Crash Course On Epidemiology

Introduction

Epidemiology is the branch of medicine which deals with the incidence, distribution, and possible control of diseases and other factors relating to health. This blog post is about the latter two: distribution and control of diseases. In particular, we will be discussing the current pandemic COVID-19.

Point 1: Disease Spread Exponentially

The figure above shows that a disease spread among humans by pathogens (i.e. viruses or bacteria) follows a curve that grows over time by multiplying a number greater than 1 by itself. That number, sometimes, referred to as R0, is the rate of growth or transmittability. It measures how contagious the disease is. For example, if a given person on average infects 2 other people, then the R0 is 2. The disease would spread over time as 2, 4, 8, 16, 32,.... and so forth. The black line shows what would happen if the disease were left unchecked. It quickly would wipe the entire human species.

Fortunately, humans have immune systems. So if a person should contract the disease and recover, that person should be protected from getting the disease again, at least partially or temporarily. Vaccines are designed to do the same thing. As a result, typically the growth curve tapers off since the pathogen is finding fewer unprotected people to infect. The curves is more "S" shaped.

COVID-19 spreads at a rate between 2 and 3; influenza spreads at a rate of 1.2 to 1.5.

Point 2: COVID-19 May Be Overwhelming

The healthcare system has only limited resources: staff, space, machines (primarily ventilators), lab tests, et cetera.

The healthcare capacity may become overwhelmed for the demands placed upon those resources. The figure above shows what happens when the pandemic overwhelms healthcare capacity. The blue horizontal line represents the current capacity. When the number of new cases exceeds that capacity, patients, even critically ill patients, may be denied treatment. The decision of who receives treatment and who must wait is called triage. With a pathogen as lethal as the COVID-19 virus, triage may be tantamount to deciding who may live and who may die.

The red horizontal line shows that the "triage territory" can be shrunk by increasing healthcare capacity. That would entail hiring more staff, enlarging hospital space, and acquiring other resources

Point 3: Mitigation Works

Without therapeutic medications or vaccines, epidemiologists must rely on social/behavioral methods to manage the onslaught of the disease. Behavioral changes include hygenic practices such as frequent hand washing, keeping hands away from the face and eyes, wearing gloves and face masks, as well as staying away from large crowds or other people. To encourage the latter, government officials may order the closing of non-essential businesses, concert arenas, theatres, or sporting events. Recreational facilities also may be temporarily shuttered.

These measures are known as mitigation. The figure above shows how mitigation is supposed to work. The graph shows the number of new cases by month. The red curve shows the course of the epidemic without employing mitigation. The green curve shows what occurs if mitigation is successful.

There are two things to notice with mitigation: 1) the curve climbs more slowly and 2) the peak is lower and later. This effect is called "flattening the curve".

The reason for these changes is that the virus causing COVID-19 is less able to find new victims thereby slowing the spread (i.e. R0 decreases). Also without new victims to infect, the virus starts to die out.

Point 4: COVID-19 Like A Nuclear Blast

The COVID-19 pandemic has been compared to the recent devastation caused by the Australian bushfires. There are many similarities: rapid spread, massive devastation and loss of live, particularly wildlife, and economic upheaval.

I think the COVID-19 pandemic is better compared to a nuclear blast into the atmosphere. A nuclear blast immediately causes wide-spread destruction. The fallout from the blast, equally deadly, may drift for many miles beyond ground zero. The fallout may contaminate thousands of square miles of land rendering it uninhabitable.

Scientists may approach the contaminated space wearing hazmat suites and carrying their geiger counters. They would attempt to set up a containment perimeter indicating the limits of safe proximity. This process is similar to what epidemiologists do to contain the spread of the virus. The geiger counters are akin to the epidemiologists' test kits.

Despite their best efforts, some people who themselves are contaminated may escape the perimeter spreading the contamination. Some may even take souvenirs, such as a pretty rocks glowing purple, not realizing their lethality.

In order to contain these leaks of radiation, the nuclear scientists might try to follow trails of radiation. The souvenir hunters may not know they are destined to suffer radiation sickness the onset of which may take several days.

Following the trails of radiation is like contact tracing. Unfortunately for the epidemiologists, they do not have geiger counters; they have only tests. Testing has two purposes: 1) to surveil the extent and demographics of the pandemic and 2) to trace who may have had contact with infected individuals.

Conclusion

We all are waiting for a vaccine which will eliminate this scourge from our lives. Until then we must follow the mitigation measures to protect our healthcare workers, our first responders, and our essential service workers. These people are facing the enemy up-close-and-personal every day.

Our friends, families, and neighbors also deserve whatever protection we can afford them by staying away.

We will survive; we will thrive; and we will prevail.