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As the novel coronavirus has rapidly spread across the world, we’ve slowly familiarized ourselves with a growing number of related terms and phrases: “social distancing,” the meaningful difference between self-quarantine and self-isolation, and what it means to shelter in place. Another such is the coronavirus’s dreaded “peak.”
While the definition of “peak” might be obvious, it’s useful to understand how that is being applied to discuss a virus: Generally speaking, a virus’s peak is the day on which there are the highest number of cases. In the case of the coronavirus, we may have just passed the grim marker. On April 17, researchers at the prominent population research center Institute for Health Metrics and Evaluation (IHME) released new data indicating that the U.S. may have hit its peak in deaths and hospital-resource use last week. In the past few days in New York State, the epicenter of the global coronavirus outbreak, the daily death toll has begun slowly falling.
However, health experts and local officials have cautioned against celebrating too soon. “We’re not at the plateau anymore, but we’re still not in a good position,” New York Governor Andrew Cuomo said in a press briefing on April 18. Earlier this month, Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, issued a similar warning: “You never want to, you know, claim victory prematurely.” However, he continued, “when you see those kinds of [downward] trends, you hope that we’ll see that curve go down and then can start to think about gradually getting back to some sort of steps towards normality.”
To better understand the general concept and what to expect, we spoke with Mark Lurie, an associate professor of epidemiology at the Brown University School of Public Health, in late February.
What does it mean for a virus to peak?
To assess the state of an epidemic, epidemiologists map out a curve by plotting the number of new cases over time. “An epidemic curve is exactly what we would want to look at to assess the state of the epidemic,” Lurie told the Cut, elaborating that the curve helps illustrate the epidemic’s trajectory. He continued: “The peak is the day on which there are the highest number of cases, after which the number of cases will start to decline. By definition, 50 percent of cases will occur after the peak.”
When estimating the coronavirus’s peak, he says it’s important to remember the pathogen’s long incubation period: “Since infections detected today were infections that actually occurred 10 to 14 days ago, we would expect the impact of interventions not to being seen at least until two weeks after they have been implemented.”
Wait, how do epidemiologists make this chart?
In general, early on in an epidemic, there is a high chance for transmission because nearly everyone is susceptible; later on, the chance is typically lower, as more people have had the disease and become immune. “In the absence of intervention, one expects the number of cases to start going down when the number of susceptible people in the population starts to decline,” Lurie told the Cut. “So, as the number of these people immune or recovered in a population grows, the number of new infections starts to decline.” Epidemiologists refer to this concept as “exhausting the susceptible” population.
In the case of the coronavirus, Lurie says: “We won’t get to a point where we exhaust all the susceptibles,” because measures to mitigate the spread of coronavirus, like shelter-in-place orders and social distancing, will likely cause the number of new cases to decline.
So how does “flattening the curve” relate to all of this?
As coronavirus outbreaks started to pop up across the the United States, public-health experts stressed the dire need to practice social distancing and precautionary measures to “flatten the curve” — referring to the coronavirus’s epidemic curve — to slow down the transmission of the contagion and therefore avoid overburdening hospitals and our health-care system all at once. Say, for the example, the United States took absolutely no measures to mitigate the virus’s spread; the epidemic curve for the coronavirus would be exponential, with a very high peak, leading to widespread preventable suffering and deaths. In short, the more strictly the population adheres to precautionary measures, the slower the coronavirus will spread and the flatter the curve will be. There’ll still be a peak, but it will be delayed and less extreme.
So has the coronavirus peaked in the U.S.?
Right now, it appears that the country may be turning a corner. According to IHME, COVID-19 deaths peaked in the U.S. on April 15 and hospital-resource use peaked on April 14. However, as Lurie stressed to the Cut, “a few days of leveling off could look like the peak and the beginning of a decline but, when seen in a larger time frame, is actually not a decline.” Therefore, health officials have stressed the continued importance of wearing a mask in public spaces and adhering to social-distancing mandates. (Last week, Cuomo extended New York State’s stay-at-home order through May 15.)
So the virus will peak in different cities and states at different times, right?
Yes. Just as the virus has peaked at different times around the world, the same will happen to locations across the U.S. According to Lurie, when each place will see its peak depends on a number of factors: “When the epidemic started there, what interventions were put in place, when they were put in place, and how stringently they were followed.”
While passing the peak is certainly significant, it’s important to contextualize the milestone. Last Friday in New York State, 540 people died from the coronavirus, and approximately 2,000 were admitted to city hospitals with COVID-19 symptoms, the New York Times reports. “If it wasn’t for the relative context we’ve been in, this would be devastating news,” Cuomo said of the death toll.
Could one city or community have multiple peaks?
“Strictly speaking, there is only one peak,” Lurie told the Cut. However, if cities lift restrictions too early, he notes that those locales will surely see a surge in new cases.
This post has been updated with new information.
