Follow-up: St. Louis County Seasonality
The jury's still out, plus a new working theory of population immunity.
On September 30, I wrote a post looking at the seasonality curves of COVID cases in St. Louis County. My prediction was:
As we saw last year, cases will begin to rise throughout October, reaching a peak before Thanksgiving.
We’re a little more than a month out from that prediction. Let’s look at how this has panned out:
Interesting, huh? Things continued to head downwards from September 30 to roughly today.
(I should state here that my approach to data for case counts is to go as granularly as I can to minimize the noise. My operating theory is that at the county level, you can pretty easily see the seasonality curves, particularly in urban counties that got exposed early. When you combine lots of counties together to get to the state level, a bunch of counties that got their first waves at different times will cloud the trends.)
There’s a couple of possible explanations I can think of for this.
Delta burned through the St. Louis area quick and hot, and infected enough people to reach “herd immunity”.
“But Joshua,” you say, “you said just two months ago that COVID herd immunity doesn’t exist! It’s right here!”
Let me refine that argument a little bit. My argument in that article was to change the paradigm for ongoing infections from a “zero COVID” platform to one where we can integrate its existence into our lives. It was making the case that a non-sterilizing vaccine — as we have with COVID and other respiratory diseases such as influenza — means that herd immunity as we have with something like chicken pox cannot exist with COVID.
But I now think we can have something like a temporary herd immunity. If Delta has indeed ripped through the area and infected the requisite number of people — I won’t deign to guess what that number would be — then enough people would still have circulating antibodies and be resistant to infection.
Note that I’m restricting this argument to only those who were previously infected. As we’re seeing in states like Vermont, having a high percentage of vaccinated individuals doesn’t seem to have any effect on case counts.
Why would this be? My theory is that because the mRNA vaccines only provide the single spike protein as catalyst for immune system reaction, it provides a priming for the immune system but doesn’t provide any meaningful resistance to infection. But if you’re exposed to the whole virus and are able to generate antibodies to all 29 proteins, then that provides stronger resistance to reinfection. Aaron Kheriaty, MD at University of California estimates naturally-acquired immunity to provide 95-99% resistance to reinfection.
(Side note: you should watch this testimony from Dr. Peter Doshi, associate editor of the BMJ, about why the COVID vaccines should really be thought of as a therapeutic instead.)
So if the recorded 130,000 cases in St. Louis County, plus the unrecorded cases — up to 24x the recorded number per this AMA article on Missouri — are enough to provide a lack of targets for the virus, that would keep it suppressed.
Seasonality is still at play here.
St. Louis weather always is hotter later into the fall than you remember. Every year, I think, “Last year was much cooler than this. This sure is a weird fall we’re having.” So I thought there was a chance that this year was unseasonably warm, pushing back an October case surge.
In 2020, we reached a monthly high of 89 degrees on October 7, and had nine days in the 80s persisting until the 22nd. This year, we had a monthly high of 88 degrees on October 10, and had eight days in the 80s persisting until the 24th.
So 2020 and 2021 are pretty close to identical. Although, the average daily temp was 56.5 in 2020 and 63.9 in 2021, so it’s possible that milder temperatures outside prevented people from going inside as much.
Maybe there was more rain last year that drove people inside? 2021 had 11 days with any recorded precipitation. 2020 had 10 days with recorded precipitation, but they occurred with much more frequency in the back half of the month. So it’s possible that the wet weather was driving people inside, causing more indoor transmission in 2020 than in 2021.
If this is true, as the weather starts to cool more considerably, indoor in-person transmission will kick in, and our winter surge will simply start later than it did last year.
For this hypothesis to be true, I’d expect case counts to kick up within the next week or so. We’ve seen our percentage of positive cases increase, but I’m not sure what it means for that to increase and the case counts to stay low. That would have to mean fewer tests are being run, which does appear to be the case from the county’s reporting.
In any event, the platform I want to switch to is getting away from case counts and to hospitalizations. The argument from public health officials is that vaccinations should lead to fewer hospitalizations as they decrease infection severity. So we should something like this chart of St. Louis COVID hospitalizations not have its peaks go as high as before.
The virus is now endemic, so the pandemic is over once enough people have gotten the virus, obtained their full memory B- and T-cell response to the full virus structure, and will now get nothing more than a cold when exposed to it again.
That leads to my updated prediction for what will happen over the next two months, based on the above observations. Either:
A. My previous prediction was correct and this will be another COVID winter, but the area’s seasonality hasn’t kicked in yet. If this is true, we should see cases rise sharply within the next few weeks.
B. The Delta wave that came through Missouri and St. Louis through August and early September provided enough infections that we should be considered “at herd immunity” for this winter. If this is true, we should see cases either stay flat or rise softly, but this would lead to the return of the flu, which should get picked up by county reporting here.
We will certainly have our answer by Christmas, but hopefully I’ll have indicators by Thanksgiving that will show one direction or another. I will keep my investigating vest on.