Only 40 percent of the visitors to President Trump’s June 20 rally at the BOK Center were from Tulsa County.
Updating a recent post, the graphic above shows the daily counts of new COVID-19 cases reported by the Tulsa Health Department through July 11, 2020. The daily case counts, represented by burgundy filled circles, are measured on a logarithmic scale, as indicated on the left-hand vertical axis. The arrow indicates the timing of President Trump’s rally at the BOK Center in Tulsa on June 20.
The Tulsa Health Department, we noted in the same post, has apparently been engaged in extensive contact tracing of hundreds of newly diagnosed cases. The results of such contact tracing, we pointed out, could be highly informative about the contribution of the June 20 rally to the continuing rise in new infections in Tulsa.
In this post, we suggest that systematically tracking down COVID-19 sufferers who were exposed at the Tulsa rally is likely to prove quite difficult.
TETRIS: Testing, Tracing and Isolation
Since April, a number of think tanks, foundations, academic institutions and other authorities have issued their own formal plans to guide the reopening of the U.S. economy in the face of the ongoing COVID-19 epidemic. These white papers, for the most part, have envisioned widespread if not universal testing, contact tracing, and isolation of infected individuals (or TETRIS) as fundamental to the nation’s recovery. Much has already been written about the benefits and costs of widespread testing of asymptomatic individuals, as opposed to the current system of voluntary, symptom-based testing now prevalent in the United States. And much has likewise been said about the ethics and feasibility of compulsory isolation, as opposed to our current system of voluntary, self-imposed quarantine. The issue here is real-world task of contact tracing.
Where Did The Attendees Come From?
The map above displays a partial enumeration of the counties of origin of attendees at the president’s rally on June 20. The map is based on our analysis of the Patterns database maintained by SafeGraph, which has been following a panel of Android and iOS device users as they enter and exit numerous points of interest throughout the United States, including – it just so happens – the BOK Center in Tulsa during the month of June 2020.
The burgundy shaded county at the center of the map is Tulsa, which was the origin of 40 percent of the recorded visitors. We have divided the remaining counties of origin into those with relatively high attendance, shaded in orange, and those with relative low attendance, shaded in mango. The orange shaded, high-attendance counties, taken together, covered 31 percent of the attendees, while the mango shaded, low-attendance counties covered the remaining 29 percent. The latter group included three remote counties not shown on the map: Seminole County FL, Cabarrus County NC, and Calvert County MD, the latter a suburb of Washington DC. As we discuss in the Technical Details below, there is good reason to believe that these estimates overstate the proportion of attendees from Tulsa County and understate the proportional attendance from the other counties.
The graphic above shows the daily counts of newly reported COVID-19 cases in the same three groups of counties, computed from the New York Times database. The three groups are color-coded to correspond to the Oklahoma counties of BOK Center attendees shown in the map above. The burgundy data points correspond to Tulsa County. The orange data points correspond to the combined daily cases in the high-attendance counties in the map within Oklahoma, while the mango points correspond to the combined daily cases in the low-attendance counties within Oklahoma.
The graphic shows that case counts have been surging exponentially, at least since the end of May 2020, in all three groups of Oklahoma attendee counties. We haven’t graphed the remaining Oklahoma counties, as we cannot be sure that they weren’t the home to some BOK Center attendees. Still, the trend in the remaining counties is parallel to that seen in the graphic.
Barriers to Contact Tracing
The data show that at least 60 percent of the attendees to President Trump’s June 20, 2020 at the BOK Center in Tulsa, Oklahoma came from outside Tulsa County. To ascertain the full extent that the rally contributed to the recent rise in COVID-19 cases – if the rally indeed did so – the Health Department will have to track down cases in surrounding counties. This will nearly triple the Department’s caseload. Failure to expand the scope of contact tracing may result in quantitative findings with too little statistical power to detect an effect, what statisticians call a Type II error.
When it comes to barriers to contact tracing, the BOK Center rally is by no means an anomaly. Unless the investigators are fortunate enough to have a restricted list of attendees, tracking down potentially infected participants in any mass gathering will have to confront significant problems of scope.
Contact tracing requires skill. You can’t just give a neophyte a battery of standard questions and expect him to come up with a reliable enumeration of contacts any more than you can give a first-year medical student the standard review of systems –Do you have headaches? swollen ankles? blurred vision? no energy?– and expect him to come up with a reliable diagnosis. It would be a profound error to assume that the “TR” in TETRIS is going to be a trivial task.
During the past four months, I have personally taken the medical history of dozens of patients who have come down with COVID-19. Occasionally, a patient will recall that her workplace has been closed because one of her coworkers tested positive. Sometimes, another will recall attending a birthday party. But a substantial proportion live in a household where multiple family members are sick and, because nearly everyone came down with symptoms at about the same time, no one is sure who gave it to whom. Some older patients will conjecture that their adult children in their 20s and 30s may have brought the virus home, but without interviewing the children, we don’t really know.
We have characterized the enumeration of counties of origin as partial because the SafeShare Patterns database covered only a sample of the full universe of attendees to the rally or, for that matter, to any of the points of interest in the database. Of the 1,434 visits to the BOK Center included in the data record for the month of June 2020, a total of 891 (62%) occurred on June 20, leaving an average of 19 daily visits for each of the remaining 29 days of the month. For the entire month of June, but not for each individual day, the database gave a breakdown of visitors by census block group of origin. These data were then aggregated into counties of origin for the construction of the map above.
Accordingly, one limitation of the analysis is that we have data on the origins of visitors for the entire month, and not just for June 20, the day of the rally. However, location exposure (LEX) data from PlaceIQ indicate that, on ordinary non-rally days, only about 4.8 percent of the devices pinging from Tulsa County did not originate from that county, while the corresponding proportion of “foreign” devices was 7.3 percent on June 20. (More precisely, other than the day when the Tulsa-based ping was detected, a “foreign” device emitted no pings from Tulsa during the prior two weeks.) This observation suggests that the inclusion of non-rally days in the construction of the map has resulted in an upward bias in the proportion of rally attendees from within Tulsa County.