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How Does Contact Tracing Work, Exactly?

Shape logo Shape 5/12/2020 Korin Miller
a woman standing in front of a computer: Experts say contact tracing is a key practice in slowing the spread of COVID-19 as states begin to re-open. But does it really work? © Aziz Karimov/Getty Images Experts say contact tracing is a key practice in slowing the spread of COVID-19 as states begin to re-open. But does it really work?

With more than 1.3 million confirmed cases of the novel coronavirus (COVID-19) across the U.S., the odds are pretty high that the virus is circulating in your area. Several states have now launched community contact tracing programs to try to track down people who may have been in contact with an infected person, with the hope of squashing the spread and helping the public understand their risk of becoming infected.

Never heard of contact tracing before? You're not the only one, but it's a rapidly growing field right now. In light of the increased need for contact tracers, Johns Hopkins University has even rolled out a free online contact tracing course for anyone who wants to learn about the practice.

Here's what you need to know about contact tracing, plus what you can expect if you're ever approached by a contact tracer.

What is contact tracing, exactly?

Contact tracing is an epidemiological public health practice that works to track down people who have had contact with a person infected with a contagious disease (in this case, COVID-19), according to the Centers for Disease Control and Prevention (CDC). Contact tracers let people know they've been exposed to an infectious disease and follow up with them regularly to provide instructions on what to do next. Those follow-ups can include general disease prevention advice, symptom monitoring, or directions to self-isolate, among other guidelines, depending on the situation, according to the World Health Organization (WHO). Contact tracing isn't new with COVID-19—it's been used in the past for other widespread infectious illnesses, such as Ebola.

In the context of COVID-19, people who have had known contact with someone with a confirmed case are encouraged to self-quarantine for 14 days after their last exposure to the infected person to try to stop the chain of coronavirus transmission, according to the CDC. (Related: When, Exactly, Should You Self-Isolate If You Think You Have the Coronavirus?)

"The basic concept is that, as soon as a patient is identified as positive for COVID-19, they are interviewed by a contact tracer to understand all the people they've had face-to-face contact with during the period of time in which they were likely to be infectious," explains Carolyn Cannuscio, Sc.D., director of research for the Center for Public Health Initiatives at the University of Pennsylvania. "We try to get that interview quickly and to do it as thoroughly as possible."

Contact tracing is done on a local and state level, so the approach can vary depending on where it's done, says epidemiologist Henry F. Raymond, Dr.P.H., M.P.H., associate director for public health at The Center for COVID-19 Response and Pandemic Preparedness at the Rutgers Global Health Institute. For example, some jurisdictions may look for everyone who has had close personal contact with an infected person in the 14 days prior to their diagnosis, while others may only consider contacts within a shorter period of time, he explains.

Who might be approached by a contact tracer?

The key here is having "close personal contact" with someone who's been infected, says Elaine Symanski, Ph.D., a professor in the Center for Precision Environmental Health at the Baylor College of Medicine.

While contact tracing is largely done at a local and state level, the CDC has issued guidance on who exactly should be contacted in the COVID-19 outbreak. Under that guidance, a "close contact" during the COVID-19 pandemic is defined as someone who was within six feet of an infected person for at least 15 minutes, starting from 48 hours before the patient began experiencing symptoms until the time they were isolated.

Close personal friends, family, and coworkers of an infected person are the most likely to be contacted, says Cannuscio. But if you just happened to go grocery shopping at the same time as an infected person, or passed them during a walk around your neighborhood, it's unlikely you're going to hear from a contact tracer, she adds. That said, if an infected person was in a small space like a public bus for a long period of time, a contact tracer may attempt to track down who was on that bus and reach out to them, notes Abiodun Oluyomi, Ph.D., an assistant professor of medicine at the Baylor College of Medicine. This is where contact tracers can get into detective-level work.

"If someone is infected, there are two ways of telling the tracer who they've been in close contact with," explains Oluyomi. Patients who know for sure that they've been in contact with certain people may simply provide names and contact info to the tracer—that's easy, says Oluyomi. But if they rode a bus for a long period of time right before they were diagnosed, and they know the bus route, the tracer can sort through historical logs and bus pass data to try to find some people who rode the bus using a reusable pass like a MetroCard. "Then, you know who they are and can contact them," explains Oluyomi. Even then, though, you can't always track down everyone, he notes. In the bus example, those who used cash instead of a MetroCard likely wouldn't be contacted, he says—you simply won't be able to know who they are. "[Contact tracing is] never going to be 100 percent foolproof," says Oluyomi. (Related: Is That Simulation of Runners Spreading Coronavirus Actually Legit?)

On the other hand, if an infected patient knows a contact's name but isn't sure of their other personal information, a tracer may try to track them down through social media or other information they can find online, adds Cannuscio.

The unknowns are a challenge for contact tracers, but they're doing their best. "At the moment, [contact tracers] have to focus on contacts one knows," says Dr. Raymond. "Potentially large anonymous exposure events would be next to impossible to trace." And given that Robert Redfield, M.D., director of the CDC, recently told NPR that as many as 25 percent of all Americans with COVID-19 may be asymptomatic, tracing every single contact just isn't 100 percent possible.

Initially, contact tracers will only reach out to an infected person's contacts and stop there. But contact tracers will start reaching out to a contact's contacts if the initial contact turns out to test positive for COVID-19 themselves—confusing, right? "It's like a tree, and then branches and leaves," explains Oluyomi.

What happens next if you're approached by a contact tracer?

For starters, you'll probably speak to an actual person—this usually isn't a robocall. "It's important that people get information quickly, but our model is that human contact is very important," explains Cannuscio. "People have many questions when they hear from us, and we want to be able to support them, provide reassurance, and to help them understand how to limit the spread of the virus to people they care about. They're anxious, and they want to know what they should do."

For the record: It's unlikely a tracer will tell you who the infected person is that you had contact with—it's usually anonymous for privacy reasons to protect the infected person, says Dr. Raymond. "[The focus is] on making sure contacts get the health services they may need," he explains.

The process is slightly different everywhere, but once you're contacted and told you've recently interacted with someone infected with COVID-19, you'll be asked a series of questions about when you may have last been in contact with the infected person (while you won't know their identity, you'll likely be given details such as whether they worked in your building, live in your neighborhood, etc.), your living situation, your underlying health conditions, and whether you currently have symptoms, explains Dr. Raymond.

You'll also be asked to self-quarantine for 14 days from the last date you may have been in contact with the infected person, which tracers know is a tough request. "There's a lot of behavior change we're asking people to do," says Cannuscio. "We ask them to stay out of the public sphere and to even limit contacts with their own household." You'll also be asked to monitor your symptoms during this time and will be given instructions on what to do if you happen to develop symptoms. (Related: Exactly What to Do If You Live with Someone Who Has Coronavirus)

The Difficulties of Contact Tracing

While the federal government's plan for reopening America includes recommendations for both rigorous coronavirus testing and contact tracing (among other measures), not all states that are reopening are actually following those guidelines. In states that have made contact tracing part of their reopening process, how effective is it really to preventing the spread of COVID-19?

The CDC states that contact tracing is a "core disease control measure" and a "key strategy for preventing further spread of COVID-19." Experts agree: "We don't have a vaccine. We don't have generalized viral nor antibody testing. Without these, it's hard to separate the infected from the susceptible without contact tracing," explains Dr. Raymond.

But Cannuscio says contact tracing will be more effective once the manpower is there. "In many situations, the number of cases is so high that it's really hard to keep up," she notes.

Plus, contact tracing isn't as technologically advanced as it could be. Right now in the U.S., contact tracing is mostly being done by people—tracers are doing the interviews, reaching out by phone, and even going to homes in some cases to follow up, explains Dr. Raymond. That involves a lot of manpower—much of which is currently unavailable, says Dr. Symanski. "It's very time-intensive and labor-intensive," she explains. "We're still at the stage of recruiting people that are able to do the work," adds Oluyomi. (Related: Your Fitness Tracker Might Help You Catch Under-the-Radar Coronavirus Symptoms)

But contact tracing has been automated (at least in part) elsewhere. In South Korea, private developers created apps to help support government contact tracing. One app, called Corona 100m, collects data from public health sources to let people know whether a confirmed COVID-19 case has been detected within a 100-meter radius of them, along with the patient's date of diagnosis, according to MarketWatch. Another app, called Corona Map, plots out where infected people are on a map so the data is more easily understood visually.

"[These apps] appear to have worked very well," says Cannuscio, noting that South Korea has kept their mortality rate down in comparison to other countries where coronavirus is spreading. "They have a very aggressive system that combines digital and human contact tracing. South Korea is upheld as one of the standards on how to do this," she explains. "In the U.S., we're playing catch-up because health departments don't have the resources to do this at scale."

That may eventually change. In the U.S., Google and Apple have joined forces in an attempt to automate the contact tracing system. The goal, the companies say, is to "enable the use of Bluetooth technology to help governments and health agencies reduce the spread of the virus, with user privacy and security central to the design."

When is the best time to do contact tracing?

In a perfect world, the best time to start contact tracing would be from the beginning of the identification of the disease, says Dr. Raymond. "However, that only works if you know when the beginning is and you have been proactively looking for [the disease]," he notes.

Cannuscio considers contact tracing especially crucial as states, businesses, and schools reopen. "The objective is really going to be able to very quickly ID new cases, isolate those people, know who their contacts are, and help those contacts stay in quarantine so they don't have an opportunity to keep infecting others," she says. "This is so critical to managing new outbreaks so we don't have the rapid escalation in cases as we saw in New York City." (Related: Will It Be Safe to Work Out at the Gym After Coronavirus?)

Still, contact tracing is not a perfect science. Even epidemiologists acknowledge the process is often very complicated these days. "It's unbelievable," says Cannuscio. "The meetings I'm in, everybody acknowledges that we're waking up and facing challenges we didn't expect to be facing now."

The information in this story is accurate as of press time. As updates about coronavirus COVID-19 continue to evolve, it’s possible that some information and recommendations in this story have changed since initial publication. We encourage you to check in regularly with resources such as the CDC, the WHO, and your local public health department for the most up-to-date data and recommendations.

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