Contact Tracing Explained

April 15, 2020

What contact tracing is, what is isn’t, and why it’s so important.


From movies such as Contagion and World War Z to the real-life story of Mary Mallon, also known as Typhoid Mary, most of us had heard the term “patient zero” long before the current coronavirus pandemic. However, the same is not true for the term contact tracing, an old-school epidemiological tool for studying and documenting the chains of transmission of infectious diseases during outbreaks.

Recent calls for contact tracing as the key to ending the coronavirus pandemic and lessening the detrimental impacts of social distancing, isolation, and quarantine have inspired well-meaning technology companies both large and small to propose technical solutions for contact tracing. As a result, the term is now being applied to very different techniques that are being facilitated by very different technologies. The result is a degree of bafflement and complexity that has left decision makers perplexed and indecisive about the nature and application of an essential toolset against COVID-19.

Let’s see if we can clear up some of this confusion. 


Selective focus photo of an outstretched hand reaching upward to several chains dangling downward against a dark background. Image credit: Zulmaury Saavedra via Unsplash
Image credit: Zulmaury Saavedra via Unsplash

Contact Tracing Defined

The “Introduction to Epidemiology” materials provided by the U.S. Centers for Disease Control and Prevention (CDC) note that infectious agents may be transmitted to susceptible hosts in multiple ways, including through modes such as direct contact or droplet spread, or indirectly via airborne, vehicle-borne, or vector-borne (mechanical or biologic) spread. The term chain of transmission refers to an infectious agent leaving its reservoir or host through a portal of exit, being conveyed by some mode of transmission, and entering a susceptible host through an appropriate portal of entry.

Contact tracing identifies these chains and modes of transmission during infectious disease outbreaks by documenting exactly how and through whom a disease has spread. This is typically accomplished through a series of interviews with patients, caregivers, and “contacts” (in other words, anyone who has been in close enough contact with a patient to have been exposed through the disease’s mode of transmission). Although contact tracing is resource-intensive, it need not be as laborious as recent reports have suggested. For one thing, targeted approaches based on scientific hypotheses and carefully crafted to be fit-for-purpose can greatly decrease the number of contacts required to understand a given chain of transmission. However, incorrect perceptions about the time and resources required for traditional contact tracing has led to a deluge of tech-based partial “solutions” that claim to be superior but that in reality range from partial solutions to new capabilities that do not provide any of the same information as traditional contact tracing.

Let’s examine the main varieties and emerging themes. 


Proximity Monitoring

Row of people waiting for a train, standing and sitting against a wall, most of whom are looking at cellphones. image credit: Jens Johnsson via Unsplash
Image credit: Jens Johnsson via Unsplash

The recent and already significantly hyped Apple/Google contact tracing solution is technically a proximity monitoring application. Simply put, the application is a way for two or more cell phones to determine and report whether or not they have been within six feet of each other using Bluetooth technology and mobile apps. Users must voluntarily (opt-in) download and install the app and then, if they have tested positive for COVID-19, disclose that to the system. The app then tracks other app users that may come with six feet of the self-identified “COVID-positive” app user. 

While this may seem straightforward, in reality systems such as this are unable to provide the truly valuable information offered by traditional contact tracing. For example, no data about the nature of the interaction that would provide insight into the mode of transmission is known or collected. And without this essential evidence, nothing about the chain of transmission by or to an infected person can be directly understood. Once we also factor in the significant privacy and security issues, the fact that only a subset of people will opt-in to such a system, and the limitations of the Bluetooth technology (including the likelihood of large numbers of false positives) the only credible way to interpret such data is to assume that it comprises a gross underestimation of the Brownian motion of humans during the COVID-19 pandemic.

Such underestimation carries with it the potential for significant harm if these flawed inferences are used to inform policies on social distancing and quarantine that ultimately result in a second wave of the outbreak. If we are to truly understand the modes and chains of transmission of COVID-19, we will need better tools and processes.


Active and Passive Contact Tracing

Traditional contact tracing can be classified as active or passive. Active tracing involves proactive data collection by health system staff, while passive contact tracing relies upon voluntary response to surveys and other calls for information. Both are extremely useful, but both also have inherent limitations. The usefulness of active tracing is typically constrained by the number of tracers, the ability to get in touch with identified contacts, and the limitations of memory and willingness of contacts to share sensitive personal information. Passive tracing on the other hand can have a very broad reach, but is limited to the subjective responses of survey participants and is dependent on the good will of the community to respond candidly and at the necessary scale. 

The importance of this cannot be overstated: when done well, active contact tracing is an intervention proven to improve health outcomes.

However, there is one other significant differentiator between these two approaches, and that is the opportunity created by the interaction between the contact tracers and those they are approaching. Active contact tracing, whether done in-person or via phone or video, not only enables objective data collection but can also constitute a highly effective healthcare intervention. For example, one tuberculosis study that compared active and passive contact tracing found that the health outcomes of the actively traced cohort were significantly better than the passively traced cohort due to improved adherence to medical guidance that resulted from the educational materials shared by and discussed with contact tracers. The importance of this cannot be overstated: when done well, active contact tracing is an intervention proven to improve health outcomes.


The Massachusetts Approach

This intervention opportunity is clearly a driver behind the major contact tracing initiative announced recently by the State of Massachusetts in partnership with Partners in Health (PIH). Having led the digital technology efforts for PIH during the 2014-2015 Ebola outbreak, I have seen firsthand how PIH uses contact tracing and other community health worker interventions to not only turn the tide during a crisis but to also strengthen healthcare systems in the longer term. This can be seen in their current initiative, which according to PIH will “…contact COVID-19 patients, learn about their recent public activities, and ensure they can take appropriate steps to get healthy and not spread the virus further.” Such an approach is well suited to improving our understanding of the modes of and chains of transmission.


A photograph showing a computer screen displaying a data dashboard tracking the number of worldwide COVID-19 cases as of April 9, 2020. Image credit: KOBU Agency via Unsplash
Image credit: KOBU Agency via Unsplash

Avoiding Partial Answers Amid Unanswered Questions

Finally, partial solutions, while well-intentioned, can add ambiguity that actually makes things worse when there are still many unanswered questions. Clinicians and public health workers battling the current pandemic are doing so without medicines that have been proven effective, in the absence of a full understanding of all modes of transmission, without knowing exactly how long the virus survives on surfaces. They also do not know whether or not patients can be reinfected, how long infected patients can be asymptomatic, and other essential details about silent spreading

All of these questions are essential to understanding modes and chains of transmission. The data that active contact tracing can provide is critical for answering them, but solutions that fall short of definitive answers or that actually introduce ambiguity and additional complexity into the mix are likely to prove counterproductive.

We desperately need conclusive, actionable answers to these questions as soon as possible. Patients are waiting.


To read more blog posts from Eric Perakslis, click here.