Post #1122: COVID-19 trend to 4/24/2021

Posted on April 25, 2021

The incidence of new COVID-19 cases per day is down about 17% from the peak of the U.S. fourth wave.  Or, at least, from what I hope was the peak, about ten days ago.

Currently, new case rates are falling about 12 percent per week.

Source for this and all other graphs of new cases:  Calculated from The New York Times. (2021). Coronavirus (Covid-19) Data in the United States. Retrieved 4/25/2021.  The NY Times U.S. tracking page can be found at

Michigan now clearly appears to be getting over its recent outbreak.  The count of new cases per day remains at a high level, but it’s trending sharply downward.  Most Midwest states also have a modest downward trend in new cases per day.

No other crisis areas appear on the horizon, but a handful of states still have a persistent upward trend in new COVID-19 cases per day.  Among these, Oregon and Washington stand out for having more than a month of steadily rising new case counts.

It’s difficult to guess what the issue is there.  First, they have a lower-than-average proportion of the U.K. variant.  As of the two weeks ending 3/27/2021, 26% of the new cases in that region were the U.K. variant, versus nearly 47% for the U.S. as a whole (per this CDC web page).

Source:  US CDC.  Arrows and red text added.

Second, they have a higher-than-average proportion of the population vaccinated.  Certainly, the vaccination rate there is as high or higher than in Texas, Florida, or Tennessee, states that all have much higher proportion of the U.K. variant, but no pronounced upward trend in new cases per day.  (Below, darker = higher vaccination rate).

Source:  US CDC.

One thing these states have in common is that they resumed all high school sports in late February.  (Per this reference, Oregon, and this reference, Washington).  The timing of that — about two weeks before new cases began to rise — fits the data.  And yet, there seems to be no news reporting that points to high schools as a main area of COVID-19 spread, as there was for Michigan.

As a bit of due diligence, let me find the data for one of those states and plot the trend in new cases, by age group.   And that’s clearly not the issue.

But one final thing these states have in common, and have in common with Michigan, is that up to now, they’ve had a relatively small share of the population infected with COVID-19.  The chart below is old (as of mid-January), but I’m sure it’s still shows the approximate relative ranking.  These two states have a much lower proportion of the total population with immunity to COVID-19, and a much higher proportion who are still susceptible, compared to the U.S.  Thus, as with Michigan, the likely story here is that they are now paying for their past success in suppressing spread of the virus.   They are farther from herd immunity than is the average U.S. state.

The only truly dark cloud for the U.S. fourth wave of COVID-19 is a falling rate of COVID-19 vaccinations.  You can’t trust the news reporting, because they are always in the business of finding and reporting the outliers.  But the CDC plot of doses administered, for the U.S. as a whole, by date reported to the CDC, does in fact show a recent peak.

The extent to which this is attributable to the “pause” in administrations of the J and J vaccine is not clear.  It looks one could plausibly attribute most of it, but not all of it, to that J and J vaccine pause, based on data from CDC:

Source:  New York Times.

That’s just something to keep an eye on.  Will that rate bounce back up, now that the J and J vaccine is available again?  Or have we actually passed the peak rate of vaccinations.  Given that there’s no hint of hitting herd immunity yet, in any U.S. state, a continued slowdown in vaccinations suggests that the end of the pandemic in the U.S. may be a long, drawn-out process.