4/25/24: Statewide Covid levels held steady last week, remaining at 1.8% positivity. If you gathered 39 residents together at random last week, there was a 50/50 chance that at least one of them was Covid positive.
Covid tracking for New York available here.
Here is more of the available data, including—for perspective—the very high peak from Omicron in January 2022, which is trimmed off in chart above:
This curve is a statewide aggregation of wastewater data reported by the Massachusetts Department of Public Health. (See below.)
In Hampshire county, where I live, wastewater is currently monitored for Covid in three sewer systems: Amherst, Northampton (including Williamsburg), and South Hadley (including Granby). The following chart shows the Covid positivity rates for each town for every wastewater sample analyzed since March 1, 2024:
At the 8% positivity level, if you gather 8 people together at random, there's a 50/50 chance that at least one of them is Covid positive.
At the 6% level, it takes 11 people to reach the 50/50 level;
at 4%, it takes 17 to reach 50/50; at 2% it takes 34;
and at 1% it takes 69 people to reach the 50/50 level.
Death Rates
Another method of tracking Covid is to look at death rates. The chart below is the weekly death rate (from all causes) in Massachusetts for 2015-2019, the five years leading up to the pandemic. It shows the normal seasonal variation in deaths (higher in the winter), and typical variations from the norm:
The gray dots are the weekly figures, the blue line is a smoothed running average, and the red line is a seasonal average for 2015-2019, repeated annually.
Here is the data for the years 2020 to present. The elevation of the blue line above the seasonal-average red line shows the effect of the pandemic:
The first peak, in April 2020, is from the original "Alpha" version of Covid. The second peak, in January 2021, is from the Delta variant, and the third peak, in January 2022, is from Omicron (BA.1). The peak in January 2023 was modest, but deaths ran 8.4% higher than normal from May 2022 through January 2023—which, for perspective, was people dying at 13 times the traffic fatality rate [5].
It is worth noting that the death rate has returned to seasonal normal levels after each of the peaks, climbing above normal levels when positivity rates rose. A fourth period of normal death rate, which began in February 2023, seems to have ended in June for unknown reasons. Covid rates were still low then; could this be long Covid catching up with us?
How Positivity Rates Are Calculated
The Massachusetts Department of Public Health used to report hospital data that made it possible to estimate the Covid positivity rate of the population at large in the state. (An essay describing this method and its importance can be found here: Hospital Covid Tracking.) They stopped reporting this data at the end of September 2023, blinding one of our eyes to the ongoing extent of the pandemic. In the wake of this loss, wastewater tracking became the only way of monitoring Covid prevalence in the state.
Covid levels in wastewater are reported in ECRA units: effective Covid virus copies per milliliter of sewage. By comparing wastewater Covid levels with hospital-derived positivity rates during the time when both of these figures were available, it's possible to choose a scaling factor that converts wastewater ECRA units to positivity percentages. The value I'm using is 1/210. Here's how the two curves match up:
The wastewater curve tends to lead the hospital data by a week or so, but they match up pretty closely otherwise. (By the way, I'm using the scaled hospital positivity data, as described in Note [6] of the hospital tracking writeup, which is my best guess at actual positivity levels. The best fit for the unscaled positivity data is ECRA divided by 180, which results is somewhat higher estimated positivity levels.)
To produce the data shown in the charts, wastewater Covid data is aggregated by week, Sunday through Saturday, and labeled with the Wednesday of the week. All readings from a particular sampling location during the week are averaged, and all the location averages are weighted by sewershed population and averaged to produce one statewide ECRA figure for the week. The sewershed population is the sum, for each town feeding into the sampling location, of the town population multiplied by the percentage of the town connected to the sewer system. The individual ECRA measurements, town populations and sewer percentages are all taken from the Mass. DPH's wastewater database.
