Flattening the curve, what does it mean?

By Lynnette Wood, PhD

Orcas Island

By now you’ve heard the term, “flattening the curve.” But what does it mean?

This term refers to the number of new cases of coronavirus infections over time. The “curve” is a graph in which the vertical axis is the number of cases, and the horizontal axis is time. (See figure below.) The idea of “flattening the curve” is to spread the number of new cases over a longer period of time, thereby reducing the number of people who need to be hospitalized at any particular time. By restricting travel and practicing social distancing and self-isolation, the hope is to slow down the spread of the virus enough to sufficiently flatten the curve to the point where our health care system can handle it.

Because CoV-SARS-2, the virus that causes the disease COVID-19, is a new virus in the human population, we do not yet have a treatment or vaccine for it. Nor have we developed “herd immunity,” a term that epidemiologists use to describe when a high percentage of the population is immune to a disease, either through vaccination or previous exposure. A high level of herd immunity makes the person to person spread of a disease much less likely. The threshold to attain herd immunity depends on how contagious the disease is. For measles, which is highly contagious, 93% to 95% of the population needs to be immune.

Although CoV-SARS-2 is also considered highly contagious, it is not nearly so contagious as measles. For CoV-SARS-2, epidemiologists are currently estimating that herd immunity can be achieved once 60% of the population becomes immune. Unfortunately, because there is yet no vaccine, that would mean that 60% of the population would first have to get sick. But some will be very sick, requiring intensive medical intervention, and even then not all will recover. Which brings us back to the need for flattening the curve.

There is no question that flattening the curve can work to save lives. During the 1918 Spanish flu epidemic, against the advice of medical professionals, the city of Philadelphia threw a parade which drew a crowd of 200,000. Three days later, every bed in all of Philadelphia’s 31 hospitals was filled with sick and dying patients. By the end of the week, more than 4,500 had died; after six months, 16,000 had died. The city of St. Louis, on the other hand, heeded the advice, banning public gatherings and closing schools, libraries and churches. St. Louis experienced a fraction of the deaths suffered by Philadelphia.

The impact that successfully flattening the curve will have on our daily lives will be profound. For this strategy to succeed, participation must be nearly universal. And, while it may be counter-intuitive, the more successful we are at flattening the curve, the longer the time period of social distancing and self-isolation. Thirdly, in the idealized curve included here (from the CDC) the line indicating the capacity of the health care system is quite high on the graph. In reality, the line is much lower. In fact, we are already hearing stories of hospitals running out of equipment and supplies, and we are still in the very early stages of this disease. ICU beds are going to be needed for other emergencies, as well as for COVID-19 patients.

Finally, the curve shows the number of new cases, not the number of total cases. Which leads to the last point: what happens when the social distancing and self-isolation mandates begin to be lifted? If we don’t have a vaccine or treatment by then or have established herd immunity, there is likely to be a second wave of illnesses. Which is why everyone is watching China closely now, to see if they will experience a second wave.

Flattening the curve is currently the only strategy open to us, and will remain so until either a treatment or a vaccine, or both, are found. We are, truly, all in this together.