Question: What is the definition of a second wave? How do we differentiate wave 2 from wave 1? E.g., Does Louisiana currently have the second wave? Or, is it just first-wave-not-properly-flattened-and-now-bursting? It’s happening in Japan and Austria too. Of course their peak is so low…

Answer: I was wondering about this too, so thanks for asking! As I understand it, there is no official epidemiological definition of a “wave.” According to WHO’s “Pandemic Influenza Preparedness and Response: A WHO Guidance Document (2009)”, a second wave is simply the resurgence of the virus after a post-peak period — e.g. resurgence in cases and/or deaths in the time after pandemic activity appeared to have been decreasing. Most of us in public health hear “waves” and think of the course of deaths in the UK (and world, which followed a similar pattern) due to the 1918 flu (Figure 1). When it comes to COVID-19, the United States still hasn’t gotten over its first wave. Instead, we had a wave of cases that never crested. Cases reached a peak, plateaued there, and then grew again (Figure 2). If we were to zoom in on different states, however, we’d see that some are in the post-peak period and are over the first wave (like New York, Figure 3) while others are still in their first wave (like Tennessee, Figure 4). When it comes to a state like Louisiana, I think it fits the criteria of being in a second wave — it was in a post-peak period and then experienced a dramatic increase in cases (check out charts here). Finally, if you’re interested in reading another article on this topic, this one from The Conversation shares an epidemiologist’s description of waves.

Figure 1. Three waves of 1918 Flu, Deaths in the UK (from Emerging Infectious Diseases)

Figure 2. Cases in the USA (excluding New York State) — first wave plateaued then rose (data from covidtracking.com)

Figure 3. Cases in New York — wave 1 over

Figure 4. Cases in Tennessee — first wave still rising