Throughout the COVID-19 epidemic, public health authorities have promoted contact tracing as a key tool to combat the spread of the SARS-CoV-2 virus. Nearly every country infected by the virus has adopted some version, though with evidently mixed results given the global spread of the epidemic. Our purpose in this essay is to make the case that, contrary to conventional public health wisdom, most of the hope placed on contact tracing efforts to control the epidemic is ultimately futile. It may be useful when the number of cases in an epidemic is very small and only if it is applied aggressively without regard to privacy rights. In cases that do not fit that description, contact tracing may make an outbreak worse.
THE IDEA BEHIND classic contact tracing is simple and intuitive. Suppose that public health officials can identify an index case of an infectious disease—by definition, the first documented patient. That patient must have been infected by someone else: the disease is contagious, after all. And that person must have been infected by someone else. Classic contact tracing is the systematic application of the idea of working recursively backward to trace the disease back to a source. All those identified along the way who are still infected can either be quarantined or treated (if there is a treatment) so that they do not further spread the disease. This form of contact tracing is used routinely for the control of venereal diseases. It is most effective when the infection would not naturally resolve itself over a period shorter than the time needed to track backward to patient zero.
A variant form of contact tracing, the one actually in use for COVID-19, works forward instead. Suppose that an index patient tests positive for COVID-19. In this variant, the contact tracer will ask the patient to report all the people with whom he has been in contact, and all the places where contact took place. The tracer will then work through the list of contacts, presumptively mandating a two-week quarantine unless a polymerase chain reaction (PCR) test, if available, shows the contact to be uninfected. The quarantine is maintained while the PCR test result comes back, which may take several days or more. In many cases, especially early in the epidemic, the PCR test was not available, so the two-week quarantine was presumptively enforced. The tracing exercise works recursively forward, identifying contacts in concentric circles of contacts on the idea that the index person may have spread the virus to them. Any contact who traces positive is also subject to the same contact tracing exercise.
The appeal of this approach is the same as the one provided by looking for one’s lost keys under the glow of a streetlight. The keys may not be there, but there is little hope of finding them anywhere else. COVID-19 is an infectious disease, so tracing and testing contacts focuses the attention of public health officials on a set of people who are more likely than a randomly chosen individual to have the disease. Since there is no effective treatment that hastens the resolution of the infection, quarantining is the only way to prevent potentially infected individuals from spreading the disease.