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Science News: Contact Tracing: Complex and difficult, but vital to suppressing the pandemic of COVID-19 caused by SARS-COV-2


photo by Dimitri Houtteman courtesy of

Science News published an article about contact tracing on April 29.  Since this article will probably be paywalled, I will include a thorough summary below.

The first step in isolating a case of the novel coronavirus is to identify the case.  This can be done best by a positive antigen test with a nasopharyngeal swab or a saliva sample (see this article from April 24 about saliva tests on the web site).  The Yale article is based on a study conducted at Yale New Haven Hospital with 44 inpatients and 98 health care workers and published on MedRxiv (prior to peer review) on April 16.

The saliva test is more convenient for the patient and can be done without risking contact with a medical professional, since it can be accomplished by simply spitting into a vial– even at home.  The study claims that the test is more sensitive and consistent than the nasopharyngeal swab.  The Food and Drug Administration (FDA) has already given Emergency Use Authorization (EUA) to the test as of April 13.

The saliva test was found to be effective at finding other respiratory pathogens (causes of airway illness) in previous studies here and more recently hereSaliva specimens were also found to have high viral loads in tests during the previous SARS epidemic in 2004.  The Yale article (the first cited above) concludes:

Once tests and laboratories are validated for using saliva, this could be rapidly implemented and immediately resolve many of the resource and safety issues with SARS-CoV-2 testing,” said Nathan Grubaugh, an assistant professor at the Yale School of Public Health and one of the senior authors of the study.

Another way to identify cases is by using the symptom checklist and making a presumption, based on agreement with most or all of the symptoms, that the novel coronavirus is the most likely cause.  This would be less specific and only preferable if antigen testing was completely unavailable; however, given the high false negative rates for nasopharyngeal swab tests, it may be necessary to follow up even negative tests if all of the symptom criteria match up.

Once a case is identified, a case worker would set about identifying everyone with whom the case has had contact within the exposure and incubation window.  Usually a case will pop up five days to two weeks after exposure to an infectious individual.  A chain of exposure and infection could then be set up, with the case individual exposing others to infection within another window until the case is isolated.

Contact tracers would have to identify the predecessors and those who follow on.  The people exposed to the case would be notified and urged to isolate themselves wherever possible.  The people who may have passed the illness to the case will also need to be isolated, if possible; tracing of those people will be attempted.  Setting up a chain of contacts and contactees, the contact tracer will notify everyone involved to the extent possible.

Contact tracing quickly becomes an insuperable task when the contacts are persons who have been widely exposed.  People who work in public-facing jobs, like bus drivers, grocery store workers, policemen, ambulance attendants, doormen, hair and nail salon employees, nursing home medical assistants, and nurses all must be identified.  If the original identified patient has had significant contact with such a person, it is critically important in order to reduce spread for that person to be taken out of circulation until their infectious status has been confirmed or refuted.

During the national isolation period of the last several weeks, we will find that many ill people have been infected by contact with someone working in a public-facing job.  If these people cannot be isolated, there is no chance of containing this pandemic.

In many cases, tracing from an ill person will lead back to a cluster of infections.  Isolation of these clusters, if done early enough, may reduce the spread to other groups and prevent new clusters from forming.

The Science News article describes the situation as of April 29: 5.8 million antigen tests had been performed, out of which 17 percent came back positive.  To make any headway, the article states that enough tests will be needed so that less than ten percent of the results will be positive.  In the last week, about 230,000 tests a day have been performed.  One source estimates that at least five million tests a day will be needed by early June, building to twenty million a day later in the year.

To go with all these tests, an army of contact tracers will also be required.  At the beginning of April, there were 2,200 contact tracers in the entire US.  100,000 tracers will eventually be needed, according to a Johns Hopkins estimate.  The State of Massachusetts is already recruiting contact tracers to follow infections in that state.

We will also need digital contact tracing with smartphone apps like the ones in use now in Singapore and South Korea.  These applications will inform people when someone tests positive who has been near the phone using the app, thus near the person using the phone.  Such apps will only need to be 50-60 percent effective to work, including the uncertainty in whether the smartphone’s owner can actually isolate themselves.

Timing is key for these apps to be effective.  The sooner a user can be notified of a positive contact, the sooner they can isolate themselves.  A system based on a smartphone app could also be effective if the user is notified as soon as another displays symptoms– thus much sooner than when a positive test is found.

Most important is public acceptance of the contact-tracer’s role and of the smartphone app.  The Science News article concludes:

[Says] Annelies Wilder-Smith, an infectious disease expert at the London School of Hygiene and Tropical Medicine, “We need a good communication strategy that starts now”… to build awareness and buy-in before systems are in place. Without widespread participation, even the most advanced technical tools won’t help curb the pandemic.

Even if the public is enthusiastic initially, buy-in may wane as the pandemic stretches on. In areas with ongoing outbreaks, it’s not inconceivable that someone could finish one two-week bout of self-isolation only to be pinged days later that they’ve come into contact with the virus again. … [D]ecisions will need to be made as to what counts as a meaningful contact.  If merely walking past someone on the sidewalk who later gets confirmed with COVID-19 sparks a message to self-isolate, many may ignore requests.  Massachusetts is trying for 15 minutes of exposure, but will transmission events be missed?  Even if a system finds a sweet spot, people may be exposed multiple times in the coming months, and asked to self-isolate each time.

That may not seem sustainable, “but right now we’re shooting blindly, and millions of people who don’t need to be quarantined are stuck at home,” Wilder-Smith says. “No solution is perfect, but of all the worst scenarios, strict contact tracing and isolation is the best scenario, and I think that’s how you have to sell it.”

In summary, contact tracing is a difficult-to-accomplish but essential component of any plan to eliminate the virus or at least reduce the impact of infections and re-open society.  Ending “social isolation” without contact tracing will inevitably lead to a resurgence of uncontrolled spread and new, overwhelming impacts upon our hospital system.

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