Interpreting the Biobot Spike

December 8th, 2021
covid-19
Boston has a really interesting program where it measures covid RNA frequency during sewage treatment. It's been running since the beginning of the pandemic:

One thing I really like about this as a source of data is that it avoids some of the problems with tracking tests. It has no dependency on the fraction of people who decide to get tested, the reasons people decide to get tested, or the timing of tests. Sometimes they are slow in getting out updated numbers, but they always know which day a sample is from.

They publish the full data (pdf), and I had to go at making a chart that's a bit less busy:

As I wrote on Saturday, we are seeing a serious spike right now, and since then we have one more day of data showing that it's continuing.

In his most recent post on Omicron, Zvi wrote:

That spike on the right has two of the three highest single-day measurements, and they were the last two days of data reported. This can't represent Delta cases alone unless it's a data error, because the rise is too rapid given what we know about conditions. If it's Omicron...

I don't think this can be Omicron yet, because MA is doing variant testing, and as of Saturday detecting Omicron was still newsworthy. To see whether this can be regular Delta, let's compare to last year:

This is a similar pattern from fall 2020, just scaled up a bit. In the last month (November 6th to December 6th) we had 4.4x growth in 2020, and 4.9x in 2021. In the last week (November 29 to December 6th) we had 1.6x growth in 2020 and 2.1x in 2021. When you consider that in fall 2020 we didn't even have Alpha yet, let alone Delta, this seems like it is probably entirely coming from the time of year (seasonal change plus Thanksgiving).

On the positive side, however, at this time last year it leveled off for the winter aside from a short post-holiday spike, and if we're lucky we'll see the same this year.

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