What Happened in South Dakota?

We’re publishing a piece today by Dr Noah Carl, an independent scholar, on South Dakota. As Noah points out, South Dakota had some of the lightest restrictions in the Western world and its death toll is high compared to other US states – the eighth highest, in fact. But it has also seen cases decline rapidly since November in spite of the Governor’s laissez-faire approach, which is puzzling given that the herd immunity threshold doesn’t appear to have been reached. In the following Extract, Noah speculates about why this could be.

So, why did case numbers fall in South Dakota? I’m afraid I don’t have the answer. But here are a few possibilities. First, the herd immunity threshold is lower than 66%. This could be because the threshold has been overestimated in general, or because it is lower specifically in South Dakota, perhaps due to the state’s geography.
Second, the Google mobility index is a poor measure of the behaviours that drive transmission (as Philippe Lemoine has suggested). Perhaps South Dakotans were extra careful to practice social distancing during the month of November, even though they didn’t stop going out for retail and recreation. Weighing against this interpretation is the fact that there were dramatic changes at the start of the pandemic. Notice the precipitous decline in the retail index, and concomitant rise in the residential index, on the left-hand side of the chart.
Third, the level of immunity at which cases start declining (even if true herd immunity has not yet been reached) is much lower than 66%. This could be the case if there is substantial heterogeneity in the behaviours that drive transmission. Suppose that 80% of infections are caused by 20% of people. (Perhaps these ‘super-spreaders’ are particularly sociable, careless, or likely to interact with others by nature of their work.) Once a large enough share of the 20% has been infected, case numbers may begin falling rapidly. (This point has been made by David Dowdy.)

Worth reading in full.

Noah’s piece originally appeared in his Substack newsletter, which is worth subscribing to. He writes regularly about the pandemic.