- Author: Kathy Keatley Garvey
“Herd immunity refers to the proportion of a population required for the rate of disease spread to equal zero,” says UC Davis distinguished professor James R. Carey of the Department of Entomology and Nematology. “This occurs at the point when each infected person is infecting only one rather than multiple susceptible persons.”
"For example, the potential rate of spread of new cases per infected person is 10 for mumps and 3 for COVID-19. Therefore, early in a pandemic when few are infected, these diseases can grow by 10-fold and 3-fold, respectively. However when 9-of-10 and 2-of-3 persons in each of these populations have either had these diseases or been vaccinated, then the infected person has only one new person in each case to infect so the rate per case is simply replacement. This is the point when the herd immunity threshold is met.”
Carey, who shared his expertise on scientific modeling and demographics at the UC Davis-based COVID-19 virtual symposium on April 23, updated his presentation April 27 to include herd immunity.
His updated presentation, “Actuarial Perspectives on the COVID-19 Pandemic,” is at https://youtu.be/aid69khJftU.
“The true COVID-19 fatality rate is of great importance because it is needed to estimate the number of persons who will die under different mitigation scenarios and along with death statistics how close we are to achieving herd immunity,” says Carey, co-author of the newly published book, Biodemography: An Introduction to Concepts and Methods. (See news story.)
"With the so-called reproductive rate, R=3 for COVID-19, and therefore the herd immunity threshold equals 2/3, this means 220 million out of the 330 million in the U.S. population are required to reach the threshold herd immunity. This is why the fatality rate estimation is so critical. If the fatality rate is only 1 out of 100 or 1 percent, then the 50,000 deaths in the U.S. implies that there are only 100 times more or 5 million who are immune. However, if the fatality rate is 1 out of 1000 or 0.1 percent, a rate suggested by the recent studies in both northern and southern California regarding seroprevalence of antibodies (albeit highly controversial yet), then the 50,000 deaths suggest that 1000 times more are immune or 50 million persons. This is nearly a quarter of the 220 million needed in the U.S. to reach heard immunity."
The virtual symposium, organized and moderated by UC Davis distinguished professor Walter Leal, distinguished professor in the Department of Molecular and Cellular Biology and a fellow of the National Academy of Sciences, is online at https://bit.ly/2VurK3Z. It drew viewers from 10 countries: United States, Germany, Brazil, France, Indonesia, India, Mexico, Canada, Colombia, and Slovakia.
The symposium opened with an introduction by UC Davis Chancellor Gary May, and included presentation by UC Davis physician-scientists Emanuel Maverakis, Stuart Cohen and Nathan Kuppermann; UC Davis veterinarian-scientist Nicole Baumgarth; and pediatrician State Sen. Richard Pan, District 6 chair, Senate Committee on Health.
Davis resident and COVID-19 survivor Marilyn Stebbins, a pharmacist who works at the UC San Francisco School of Pharmacy, told her story in a pre-recorded interview with Leal.
Online interviews also included Michael B. A. Oldstone, M.D., of Scripps Research Institute; professor emeritus Niels Pedersen, DMV, of UC Davis School of Veterinary Medicine, Anne Wyllie, PhD., Yale School of Medicine; and You-Lo Hsieh, UC Davis distinguished professor in the Department of Biological and Agricultural Engineering, and an expert on textiles and clothing.
As of 4:30 p.m., April 27, nearly 3000 had viewed the COVID-19 presentation on YouTube.
An unsolicited comment to Leal about the symposium:
“I just wanted to thank you! You are my heroes."
“This give me a sense of hope and calmed my anxiety like nothing else,” letter writer Kim Allen continued. “Part of what has been so hard is all the disinformation and complete lies and contradictions that are happening daily. To hear people, real doctors and scientists who are so knowledgeable talk about what is going on and why, is so appreciated. We need to know what we are contending with to fight it and be safe. You are all so much appreciated!”