Understanding ‘Clusters’

There is still a lot we don’t know about COVID-19, but we do know that the riskiest situations involve poorly ventilated indoor spaces, crowds, and prolonged close-contact settings. These situations can often lead to clusters or outbreaks of cases, which we have been tracking throughout our local epidemic. Tracking these is important because in order to control spread, we have to break the chain of transmission. This involves identifying locations where multiple cases are positive so that other people at those locations can be contacted, tested, and quarantined. It also involves working with the location to identify why spread might have occurred and to prevent future spread.

The Wisconsin Electronic Disease Surveillance System (WEDSS) is the system all Wisconsin health departments use for the reporting, investigation, and surveillance of COVID-19 and other communicable diseases. In this system, we use a tracking mechanism to identify and link clusters of cases, and initiate a facility investigation when one is required.

Clusters

There is no national standard for a definition of “clusters” or “outbreaks” (we use these terms interchangeably; in our data products and communications we will refer to them as “clusters”). CDC states that an outbreak “indicates potentially extensive transmission within a setting or organization” and the definition for outbreaks are “relative to the local context.”

At PHMDC, we define a cluster as two or more cases associated with the same location, group, or event around the same time. A recent example of a cluster that occurred in Dane County is an office building where staff have been working in person. One of the staff went to work while they were infectious(1) with COVID, and then two of their coworkers subsequently tested positive the following week. We then linked these three cases in WEDSS so that we know they are part of a cluster. We also initiated a facility investigation, which is described in the next section.

The number of clusters that we have identified is almost certainly an undercount of the true number that have occurred. Oftentimes we’re able to first identify a cluster if one or more workers at the same establishment get sick around the same time. It is much more difficult to identify patrons that are part of a cluster because they could have many potential sources of exposure (or many places they went while they were infectious(1)), and we don’t always have details on the specific places they went or the specific dates and times they were there. Therefore, the number of cases we have linked to clusters in businesses such as bars and restaurants are mostly linked to staff members and don’t capture the full extent of transmission that is likely occurring in these spaces.

Facility Investigation

A facility investigation is initiated when there is evidence of a cluster of cases or a strong possibility for a cluster to emerge from a single facility or setting.

We initiate a facility investigation when there is just a single case in a congregate living facility (such as a long-term care facility, skilled nursing facility, or fraternity/sorority), childcare facility, or school because these types of facilities are prone to outbreaks and can contain vulnerable populations. The Wisconsin Department of Health Services lists facility COVID-19 investigations on their website.

When we start a facility investigation, one of our staff is assigned as the investigator and works with the owner or manager of the facility to identify contacts of the person(s) within the facility who have tested positive, and ensure they are following best safety practices and taking precautionary measures to prevent spread throughout the facility. The goal is to prevent a cluster of cases or additional spread from the current cluster of cases within the facility.

A facility investigation does not have to involve just a single building or location, it could also involve a group or event. A recent example is a club sports team that had two players test positive on the same day, with a third awaiting results. They practiced together in a single facility but also had games in other facilities, including a tournament outside of Dane County. We initiated a facility investigation to make sure the players and staff are being protected with appropriate infection control practices and that any close contacts of the players who tested positive are placed into quarantine. We also investigated whether these cases were part of a cluster and whether the likely source of infection was the practices and/or games within the sports facilities.

Where did someone get COVID-19?

We can never definitively prove that an individual contracted COVID-19 from a specific person, facility, or event without genomic epidemiology, which is outside of our capabilities as a local health department. Genomic epidemiology uses a laboratory tool called “sequencing” to detect tiny differences in the genetic code of viruses. This tool can tell us if the virus in two different people is similar enough to have come from the same source. Then, using traditional epidemiology, interview data can help us identify the likely source of exposure. There is work happening on this locally in multiple labs at UW-Madison.

We use the best evidence we have available to determine whether cases are likely linked, based on multiple factors such as the date of onset of symptoms, whether someone had known contact with someone else who had COVID-19, the date and time that people who tested positive reported being at a facility or event, and whether the people who tested positive had other potential exposures.

When we say that cases occurred “in” a facility or are “associated” with a facility or event, we mean that either:

• They were at the facility or event while they were infectious(1), or
• Someone at the facility or event was their likely source of exposure to COVID-19.

Clusters and Facility Investigations in Dane County

As of October 14, we have had 3,450 cases associated with 555 clusters or facility investigations in Dane County.

Over the past month, from September 13 to October 13, we had 138 unique clusters that resulted in 428 cases among Dane County residents. Most of these clusters also involved facility investigations. Twenty of these clusters occurred in a different county, so these numbers do not show the full extent of cases resulting from these clusters:

*This does not include dorms or campus-area apartments. While we track cases that occur in these residences, it’s difficult to determine whether spread actually occurred within the residence itself.

Over the past month, from September 13 to October 13, we had 90 unique facility investigations that involved 128 cases among Dane County residents (ten of these facilities were located in another county, and that county would be responsible for the facility investigation):

How this guides our local response

During the course of the pandemic we have talked about the variable R, which is the average number of secondary cases caused by an individual with COVID-19. It’s also known as the infection rate, and is tracked here for Dane County. For example, if the average person spreads the virus to two people, the R would be 2. Looking at R can help us measure contagiousness, and we have been monitoring its value in Dane County throughout the epidemic.

Emerging studies suggest that as few as 10-20% of people infected with COVID-19 may be responsible for as much as 80-90% of transmission, and the majority of people may not infect anyone at all. If COVID-19 is a super-spreading disease, then the key to getting it under control is to prevent super-spreading situations and events.(2)

Identifying large clusters has been key in how we have approached our local response and orders throughout our local epidemic. Early on, our largest clusters were occurring in long-term care facilities. Thankfully, with the implementation of stricter guidelines and protocols, the number of clusters in these facilities has declined significantly.

In early summer, we began to see large clusters emerging from bars/restaurants: 405 cases were associated with seven bars/restaurants near the UW-Madison campus over the course of two weeks, between 6/22/20-7/5/20. These clusters were a textbook example of a super-spreader event: people were crowding into indoor spaces, not wearing masks because they were drinking or eating, and talking or yelling closely to one another. A single highly infectious person in that type of setting could have infected nearly everyone in the room. Our response of issuing a new order limiting gatherings and bar and restaurant activity was effective in curbing associated clusters.

While certain factors increase the chance of spreading COVID-19—like lots of people being indoors, close together, and not wearing masks—we can’t predict yet which events will be super spreading events and which will not. That’s why it’s so important that people avoid going to gatherings when possible, wear masks, and stay at least 6 feet from others. Even smaller gatherings can end up infecting many people, so everyone needs to be constantly following precautions so that there aren’t opportunities for superspreading events to occur. And that’s also why contact tracing is so important—we can identify and quarantine people infected from events before clusters grow out of control.

We will continue to track where clusters are occurring, keep up with the latest research, and tailor our local response based on our findings. However, we know that our data is incomplete, we’re unable to identify every cluster that occurs in our county, and there are still many unanswered questions about COVID-19. This is why it’s so important that we continue to follow all precautions, limit our exposures, wear masks, and stay home as much as possible, for the safety of everyone in our community.

1. When someone with COVID is “infectious,” that means they could be spreading COVID to other people. The period of infectiousness is defined by the CDC as two days before symptoms began (or two days before the day of their positive test for people who don’t have symptoms) to ten days after symptoms began (or ten days after the day of their positive test for people who don’t have symptoms).

2. For a look at this topic outside the scientific literature, we recommend checking out a recent article in The Atlantic by Zeynep Tufekci. Here, she discusses a lesser known variable k, which is the measure of the dispersion of a pathogen. Zeynep writes that k is “a way of asking whether a virus spreads in a steady manner or in big bursts, whereby one person infects many, all at once. After nine months of collecting epidemiological data, we know that this is an overdispersed pathogen, meaning that it tends to spread in clusters.” Another helpful article is from The Guardian.