COVID-19 Data Dives: It's a Wildfire, Not a Wave

William P. Hanage, PhD


July 07, 2020

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William P. Hanage, PhD

The language we use to discuss the pandemic matters. We hear a lot about "second waves," which I think has the capacity to seriously mislead. Talk of a wave suggests that you can avoid it by sheltering until it passes. That's not the case for the COVID-19 pandemic. Here's why.

Think about waves in the ocean: They rise and fall independently of us. The surfer does not create the wave; they only ride it. In contrast, the falling numbers of cases in the Northeastern United States (and Europe) are the direct consequence of human actions to limit transmission. It's not independent of us; we made it happen. 

The roots of the confusion lie in the 1918-19 influenza pandemic, when there were distinct waves of disease before and after relatively little activity in the summer. That is not a good model here because influenza has trouble transmitting in the summer. It is subject to strong seasonal forcing. That's not the case for SARS-CoV-2. Far from melting away in the sun, we now see record numbers of COVID-19 cases spreading across the South of the US in spite of the summer heat (see also Brazil). 

I wonder out loud how much this has to do with the summer heat driving people into indoor spaces as they emerge from shutdown. Once indoors, transmission is more likely, especially in absence of mask use. 

This is why I prefer to talk about surges rather than waves of disease. "Surges" emphasizes that increases in pandemic activity are not seasonal and can happen at any time (even if they may be more likely in the fall and winter). Talking about surges also focuses on the most damaging aspect of a rise in cases: the threat to healthcare.

A wildfire is a better metaphor for this pandemic. Even if we have doused the area around us with water, we should not be surprised to find the fire still burning elsewhere. If there is dry brush and oxygen, there will be more fire. In this metaphor, the virus is the fire, we are the dry brush, and opportunities to transmit are oxygen. An ember can be carried a long way before it sparks another blaze. We should get used to this and accept that complete normality is a long way off.

We need to think about what the situation now means for the future. The relatively young median age of cases suggests better testing right now than in the spring (it could hardly be worse), which means we can identify milder infections. This is good, but it's not altogether comforting. 

It suggests that we are catching outbreaks at an earlier stage because we are detecting them before people become severely ill. But it's crucial to remember that increases in hospitalizations and death reflect transmission in the past. Given how long it takes people to get seriously ill, even if all transmission were stopped right now, we would expect hospitalizations to keep climbing for a few weeks. Indeed, the increases were noted and the alarm raised in the Sun Belt about 3 weeks ago. Sure enough, the hospitalizations have followed. Systems are now experiencing or approaching surge in multiple cities.

We will need to hope that the virus does not find its way into older, more vulnerable age cohorts. And we need to remember that there's not only a long summer ahead; after that comes the fall and then winter. 

Bill Hanage is an associate professor at the Center for Communicable Disease Dynamics in the Department of Epidemiology at the Harvard T. H. Chan School of Public Health. He specializes in pathogen evolution. Follow him on Twitter.

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