By now, everyone has heard the term “social distancing” and should be practicing it. Keeping our distance from others during the COVID-19 pandemic can slow the spread of the virus and save lives.
That’s the story the chart below tells in one simple image. The chart, based on one created by the Centers for Disease Control, contains two curves. The first curve’s steep peak represents what would happen if we do nothing to stop the spread of COVID-19 disease: we’d see a surge in the number of cases over a short period of time. The second curve’s flatter peak represents a slower rate of infection occurring over a longer period of time. This is what health experts are referring to when they say our goal is to “flatten the curve.”
Drew Harris, a population health researcher at Thomas Jefferson University, in Philadelphia, made the CDC chart newly relevant by adding a simple, yet significant feature. The CDC chart shows how an outbreak can be controlled. Harris added the line representing the number of people our health-care system could care for at one time. I talked with him about how the chart teaches us to flatten the curve, its relation to past pandemics, and how educators can explain the chart to students.
Brenda Iasevoli: How can teachers explain the chart to students?
Drew Harris: The first curve shows what we don’t want to happen: a swift rise in the number of people falling ill with the virus [y-axis] because we aren’t taking any measures to stop transmission. The second, lower curve shows what is possible if we are able to spread out the transmission of COVID-19 over time [x-axis].
Here’s an analogy that teachers can use to explain this to students: it’s the difference between a one-to-one conversation and a group chat. Let’s say you tell your friend something, and that friend tells another, and so on. That is a much slower way of sending a message around. But if you tell a lot of people at once, and they tell a lot of people at once, then you wind up with a much faster spread of the news. A virus essentially is a piece of information, and the virus wants to spread that information around as quickly as possible so it can make more of itself. It’s like when we say a message goes viral on the internet.
BI: How do we get to that second, flatter curve?
DH: The way we stop the speed of the spread is by social distancing: keeping people further apart. We separate people in terms of physical space: stay six feet away from others because the droplets that come out of your mouth can be breathed in by someone else, and that’s how this virus spreads. We also separate people in terms of time, making sure that not everyone is in the same place at the same time. Some people, for example, might start work earlier while others start later.
BI: What are some other ways that we can slow the spread of disease?
DH: We identify people who have the disease and isolate them so they can’t spread it to others. That term isolation doesn’t mean you lock them in the closet somewhere. It means they are in a separate room or facility where the air is purified so the virus can’t spread. We also quarantine people who might have the virus because they talked with a friend or neighbor who tested positive. A quarantine takes people who may be sick and separates them, and a so-called reverse quarantine takes people you don’t want to get sick—the elderly, people with underlying health conditions—and separates them for the duration.
This is why testing is so important. It can tell us who does and does not have the virus, so we can take the appropriate measures. The unfortunate thing about this virus is that it seems to spread before people even know they are sick.
BI: What did you hope to show by adding the line to the chart?
DH: The chart shows the number of new cases over a period of time. The faster the virus spreads, the greater the number of cases we have out there at any given moment. And because we know this virus results in about 20 percent of people being hospitalized, that means lots and lots of people are going to want to get into the hospital in a short period of time. The danger in that is that hospitals have a limited capacity. They can’t treat everyone at once. So, some people may not get the treatment they need and may die as a result. That’s why we want to spread out the infections over time. That limits the surge that overwhelms the system.
BI: Why aren’t there any numbers on this chart?
DH: People get confused by that. This is a model, a representation, even though it looks like a graph that kids can use. Where I drew the line was entirely arbitrary. The curves are meant to be relative to each other, nothing to do with actual numbers.
The chart doesn’t necessarily represent the COVID-19 outbreak. It represents the way an infectious-disease outbreak works. It’s a conceptual model explaining essentially three dimensions—the number of cases, time, and the capacity on the health system—and how they relate to one another. When teachers talk to their students, they need to explain that you don’t need numbers to show the relationships between different concepts.
BI: What would you say to people who might see the idea of spreading out infections over time as counterintuitive?
DH: The step we want is containment. Ideally, when a new virus pops up, we should contain it—don’t allow it to spread anywhere; stop it completely. That would be perfect. Then you don’t have to worry about it. But containment can fail for a variety of reasons, especially with a virus that is easily transmitted, like this one. So, when containment fails, you go to mitigation.
There are two goals of mitigation: one is to prevent further spread to other areas. We don’t want to overwhelm the response systems. That’s the reason I published the graphic with the line representing our health-care capacity. And it’s important to remember, it’s not just the hospitals. It’s the people who run the power plants, the people who help move food around this country—all the vital services we have in our society depend on people doing this work. We haven’t replaced them with robots yet. People are not expendable. They are necessary. If too many people don’t show up to work because they are sick or fearful of catching the disease, then our whole society will grind to a halt. So, we want to make sure we are not overwhelmed.
Best case: containment. Second case: let’s just spread it out so this thing can be managed over a longer period of time.
BI: Does spreading the disease out mean we get fewer infections and deaths overall?
DH: It could mean that. That is the hope. Another reason for spreading the disease out over time? It gives us time to come up with a vaccine. Pandemics like this tend to travel in waves. They can move around the globe for years. That’s what the 1918 flu did. It came back in subsequent years because people became complacent and stopped all the things they were doing to prevent the spread of the flu in the first place. There were vulnerable people out there who didn’t get the flu the first time. But we now have the ability to create vaccines to shut this down, because ultimately what we want is an immune population. If enough people catch this, the population is immune. It’s what we call herd immunity. When you have enough people in a population who are immune to a disease, then it cannot spread.
BI: Has this chart ever played out with real numbers in history?
DH: There’s one real example in the 1918 flu pandemic. We have examples of three cities: Philadelphia, St. Louis, and Denver. Each experienced a flu outbreak in different ways. In Philadelphia, officials decided not to cancel a bond parade to raise money for World War I. That resulted in up to 200,000 people in the city at the beginning of the flu spread. Public health people warned that this was not a good idea, but the parade went on anyway, and within 48 hours, people around the region began to die. The death rate was very high, and systems were overwhelmed.
On the other hand, St. Louis had a strong public health official who locked down the city, told people to socially isolate, and did all the things necessary to ensure the flu didn’t spread. St. Louis had a much flatter curve. What I describe abstractly in that chart they were able to do in reality in St. Louis.
Denver essentially did what St. Louis did, but then let the lid off the pot too early, and the city saw a second peak in cases, sort of like stopping chemo early and the cancer comes back quick.
BI: What do you wish people knew about preventing the spread of disease from a public health perspective?
DH: When we’re effective and people heed our warnings and the disease does not spread as we expected or anticipated, nothing happens. Then it looks like our warnings were too hysterical and people say, "Oh, you public health people, you always exaggerate. It wasn’t that bad." Yeah, it wasn’t that bad because people heeded the warnings and did what they were supposed to do. That’s what I call the public-health conundrum. When you’re effective, nothing happens.
The views expressed in this interview are those of the interviewee and do not necessarily represent those of HMH.
Teach the “Flattening the Curve” Chart
Provide students with a list of words and phrases related to COVID-19, such as pandemic, quarantine, isolation, lockdown, community spread, and social distancing. For each, have them write a definition and draw a picture that shows the meaning.
Analyze the Chart
Show students the chart and ask them to describe what they see. Ask: What do the two curves represent? Why is one curve steeper than the other? What is happening with the first curve in relation to the health-care system? What is happening with the second curve in relation to the health-care system? Point out that the second curve is spread out over a longer period of time than the first curve. How does that extended time benefit the health-care system?
Compare the Pandemics
What do COVID-19 and the flu pandemic of 1918 have in common? Read about them and discover key similarities and differences. Share your findings in a comparison chart. Download the student resource here.
Model an Epidemic
Challenge students to model an epidemic moving through a community. Give them these criteria:
- On Day 0, 10 people are infected.
- Each day, the infection rate increases by 30%.
Have students model the epidemic through 5 days, using circle or tick marks to represent the number of people infected. Ask: How many people do you think will be infected after two weeks? (394 people) Challenge them to graph the two-week period in Excel or another program. Explain that the graphs show the rate of infection of COVID-19 rising exponentially.
Demonstrate the difference between exponential and linear growth by showing students the graph below. The yellow line shows the two-week rate of infection described above. The blue line shows a two-week rate of infection increasing by 3 each day after the initial 10 infections on Day 0. Have students compare the graphs. How are they alike? How are they different? What is the difference between exponential and linear growth? What would have to happen in order for the yellow line to start curving downward? Be sure students justify their responses using the graph.
Write an Opinion Piece
Some leaders want to reopen the country’s economy sooner than later, arguing that the pandemic’s financial damage will hurt us more than the virus itself. Drew Harris says the pandemic therefore raises an “interesting ethical question: Do people exist to serve the economy, or does the economy exist to serve people?”
Have students write an opinion piece responding to Harris’s question. They might begin by thinking about which of these statements they agree with:
- The government should allow businesses to reopen despite the risk of spreading infection.
- The government should provide people with income to help cover basic needs while they can’t earn money.
To help you continue teaching and learning during the current outbreak of coronavirus (COVID-19), visit HMH's At-Home Learning Support page for free resources.