What can school boards learn from IDPH’s contact tracing data?

The Illinois Department of Public Health (IDPH) has started to release COVID-19 contact tracing data ( As the IDPH explains, COVID-19 contact tracing is conducted mainly by local public health departments and involves identifying people with confirmed COVID-19 cases and their close contacts and asking those people to quarantine at home. A close contact is a person who was within 6 feet of a confirmed case for at least 15 minutes at any time starting 2 days before the person with the confirmed case first experienced symptoms or, if the person with the confirmed case is asymptomatic, starting 2 days before the person with the confirmed case first took a positive COVID-19 test. Of the 191,960 confirmed cases in Illinois from August 1 through October 24, health departments attempted to contact 69% of those people and actually interviewed 54% of those people, representing 102,864 people with confirmed cases who were interviewed. Those interviews identified 171,905 close contacts and health departments attempted to contact 70% of those close contacts and actually interviewed 57% of the close contacts, representing 97,314 close contacts who were interviewed (

IDPH is releasing data on the locations of actual COVID-19 case outbreaks and potential COVID-19 case exposures that were identified through the contact tracing interviews. An outbreak means that five or more cases from different households are linked to that location within a 14-day period ( An exposure means that a person with a confirmed case visited that location during the 14 days before the person with the confirmed case first experienced symptoms or, if the person with the confirmed case is asymptomatic, during the 14 days before the person with the confirmed case first took a positive COVID-19 test ( In its initial release, IDPH provided data on outbreak locations for two different time periods – from July 1 to November 6, and from October 8 to November 6. IDPH provided data on exposure locations only for the period from October 8 to November 6. (IDPH did not explain why the contact tracing data were updated only through October 24, while the outbreak and exposure locations data were updated through November 6.)

In addition to the general data that identifies categories of locations with outbreaks and exposures, IDPH released more specific data for the outbreaks that were linked to schools and the potential exposures that occurred in schools. The outbreak data identify the school and the approximate number of cases involved in each outbreak ( The exposure data identify the number of potential exposures for each school (

Although it is certainly helpful that IDPH has started to release these contact tracing data, it is difficult for school boards to use these data to make decisions about school instruction types without additional analysis. I have already seen several news articles with statements that schools are the third largest exposure risk or that schools are the largest source of new cases. Both of those statements are misleading, as I will discuss below.

The potential exposure data are particularly difficult to use. To consider the potential exposure risk at a location, a person would like to know the probability of contracting the virus at that location if the person follows recommended safe practices. But IDPH’s exposure data tell us only the number of people who visited the location within 14 days of having symptoms or taking a positive test, which has two significant limitations.

First, the 14-day period used by IDPH is very conservative. Researchers believe that people become contagious only 2-3 days before they develop symptoms and that people who test positive but never develop symptoms are not likely to be contagious after 10 days ( So, it seems very unlikely that someone who first developed symptoms or took a positive test 14 days after visiting a location would have exposed people at that location.

Second, even if a person is contagious and visits a location, the risk of another person contracting the virus from that person at that location obviously depends on many other factors, especially how closely and how long those two people interact with each other, if at all, and the safety practices that those two people observe. And those factors vary significantly by location. Combining the lists of exposure locations and outbreak locations helps to show the relative risk of each location, although this task is made a little more difficult by the fact that IDPH uses slightly different categories for the locations of exposures and outbreaks. However, I matched the two lists as closely as I could and the table below shows, for the 19 location types with the most potential exposures, the number of potential exposures, the number of outbreaks, and the percentage of potential exposures resulting in outbreaks over the 30-day period from October 8 to November 6.

We cannot make too fine of distinctions based on only one month of data. However, there appear to be three basic categories of risk in this table. Community events and mass gatherings seem to be the highest risk locations, with almost 2% of potential exposures leading to outbreaks. Places of worship and correctional facilities seem to be the next highest risk locations, with almost 1% of potential exposures leading to outbreaks. And all other locations, including schools, seem to be of lower risk, with no more than 0.5% of potential exposures leading to outbreaks. Most locations fall into this last category; with more data, we might be able to make further distinctions within that category. Therefore, although it is true that schools had the third most potential exposures during this period, schools seem to be in the lowest risk category in terms of the percentage of those potential exposures that resulted in actual outbreaks.

The outbreak data are also difficult to use and there is a risk that school boards will place too much importance on the relative size of each pie slice, without considering the overall size of the pie. It appears from the IDPH data that relatively few new COVID-19 cases are associated with outbreaks of five or more cases at a single location, other than long-term care facilities. The IDPH data counted 76 outbreaks from October 8 to November 6. However, those data did not include outbreaks at long-term care facilities, which are reported on a separate IDPH page ( And it seems that those data also did not include outbreaks in the City of Chicago, which were reported in a separate document prepared by the Chicago Department of Public Health ( Congregate & Community Outbreak Responses 11062020_FINAL.pdf). IDPH did not report the number of cases that were included in those 76 outbreaks, but it seems likely that fewer than 1,000 of the new cases reported from October 8 to November 6 were associated with those 76 outbreaks. Over that 30-day period, more than 155,000 new cases were reported in Illinois; about 36,000 of those cases were from the City of Chicago, so about 119,000 new cases were reported in the rest of the state. Thus, it seems likely that less than 1 percent of the new cases in the rest of the state over that 30-day period were associated with the 76 outbreaks. So, the data on the locations of outbreaks are not that helpful, considering that they seem to include the sources for less than 1 percent of new cases. And while it is true that more outbreaks occurred at schools than at any other type of location (other than long-term care facilities) during this period, the number of new cases associated with those school outbreaks was still very low – likely fewer than 100 new cases, which represented less than 0.1 percent of the new cases in the state (other than Chicago) over that period. To know how significant schools are as a source of new cases, we would need source data for the 99% of new cases that were not associated with these 76 outbreaks.

The data on the specific schools with outbreaks are also not as helpful as they could be, both because such a small percentage of new cases are associated with outbreaks and because the data don’t show what type of instruction each school was using when the outbreak occurred. Of the nine schools that were listed as having outbreaks, it appears from their websites and from news sources that all nine of the schools were using primarily in-person instruction at the time of their outbreaks. But it would be helpful if IDPH would provide that information, so school boards could better evaluate the risks of the different instruction types.

In summary, none of the contact tracing data released by the IDPH are very helpful for school boards in making decisions about instruction types. But, in general, nothing in the contact tracing data that have been released so far disagrees with the general conclusion that experts are increasingly reaching, which is that K-12 schools have not been a major contributor to the spread of the pandemic. The clearest support for this conclusion in the IDPH contact tracing data was a chart that was released with those data, but that was not actually derived from the contact tracing data, which showed weekly new cases among different age groups of children and young adults in Illinois ( That chart shows that there was a significant spike between August 24 and September 6 in the number of cases among people 18 to 22 years old; this spike is consistent with news reporting about case spikes among students at colleges after classes resumed in the fall. However, there were no similar spikes in the number of cases among the two K-12 school age groups. All of the case numbers do trend upwards starting in early October, but that pattern is consistent with the overall upward trend in case numbers since then. I have reproduced IDPH’s chart below for convenience.

For more information on the effect of K-12 school instruction types on new COVID-19 cases in Illinois, please see my other blog post ( and the NPR news article that I link to at the end of that post.