The Models are Changing

Switzerland is doing particularly well today, with no reported growth in active cases for two days in a row. The rest of the countries are holding steady, with the exception of France. France's numbers today have a big asterisk, as they reported a whole new host of 'updated' numbers based on a difference between how they are tracking these numbers versus the international standard. For instance, deaths of COVID-19 patients in nursing homes were not being counted. It will take me a little while to integrate that new information into the reports.

In the States, MS, OK, TN, and TX have become the latest states to enact state-wide stay-at-home orders. The governor of Oklahoma actually issued their order yesterday, but I had missed it at the end of the day. That leaves five states with partial coverage of their stay-at-home orders, and four more with no orders at all. At this rate, Missouri and South Carolina are due to change their mind in the day or two. For the Dakotas and Wyoming, it may take awhile before they feel threatened enough to act.

Also of note today, Florida's governor quietly issued an edict stating that no local governments could enact tougher rules on shutdowns or stay-at-home orders than the order he had (belatedly) issued for the state yesterday. This is notable, since throughout March, Florida only kept its numbers down through a series of mayors and county commissions closing down many of the most populous cities, including the hot spots in South Florida. For instance, this has the effect of lifting the curfew that was in place in Leon County (which happens to contain Tallahassee, the state capital). The fact that most of these locations trend Democratic is just one of those odd coincidences. Their display of initiative was not taken well and such leadership will not be allowed to continue in the future. It will be interesting to see if Governor DeSantis is challenged on this order and how it is enforced.

Louisiana is seeing a major jump in active cases over the past few days, rising to 43% growth today. The governor is claiming a "log jam" of testing is the reason behind the sudden increase, though the Mardi Gras celebration is definitely on everyone's mind. Given that the stay-at-home order was enacted 10 days ago, this surge is definitely unexpected.

For the most part, however, the major infection sites are maintaining steady growth numbers, and typically coming in under my model expectations. Which brings me to the main topic of today's post: the models are changing.

Previously, most regions were following an exponential growth model. This makes sense, as each individual has a chance to infect N people around them every day. Once infected, those individuals can infect others around them, too. This leads to more people, each of which can spread the infect to N new people, on top of the ones that were already infected and spreading the disease. This model matches the data very well in regions that have not taken steps to protect themselves. For instance, look at Tennessee or South Carolina, which only enacted a stay-at-home order today. The blue line is the actual cases over the past ten days, while the red line is the predictions of the exponential model. For reference, the gold line is the best-fit linear model.

Now look at a region like NY, which enacted statewide protections on March 22, with additional measures taken even before then. The data is almost a perfect linear model. The best exponential model is still shown in red.

Finally, to round out the point, examine a state like Florida, which has seen a federation of local protections with a lack of any statewide protections. The growth sits perfectly between the two models.

The same is true in other countries as well. So what does this mean? What is causing the residual linear model? What drives the slope of this linear growth? These are questions that I would really like to answer, as they may help reveal when this will peak, and what measures need to be taken to finally reverse course. Stay tuned.