What Is Contact Tracing?

Contact tracing is a critical tool for containing a disease outbreak. For COVID-19, it’s also a key part of plans like the World Health Organisation’s guidelines for easing distancing restrictions. So what is contact tracing and how does it work?

Contact tracing finds people who are at increased risk of contracting an illness

If we know that one person is sick, it’s likely that they passed the disease to others. Contact tracing is a classic epidemiology tool for finding those people. It’s simple and typically low tech: you interview people to find out who the infected person was in contact with.

Contagion, a movie that is holding up eerily well these days, has a few scenes that show a dramatized version of contact tracing. Kate Winslet’s character asks a late patient’s colleagues and partner about who she had been in contact with during a certain time period before her death.

A contact tracing protocol for COVID-19 cases is here (from nonprofit Vital Strategies, which the CDC links from their COVID-19 contract tracing page).

According to this draft protocol, after a person is identified as a positive case, they should be interviewed to determine when they were likely to be infectious. At that point, the interviewer should try to identify a list of people who were exposed, according to these criteria:

Contacts from 48h prior to symptom onset thru beginning of isolation period or thru 7 days after symptom onset and 72h after fever resolved:

1. Household members

2. Intimate partners

3. Individual providing care in a household

4. Individual who has had close contact (30 minutes as an initial threshold)

After that, the person doing the contact tracing would approach these people and notify them of their risk. Ideally this would be done anonymously, without revealing who exactly is sick (since that is another person’s private medical information). And then they would follow up with those contacts over time, to find out if they got sick. If so, they would trace that person’s contacts, too.

If an infected person went to any large gatherings while contagious, people who were also at those gatherings might need to be contacted as well. Part of the point of contact tracing is to find out who is sick, but the information goes both ways: it’s also important that people learn that they were exposed, so they can quarantine themselves and so they can watch out for symptoms.

This is a huge job

Contact tracing is low-tech, but labour intensive. Typically, health departments are trying to stop outbreaks before they spread very far, so they’re chasing down handfuls of contacts and ideally containing the outbreak quickly.

For something like COVID-19, where there are many thousands of cases, there needs to be a way to do a lot of contact tracing. The CDC and WHO have teams doing contact tracing, but they can only do so much.

China has been using an app to track people on a massive scale. The system links a person’s identification number, phone number, and information about their health. The app colour-codes your status: green if you’re fine, red if you need to stay home, and yellow if the system determines you’ve been close to someone who is sick.

But there are reasons to doubt that apps can effectively trace contacts. Does your phone really know who you’ve been in potentially infectious contact with, or does it only have a vague idea of where you’ve been? And then there are privacy concerns. Who will hold your data? What will they do with it? And what happens if you decide not to use the app? The massive-scale, 2020 version of contact tracing may be different than traditional interviews, but the details are yet to be seen.