Young adult looking at phone while sitting on steps outside. They are wearing a mask.From September 1 through September 14, 2,380 people tested positive for COVID-19 in Dane County. Of those, 76% were from UW-Madison students and staff. We are closely monitoring data for evidence that those cases have led to spread of the virus outside of UW students, which we are calling “spillover.”

Assessing spillover is challenging

Assessing the timing and direction of disease spread is extremely difficult and very hard to prove. This is why you will probably not ever hear us say there is clear-cut evidence of spillover. One of the reasons is that people often have many different possible exposures. Someone may have gone to a family get-together but also went to a sports team practice. Teasing out with absolute certainty where they got the virus and to whom they spread it to is nearly impossible without using advanced molecular sequencing techniques.

While we can’t be definitive about spillover, we do closely monitor three key data sources for evidence it might be happening. Looking at any one of these sources independently can’t give us the answers we need. We have to assess them altogether.

Case counts are one tool for assessing evidence of spillover

We are monitoring a comparison of daily, 7-day, and 14-day average case counts with and without UW-affiliated cases included. If the trends not including UW-affiliated cases are stable or decreasing, this may indicate a “contained” UW outbreak. Keep in mind that if we see increasing trends without UW-affiliated cases, this could mean there is some spillover, but it could also mean we’re just seeing increased transmission elsewhere, for instance, with the arrival of colder weather, people decide to still host gatherings, but take them inside.

Heat maps of age groups can help us visualize transfer of cases from one age group to another

We look at age heat maps to further tease out evidence of spillover. We map out the age distribution of cases across time, which helps us understand if there may be transfer from one age group to another. For example, the heat map from June 1-September 7 clearly shows the majority of recent cases are among people in the 18-24 year old age group. If we start to see cells representing other age groups get darker, that could mean there is spillover happening between age groups.

Analyzing interview notes helps us better understand the nuances of possible disease spread

We also deeply analyze interview notes to understand how spread may be happening. This is extremely time-consuming work because there is no algorithm or code that can spell out exactly what’s happening. Our team has to take the time to read the case notes—which are detailed and extremely helpful!—of people who have tested positive for COVID-19 and have been interviewed to learn circumstances of how they might have gotten the virus. Our team members doing this work are acting like detectives, trying to identify places of possible spread. If someone says, “I went to a wedding,” our team will then search through records to see if anyone else from this wedding also tested positive.  Keep in mind that a limitation of studying case notes is that they rely on what the interviewer writes down, as well as peoples’ memories and willingness to recount their potential exposures.

So are we seeing evidence of spillover?

The case count trends for Dane County without UW-Madison have increased over the past two weeks. We cannot say for sure that there is—or isn’t—spillover. We know that some of these cases are due to clusters without a clear link to UW cases—one cluster in a healthcare facility, college students from schools other than UW-Madison returning to Dane County, and daycare settings. Our heat maps still indicate the vast majority of cases are in the 18-24 year old age group, and our analysis of case notes has not clearly illustrated strong evidence of spread from UW students to the larger Dane County community.

We will continue to assess these data sources for evidence of spillover and will keep the community up-to-date in our data blog posts. It’s important that everyone across the county continues to keep up strong prevention behaviors to minimize the spread of the virus, like staying home when you can, avoiding gathering, wearing a mask, and staying at least six feet from people you don’t live with.

Image 1. Heat Map

Heat map showing cases by age group over time. The darkest cell shows people 18-24 make up the largest portion of cases in September.

Image 2. Case Counts Over Time

Chart of cases over time: The 14-day average of Dane County case counts without UW-Madison have increased over the past two weeks. There is a steady line with a slight uptick over the past few weeks.

This content is free for use with credit to the City of Madison - Public Health Madison & Dane County and a link back to the original post.