The rise and (hopefully) fall of the Delta variant in the U.S.

Using the excellent data visualizations on the covidestim.org site, I noticed an interesting “wave” effect as the Delta variant started to take hold in the United States in late spring. The first reports of significant spread of the highly contagious variant were in Missouri and Arkansas in mid-June.

The signs were already there in the data, however. The effective reproduction number (R_t), which we’ve discussed before, is “the average number of people who will become infected by a person infected at time t. If it’s above 1.0, COVID-19 cases will increase in the near future. If it’s below 1.0, COVID-19 cases will decrease in the near future.” By May 23, both Missouri and Arkansas were seeing Rt numbers above 1.2 (orange and red on the map).

By July 4, ironically at the same time as President Biden was suggesting that the nation was declaring independence from COVID (which this New York Times article by Sheryl Gay Stolberg correctly questioned as being too soon), the R_t numbers throughout the southeastern U.S. were very high and most of the rest of the country was following.

Per capita infections were still low on Independence Day, but as predicted by the high Rt numbers, within three weeks that had changed for the worse.

The spread from the southeastern states, where vaccination rates have been the lowest and several Republican governors have deliberately worked to thwart public health measures including mandating (or even promoting) vaccinations and wearing of protective masks, through the southwestern and midwestern states, the Pacific Northwest, and even Alaska, happened next. Here’s the map on August 29 with Delta at what appears to be its widest extent.

Notice that at this point 18 days ago, Michigan was still experiencing lower rates of COVID infection (as was most of the rest of the upper midwest and New England). The state is still lagging in the percentage of residents who are fully vaccinated, unlike New England which has some of the highest vaccination rates.

At this same time (August 29), R_t numbers had begun to decline significantly in states that were still bright yellow and white on the infection per capita map.

Perhaps some of this was the increase in vaccinations that followed the Delta outbreak in the southeastern states, or perhaps it’s a result of there being fewer people left to infect. In any case, with R_t numbers declining back under 1.0 in those states, a predictable decrease in infections per capita should happen… which it has, according to the most recent map from September 14.

Here are two animated GIFs of both sets of data covering the period from May 2 to September 14:

There’s always the possibility that another variant could emerge and the whole pattern starts over again. However, the combination of vaccine mandates – which have started to have a positive effect as they expand to more businesses and organizations – and so-called “vaccine passports,” already established in Québec, coming to Ontario, and being considered in several U.S. states, which restrict access to non-essential activities such as bars, restaurants, sporting and music events, and more. Ultimately, it will be the level of vaccination that will move COVID-19 from a pandemic to an endemic virus that will likely require periodic boosters, similar to an annual vaccination for influenza, pneumonia, or other already-available prevention techniques.

Watching the R_t numbers, though, is an important way to cut through the media hype and misinformation (both intentional and well-intentioned) that continues to dominate our discussion about what the future holds for the COVID-19 pandemic.