Wednesday, August 5, 2020

Contact Tracing: Tracking the Virus To Clusters of Infection

The purpose of Contact Tracing is to find possible infections and stop the spread before it goes further. When someone is tested and the result is positive for COVID-19, the case is turned over to the tracers and the investigation begins to find those contacts who may be infected, but do not know it yet. If the cases are found soon enough and the patients are quarantined quickly, they either get better or are provided more intensive care (Tracing Principles). The important thing is that they would not be able to transmit the virus to one of their contacts. If not enough of these cases are found, the virus continues to spread and, at some point the hospital beds fill up, treatments become depleted, more health workers get sick, and the system breaks down.

In the last post, I introduced the idea of Contact Networks as a tool that can be used to visualize an infection and from which certain conclusions can be drawn that might help mitigate the spread of COVID-19. The networks are built using data from the tracing investigations. Groups of individuals can be assembled, branching out from the original patient. The map of Winnipeg in the last post showed a way to add a geographic dimension to the network in order to highlight certain influences that are spatially coincident with patients. Sometimes, however, it is not the physical distance between these features and patients that is important. By incorporating sites as nodes in a Contact Network along with contacts, a clearer picture of their relationships becomes apparent.

An example of this sort of graph was created by researchers at the University of Arizona, Tuscon, They used data from the SARS virus outbreak in Taiwan in 2003 (SNA For Tracing).


The network focuses on patients who have had contact with hospitals where outbreaks of the virus were active. In the graph above, major clusters of patients surrounding each hospital have been removed. What remains are those individuals who act as bridges between clusters. This additional interaction adds to the possible spread of the infection.

For COVID-19, as contact tracing data is analyzed, it is becoming more likely that most of the transmission of the disease occurs at places where people gather. This includes workplaces, special events, recreation areas, bars and other social gatherings. The exception to this trend is when the majority of the attendees are wearing masks, social distancing or outside with free air flow.

The evidence for these conclusions comes from a number of reports made by health departments regarding the status of the disease in their area. On July 1, after a month of starting to reopen, the director of Public Health Dane County in Wisconsin announced new restrictions on bars in response to an increase in cases (Public Health). Data from contact tracing of 614 new cases for two weeks in June indicated that 45% of patients had attended parties outside their home. The Tavern League of Wisconsin criticized the new order, saying it was unfairly penalizing bars over other activities like protesting. It was during this time that nightly protests in Madison were drawing hundreds of people in response to the police custody death of George Floyd.

The county released more detailed data, showing that 21% of the patients said they had been to bars during that time, but only 2% had attended the protests.


Many of the protesters were wearing masks in news footage and the crowds were outside in the street with a free flow of air. This is just another indication that personal preventive measures can help bring the positive cases down. Gatherings without protection and inside allow the virus to spread quickly.

In July, the Lincoln County, Oregon, public health director presented data to the Board of Commissioners showing that most local transmission was due to outbreaks, rather than out-of-county visitors (sources of infection). An outbreak was defined as two or more cases in separate households linked to a single event or location, To illustrate the spread of the disease, the health department produced a chart using contract tracing data for four individuals. These original cases were responsible for the infection of 58 additional people over several weeks.

It was not known how the original patients got the virus, but three of them were responsible for 10 workplace outbreaks affecting 39 people. These and other outbreaks resulted in 73% of the positive cases in the county. Additional cases came from household transmission or sporadic instances of community spread. The position of nodes in the diagram above has no relation to their actual location relative to each other within the county. This helps visualize how the links between cases and gathering places are related.

When restrictions are lifted on residents of different jurisdictions, it is important for people to remember that it does not mean the virus is gone. The virus, according to most experts, is here to stay for quite a while, even after the vaccine becomes available. No matter how young or old you are, or how unafraid you are, if you catch the disease you may be lucky and not suffer a long illness. You will, however, spread the virus to anyone you come in contact with unless you are careful and protect them from possible infection: wear a mask, stand apart, wash your hands.

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