are two large impediments to reaching that goal: the fraction of the 170 million willing to be vaccinated and the effectiveness of the vaccine. In the case of influenza, less than 50 percent of the adult population take the vaccine. If we argue that this infection may be more compelling and we get 50 percent of the non- immune or 270 (330-60) million to take the vaccine, that would add 135 million to the 60 million immune by infection or 195 million immune Americans. Vaccines for respiratory infections are rarely if ever over 70 percent effective. The FDA has agreed to license a “safe and effective” vaccine if it provides 50 percent protection. But if the vaccine is only 50 percent effective that would reduce the 135 million to 67.5 million. Thus, only 60 million naturally immune and 67.5 made immune be vaccination would result in 127.5 million immune, far from the 270 million required for herd immunity. An additional scenario with slightly more optimistic assumptions (65% effectiveness of the vaccine) is shown in the Table.

We can hope more persons will take the vaccine than projected and that the vaccine has a higher effectiveness, getting us closer to 70 percent of the population having immunity. This goal is, however, a stretch with some daunting impediments. That is why the race to finding therapies is so important, including (i) more effective antiviral compounds that have a low toxicity, (ii) the application of passive immunity, especially engineered potent monoclonal or polyclonal antibody formulations that can be administered very early after diagnosis to reduce severity of the infection and the frequency of its complications and (iii) novel approaches, such as engineered decoy receptors that when injected interact with the virus and prevent it from attaching to actual cell receptors and entering and infecting cells.

A vaccine would, however, be of enormous benefit. Children under 10 years old are very uncommonly seriously affected. By providing immunity to an increased fraction of 18-65 year olds, school openings would be facilitated and economic activity could be increased. If the vaccine provided immunity in those over 65, life style would be improved at much lower risk for seniors. So, one can look at the approach to population immunity quantitatively as in this fictitious example: >30% immune, better that <30, >50% immune, very good, > 70% immune, outstanding. The mathematical statistics surrounding the guestimate of what fraction of the population needs to be immune to achieve herd immunity are complex and controversial, with figures lower than 70% being espoused by some. In addition, some epidemiologists believe the analysis of spread is complicated if one wants to include super-spreader events in addition to random interactions.

The use of herd mentality by Trump, indicates his unfamiliarity with the details surrounding this virulent, highly contagious, and lethal infection. We are at war with this virus. We need a commander-in-chief who is using all of our weapons to subdue it. A lot is at stake. Hundreds of thousands of lives, colossal disruption of education, the economy, social intercourse, travel, international relationships and more. Our way of life has been severely disrupted. We need someone at the top who is smart, wants to learn more and get his head into it, enlists experts and uses their best ideas to formulate policy, develops a national plan for behavior, testing, tracing, and careful reopening, pours in federal resources in the right places to deal with the epidemic, lead the governors in introducing critical control measures in their states, be an example for all the country on safe behavior, show an interest in controlling this epidemic and showing empathy for those who have suffered from it. The use of the term herd mentality was with a tone that was dismissive of the problem. The federal response led by Trump has been an abject failure.  

Written October 2020