Module 2: Epidemiology Principles

Apply epidemiological principles to describe the spread of COVID-19, and evaluate the potential impact of public health interventions via modeling and historical and contemporary examples.

Authors: Jakub Glowala; Kiryung Kim; Katherine McDaniel, MSc; Katherine Nabel; Himaja Nagireddy; Deborah Plana; Emily Rencsok; Lily Zhong

Editor: Himaja Nagireddy

Reviewers: Wolfram Goessling, MD, PhD; Andrea Wershof Schwartz, MD, MPH; William Hanage, PhD; Rebecca Kahn, MS; James Hay, PhD

Highlight of Updates (5/1/20): (Reviewed by Rebecca Kahn, MS; Andrea Wershof Schwartz, MD; Wolfram Goessling, MD, PhD)


In this module, we move from the biology and pathophysiology of SARS-CoV-2 to its implications at a population level. We start with an introduction to epidemiological terms. To understand where the epidemic is now, we link a curated set of continuously updating resources. Current estimates for the U.S. indicate a caseload 10-50x of what is currently recognized, with asymptomatic and mildly symptomatic people as a major contributor to transmission. Next, we give an overview of the factors used to predict where the epidemic is going in the U.S., focusing on the concept of exponential growth. Mathematically and empirically, small modifications to the parameters of this growth can “flatten the curve," which lengthens the time over which severely ill people present, providing the healthcare system more time to prepare to treat patients and scientists time to test and optimize new treatment strategies to reduce mortality. At this phase, the U.S. is primarily attempting to flatten the curve by “social distancing.” Modeling from the UK indicates social distancing may be required for months.

We end with three case studies to contextualize these epidemiology principles. The influenza pandemic of 1918 prompted different responses from three U.S. cities, with three dramatically different outcomes for morbidity and mortality. The 2009 H1N1 pandemic, a frequent foil to COVID-19, required less disruption to control due to a lower R0 and case fatality rate, as well as faster testing, prior population immunity and pre-existing antiviral treatments. In confronting COVID-19, South Korea presents a contemporary example of a country that rapidly scaled up testing, contact tracing, and social distancing without nationwide lockdown, and has brought new cases to a minimum.

Learning Objectives

By the end of this module, medical students should be able to:

  • Define R0, Re, incubation period, serial interval, epidemic curve, community transmission, social distancing, and flattening the curve as they pertain to COVID-19

  • Identify and access a reliable source of the latest epidemiologic information about COVID-19

  • Describe how changing epidemiological parameters changes disease dynamics

  • Contrast three cases that illustrate how flattening the curve saves lives in a pandemic

Core Materials