July 14, 2020

Herd immunity. You hear it a lot these days, but what does it mean? Such a bucolic term–first used for livestock, then for lab mice, and then for us. How much does it actually apply to human beings in a pandemic?

Back in 1894, the United States Veterinary Association noted that different populations of swine had different levels of susceptibility when exposed to the same disease:

These facts show something besides individual immunity. They demonstrate the possibility of obtaining herd immunity. There is no doubt much still to learn about this subject, but it may be safely assumed that with hygienic surroundings, proper exercise, proper food, and by practicing the principles of breeding . . . much can be accomplished in developing animals with a more than ordinary power of resisting . . . diseases.

Proceedings of the Thirtieth Annual Convention of the United States Veterinary Medical Association and First Veterinary Congress of America. Philadelphia, Printed for the association. 1894.

By the 1920’s herd immunity had entered the lab as a concept related to, but separate from, individual immunity. Topley and Wilson at the University of Manchester introduced a varying proportion of immunized mice into a population of normal susceptible mice. They wanted to find out what proportion of immunized mice would confer protection on the group as a whole. Studies like these were used to develop immunization plans for vaccines.

Note that the livestock and the mice have something very important in common: standardization.

Herd immunity is a useful mathematical model for calculating how many people you need to vaccinate. It assumes a standard dose administered by a standard route. With cattle and pigs you also have a population with standard breeding, exercise, diet, and age.

With lab mice, the population is even more standardized genetically as well as for age, health, and diet.  Part of the test is to infect mice with a standard inoculum and introduce them into a mixed population of normal and immunized mice of known proportions. A variable number of infected mice are introduced to see whether and how much the infection spread to the healthy mice.

Calculation of herd immunity, Topley and Wilson, 1923

In a good experiment, you reduce the variables to the one you’re testing. Nature is much more messy. And for good reason. Variety is important for survival–of the herd, not the individual. In a real-life pandemic you have a random dose of exposure delivered by random routes to a random population. Plus, being human, we tend to care about herd immunity as a way of protecting ourselves and people we care about. In other words, we protect a population because we care about the individual.

The immunity you might get as an individual depends on:

  1. The viral load you’re exposed to and how it’s carried.
  2. How it enters your system. (nose, throat, eyes, blood, etc.)
  3. Your own immune system, which varies with age, health, nutrition, heredity, exercise, habits, stress, and so on.
  4. Environmental factors such as air quality, weather, hygiene, crowding.

Historically, herd immunity is a slow, unreliable, and temporary process at best. A new disease is introduced. It spreads quickly in an unexposed population.  Some people die. Some survive, but are weakened or disabled. Some recover. Some survivors develop immunity.

Let’s assume that immunity is robust and lasting. And that a large enough proportion of the population actually get the disease. Moreover, our community is an isolated population, with nobody coming and going.

The immune survivors struggle with a breakdown in society, the food supply, and the economy. The ones who get through this reproduce. But the virus is still around, in animals, in people, in the environment.  Within a few decades you have an aging population of resistant people and a younger unexposed population.

COVID-19 has been around for about six months. We don’t know if having it, with or without symptoms, confers a robust and lasting immunity. There is some evidence that it may not. That may depend on the four variables listed above. Or maybe it’s just the nature of the virus.

Fortunately, we do not have to rely on chance. The international projects in vaccine research sounds promising and specific effective therapeutics are in clinical trials. We have more understanding of epidemiology and prevention than our ancestors a hundred years ago.

I come from the generation that saw polio and smallpox eradicated.  That grew up in an America safe from diphtheria, measles, and mumps. My parents didn’t helplessly watch their children gasping for breath from whooping cough or dying from dysentery. But their parents did.   

On New Year’s Day of 1929, my father’s mother Lottie wrote a friend about the events of the previous April:

  That night little James (Stella’s baby, 14 mos) was taken very sick with dysentery (Saturday) he only lived until the following Friday. died in convulsions. Stella managed to take care of him all the way through but she was coming down with the same disease. Friday night little Pauline was taken sick (the 3 year old one) and became very bad and the next day Sarah the 4 year old came down. . . . .

You see in our family in about a week there was one in the cemetery, one in the undertaking parlor, one expected to go any minute and two in the hospital . . .

Letter from Lottie Woods to Elsie, Huntington Park, California. Jan. 1, 1929.

Excerpt from Lottie’s letter. Stella was her sister. Jan. 1, 1929.
Lottie and children, 1912

 Lottie died of tuberculosis at the age of 47. How different things were then! And how many more options we have now. 

Today’s Notable Headlines

“We’re wasting time talking about herd immunity,” CNN Opinion by William Haseltine, July 13, 2020. https://www.cnn.com/2020/07/13/opinions/herd-immunity-covid-19-uncomfortable-reality-haseltine/index.html

“The many estimates of the COVID-19 case fatality rate,” The Lancet: Infectious Diseases, July 1, 2020. https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(20)30244-9/fulltext

“A New Understanding of Herd Immunity,” The Atlantic, July 13, 2020. https://www.theatlantic.com/health/archive/2020/07/herd-immunity-coronavirus/614035/

“Masks offer much more protection against coronavirus than many think,” Los Angeles Times, July 14, 2020. https://www.latimes.com/california/story/2020-07-14/evidence-mounts-that-masks-help-lower-your-exposure-to-the-coronavirus


“What is Herd Immunity and How Can We Achieve It With COVID-19?” BY GYPSYAMBER D’SOUZA AND DAVID DOWDY | APRIL 10, 2020, Johns Hopkins School of Public Health. https://www.jhsph.edu/covid-19/articles/achieving-herd-immunity-with-covid19.html

Proceedings of the Thirtieth Annual Convention of the United States Veterinary Medical Association and First Veterinary Congress of America. Philadelphia, Printed for the association. 1894. https://www.google.com/books/edition/Scientific_Proceedings_of_the_Annual_Mee/oEpEAQAAMAAJ?hl=en&gbpv=1

“THE SPREAD OF BACTERIAL INFECTION. THE PROBLEM OF HERD-IMMUNITY,” W. W. C. TOPLEY AND G. S. WILSON. J. Hyg., 21, 243-9. 1923. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2167341/pdf/jhyg00291-0051.pdf

Why am I doing this?

The coronavirus pandemic will be indelibly written on our memories just as the Great Depression or the Battle of Britain left their mark on past generations. It is my intention to journal the events of these days from three perspectives: as a retired medical technologist, a historian (Ph.D., 2014), and an ordinary person living through an extraordinary crisis.

You are on History’s Edge.

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