How Does Contact Tracing Work? Answers from a Stanford Epidemiologist

As states continue to reopen, administrations are using contact tracers to track down and control the spread of COVID-19. We speak with Dr. Yvonne Maldonado, an infectious disease epidemiologist and professor of public health at Stanford University, about contact tracing, stopping the spread of the virus and reopening.

In April, former CDC director Dr. Tom Frieden shared a 4-step plan to reopen and slow the spread of COVID-19: test for COVID-19, isolate infected persons, find and trace down their contacts and quarantine those contacts for 14 days. This plan involves at least two key administrative endeavors being carried out on a massive scale: testing (read more here) and contact tracing. 

The latter is an increasingly popular term thrown around by experts and politicians alike as a tool to reopen the country. As testing increases, the need to utilize that information to stop further spread has led to efforts to greatly increase the number of contact tracers in each state.

So how does contact tracing work? Dr. Yvonne Maldonado, a professor of infectious diseases at Stanford University and chief of pediatric infectious diseases at Lucile Packard Children’s Hospital at Stanford, said it’s all about controlling the chain of transmission.

“The idea here is that if you can find those infected people, you can track down the people who they expose, so they can be protected […], and also to keep them from exposing other people so you prevent a chain of transmission,” she said.

The U.S. is still seeing tens of thousands of cases daily, and without a vaccine available for at least another 12 to 18 months, contact tracing is one of the only tools available to limit transmission of COVID-19 while attempting to get back to some sense of normalcy.

“The lack of a vaccine or any treatment right now really means that [contact tracing] is pretty much the only way we’re going to be able to track and prevent disease. It’s really by identifying infected individuals and their contacts and making sure that we can keep them from spreading the disease to other people—that’s almost all we can do right now,” she said.

Contact tracing has found success internationally, with some countries able to reopen with minimal risk to the public. For example, South Korea has been able to wind down social distancing measures due to intensive testing and contact tracing with data showing the peak number of daily cases never peaking above 1,000 people and total number of deaths under 300. In a recent incident in late May, through contact tracing, South Korea found over 45,000 people connected with a small surge in COVID-19 cases. 

Taiwan is another country that doubled down on contact tracing efforts early on; according to Taiwan’s CDC, the total number of COVID-19 cases has not exceeded 500.

The U.S., however, is still working to ramp up its contact tracing and set standard protocols. Maldonado said that the CDC has set guidelines for contact tracers to more easily narrow down contacts, defining a close contact as someone who has been within 6 feet of and exposed 15 minutes or more to an infected individual. 

“You really have to engage people and know how to really track down specific details: Where were you? Who were you with? How long were you with them? Those are the kinds of things that are very simple, but at the same time very detail-oriented, [that] will really help you identify those individuals,” she said.

The CDC has also set recommendations for the window of time that contact tracers should use to find and quarantine contacts of infected individuals. The period begins 2 days before either infected individuals start showing symptoms or asymptomatic carriers test positive for COVID-19. A paper produced by John Hopkins estimated that, excluding asymptomatic cases, 50% of infected individuals will show symptoms by day 5 of infection. Quarantining contacts would reduce potential transmission before symptoms even start to appear. 

To understand how the virus is spreading, we can look at a value known as the effective reproduction number, Rt, which represents the number of new infections caused by an infected individual. This number will change as a result of gained immunity to the virus (vaccines) or public health interventions (social distancing, stay-at-home measures, etc.). This should not be confused with the basic reproduction number, R0 (pronounced R naught), which assumes no existing immunity or public health intervention. The R0 of COVID-19 has been estimated to be around 2-3, but some estimates say it could be higher.

The goal should be to reduce Rt below 1, which would indicate a reduction in the number of new cases of infections from existing infections. Over the next few weeks, the Rt in each state will also give a picture of how preventative measures like contact tracing have been working to combat virus spread.

“If we want to keep those numbers low and dropping overtime, you want to make sure that you start finding those contacts and making sure that they don’t continue to create that chain of transmission,” Maldonado said.

Many states are reaching out to hire contact tracers and recruit volunteers. Ohio currently plans to hire close to 2,000 contact tracers (17 workers per 100,000), Recommendations from the National Association of County and City Health Officials, however, suggest that 30 professionals per 100,000 would be effective in this pandemic.

“I think we clearly could use more [contact tracers]. I think that’s really going to be critical as we move ahead because we know the virus is still spreading, so we know people are still getting infected,” she said.

There has been increased interest in contact tracing with tens of thousands of people applying for these positions and still others interested in volunteering. Maldonado said that one thing that needs to happen is mass training of contact tracers. The University of California, San Francisco has a program that is currently repurposing and training 10,000 state workers to be contract tracers.

The government and other institutions are working to make training more easily accessible; the CDC has also provided training resources, and John Hopkins is providing a free course on contact tracing. 

But while these initiatives are focused on limiting the spread of the virus, contact tracers do more than just ask questions; in each interaction with a potentially infected person, they are tasked with engaging and calming the fear of people in their communities.

“You have to have the diagnostic procedure to identify infected individuals and then you really have to talk to them and, really, a lot of this contact tracing is a gift of people learning how to engage others,” she said.

While we are consulting experts in the field to get answers to important questions during this crisis, new information and studies come out almost every day and much remains unknown regarding COVID-19. Midstory encourages everyone to follow all public health and safety protocols and exercise extreme caution.


Please enter your comment!
Please enter your name here