Health & Health Care, Science, Pseudoscience, & Media Literacy

What’s This Curve We’re Flattening?

In the age of COVID-19, everyone keeps talking about flattening or bending the curve.  But what the heck kind of curve are they talking about?

Epidemiology looks at the patterns of diseases across populations.  This is a fundamental part of public health.  Epidemiological curves, also known as epidemic curves or epi curves, are used to visually display data about a condition.


epi curve showing lower and delayed epidemic peak
Esther Kim@k_thosandCarl T. Bergstrom@CT_Bergstrom / CC BYWikimedia Commons

This is a generic epi curve.  In this particular example, the horizontal axis represents time, and the vertical axis represents the number of cases.  An actual epi curve would give more specific information, such as dates, active or resolved cases, etc.

The dashed horizontal line represents healthcare capacity.  The healthcare system is capable of handling a finite number of patients at a given point in time, and that limit is stable unless resources are added to or removed from the system to change the capacity.

The large peak represents uncontrolled transmission.  The number of cases goes up so sharply because, without effective measures in place to control the spread of a highly contagious illness, each person who gets sick infects 10 other people (the 10 in this example is a totally arbitrary number), and each of them infects 10 other people, who in turn each infect 10 more, etc., etc, and a lot of people get sick very quickly.

Where the shit really hits the fan is when the number of cases far exceeds the flatline healthcare capacity.  More people are getting sick and dying than hospitals can handle.

On the other side of this large peak is a sharp drop in the number of cases, because by that point most people will have gotten sick and either died or recovered and developed at least some degree of immunity.

By implementing control measures to “flatten the curve”, you get the second peak shown in the above graph.  There’s a more gradual increase in cases, and the number of cases at any point in time doesn’t greatly exceed the healthcare capacity.


GIF showing the flattening of an epi curve and raising the line of healthcare capacity
RCraig09 / CC BY-SAWikimedia Commons

This fancy little GIF from Wikipedia shows how the curve can be flattened with mitigation measures to decrease transmission.  It also shows increasing the healthcare capacity, but that’s not necessarily easy to do.  In the case of COVID-19, the production of more ventilators allows for increased capacity to deliver critical care.  However, healthcare personnel numbers can only increase so much.


epi curve showing resurgence with inadequate mitigation
RCraig09 / CC BY-SAWikimedia Commons

As many areas are talking about “reopening,” that’s where this fancy GIF is waving a figurative caution flag.  This graph doesn’t show a clear line for healthcare capacity, but the green represents capacity and the pink/red area exceeds capacity.

Just because the curve of the initial surge has been flattened doesn’t mean the number of illness cases can’t flare up again.  If there aren’t enough mitigation strategies in place to keep the number of new cases down, more people will start to get sick, and each of them will get 10 more people sick, etc., etc.


epi curve showing forecast COVID-19 deaths in the U.S.
U.S. Centers for Disease Control and Prevention

The first three graphs weren’t COVID-19 specific, but this one from the CDC is.  The horizontal axis shows time, with specific dates.  The vertical axis shows the cumulative number of deaths in the United States (so, for example, the number of deaths May would include all deaths from March and April).  This is a national forecast; forecasts for individual states are also available from the CDC here.  The CDC page notes that this information was last updated on May 6, so it doesn’t incorporate any changes in either case numbers or mitigation measures that have happened since then.

The data points connected by solid lines up until just after May 1 represent actual data.  Beyond that is the forecasted deaths based on epidemiological modelling.  This kind of modelling is very complex, and since no one knows the future, models need to be based on certain assumptions about different factors that will affect how many people the virus kills.  The curve on the left breaks it down so the projections of each model are shown and labelled in different colours, while on the right the various models are combined.  The models come from various sources; for example, the ones labelled CU-20 come from Columbia University.

Just like a weather forecast, the ability to make accurate predictions decreases the further into the future that you’re looking at.  That, combined with the different assumptions made in different models around things like social contacts and speed of restrictions lifting and businesses reopening, ends up giving a sort of fan shape to the projections.

The take-home, though, is that even in the best-case scenario projections, it looks like more people are going to die.  In the worst-case scenario projections, a whole lot more people will die.

I wonder if the people who’ve been protesting because they want haircuts, or because they think restrictions are communist measures that take away their freedom, are keen to volunteer their parents, grandparents, and immunocompromised family members to be among the potentially 70K+ more people who are going to die if restrictions are lifted too much, too quickly.  Priorities, right?



You can find more on public health on The Science Corner page.

31 thoughts on “What’s This Curve We’re Flattening?”

  1. I’ll admit, I am definitely concerned with NJ/NY area reopening certain areas because of them already being hot zones. At the same time, If people continue to keep wearing mask and social distancing, I don’t feel that all parks and beaches remain closed. We need some sort of normalcy. We must remain diligent and proactive and make sure we follow guidelines, not just run out without taking precautionary messures to protect ourselves and others.

    1. I think the key is for governments to be listening to what public health officials so they can be really clear on what activities are safest to resume first (like outdoor activities) and what precautions should be taken.

  2. I get news from Las Vegas on my google feed, and it is costing one casino chain 3 million dollars a day to be closed… but, I would rather see them closed than everyone putting their grubby hands on chips and machines, and potentially ingesting everyone including employees!

  3. I don’t understand the people in the US protesting against restrictions. Here in the UK, if anything some people are worried that the government is about to ease some restrictions a little bit and they think it’s too soon. There’s a disturbing anti-science streak in American conservatism, which here meets a similarly extreme libertarian streak and this is the result. Sadly, I think some of the “give me liberty or give me death”-types would volunteer elderly relatives to die.

    1. There are some very strange attitudes in the U.S. The staunch individualism regardless of collective well-being, the idea that science and government are, by default, against the people… It all just seems so bizarre. And I agree, that type may well volunteer their elderly relatives, and then blame China.

  4. Over here the restrictions are more lose. Shops are open (with social distancing and mouth masks, limited time in stores and only 1 pers per household) but still … Am I worried as there is no way you can keep social distancing where the streets are narrow. But I feel that the economy is a huge stake holder here and this is where we ended up.
    I hope that the curve keeps flattening but I worry for people as not all are going to make it.
    I see some people not following the rules (no mouth masks on public transportation for example but police reprimands them) and I just can’t understand how you can be so selfish! On the other hand, I go outside when I need to to (for me that’s therapy) and not when they decide to open the shops. Nobody forces us to socialize more or to mingle more)

    1. Where I am we never had full lockdown, and no requirement to wear masks. I can see how with narrow streets distancing would be challenging.

  5. Governor Newsom here in California appears to be taking a sensible approach, but of course people are protesting just because he’s a Democrat. He also said the initial infection began in a nail salon, so now salon owners hate him. I guess he was supposed to hide that from us? People will complain whatever…

  6. Hi Ashleyleia, good to hear from you here. A well-researched piece. Very educational. Thanks for sharing the important piece at this right moment. I wish you Happy Nurses Day <3

  7. I’m also afraid now with things getting open again that the second wave will be worse and more people will die. I didn’t go to the hairdresser yet. I’m anxious. I’m just doing it at home. You really described it very well ❤️

    1. Thanks! I think what’s really important is that governments listen to public health officials and open things up at a pace that matches what the science says is appropriate.

  8. “I wonder if the people who’ve been protesting because they want haircuts…”

    😆. Growing your hair longer’s fun! It’s like putting on a costume lol.

    Yes, it’s odd how talk of easing lockdowns just increased and increased from nothing, and as far as I can tell it was arbitrarily dictated by the media wanting something new to talk about 🤷‍♂️. Imagine if you went back to the beginning before everybody ‘got used’ to these numbers, and with the current numbers of active infections talked about NOT locking down. Everybody including the media would be outraged!

    And here in the UK, lockdown never even happened 🤦‍♂️. Airports are still open like normal and there’s no plans to close them! Just look at the number of comments on this BBC News article about it (from May 9th):

    1. I say before every body ‘got used’ to the numbers, it’s probably again only the media who ever got used to them! As well as all the idiots who never cared about them to begin with, of course.

    2. Some flights are still happening in Canada, although international arrivals are required to quarantine. It makes me wonder who the hell are these people that are flying? Where are they going, and why? And planes are a germ soup at the best of times, much less in the middle of a pandemic!

      1. “And planes are a germ soup at the best of times, much less in the middle of a pandemic!”

        Exactly! I was thinking earlier that a lot of these countries that have had big outbreaks also have large financial centres.

        I remember reading about how many Britons had decided to go on holiday during the early days of the pandemic, then got stranded abroad. It was hundreds of thousands! So these flights coming into the UK could conceivably be filled with a lot of those!

  9. In the Uk, Boris is starting to ease some of the restrictions placed on us, slowly and with conditions – if cases of the virus increase, he’ll go back to the ‘lockdown’ we have. I’d rather we go slowly too.

    1. That’s what’s happening where I am. They’ll go slow, and if numbers start to increse, they’ll increase restrictions as needed.

      1. Lol, though not sure about flights landing and boats coming over from France where migrants have been travelling through how many different countries to get here tho 😉

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