Wednesday, October 14, 2020

Food For Thought: "On the Futility of Contact Tracing"

 An overview of contact tracing that I've not seen elsewhere.

For folks who like a little "Appeal to authority" argumentum, the authors, Jay Bhattacharya & Mikko Packalen are respectively Professor of Medicine at Stanford University and a Research Associate at the National Bureau of Economic Research, and Associate Professor of Economics at the University of Waterloo.

Otherwise here's the straightforward argument. From Inference Review:

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.

Contact Tracing

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.

Contact Tracing in History

Contact tracing has a long and checkered history and was in use even before the germ theory of disease. In sixteenth-century Malta during a plague epidemic, city authorities quarantined newly arriving sailors, forcing them into lazarettos (plague hospitals).1 The physicians treating the sailors were isolated, even if they showed no symptoms, since they were in close contact with the sailors. Since Yersinia pestis, a bacterium which infects fleas and is carried by rats, causes the plague, the quarantine could not have slowed the spread of the disease, and, indeed, may have intensified the epidemic given the poor sanitary conditions of the lazarettos.

Eugenia Tognotti has described an early version of the techniques used to control a cholera outbreak in Europe during the nineteenth century:

In cities, authorities adopted social interventions and the traditional health tools. For example, travelers who had contact with infected persons or who came from a place where cholera was present were quarantined, and sick persons were forced into lazarettos. In general, local authorities tried to keep marginalized members of the population away from the cities.2

As with the plague, contact tracing in this context likely had little effect on the spread of cholera—a disease caused by fecal contamination of the water supply—and may have served to prolong the epidemic because of poor sanitary conditions in the quarantine hospitals.

By contrast, public health used contact tracing with considerable success in the conquest of smallpox in the late twentieth century.3 After the development of an effective smallpox vaccine, the World Health Organization in 1959 developed a plan to eradicate the disease through worldwide mass vaccination. By the mid-1970s, the campaign had worked to sharply limit the spread of the virus, but there were still occasional outbreaks in places where there were unvaccinated people. Whenever authorities became aware of a case, they would quarantine the index patient and initiate a campaign to vaccinate every person within the vicinity who may have been exposed. Contact tracing worked to help eradicate the disease because, by the mid-1970s, the vaccine had limited the epidemic to a few localized, disconnected outbreaks. Of course, there is no known effective SARS-CoV-2 vaccine, and the disease is widespread and global. The smallpox example provides no support for contact tracing as a tool for disease eradication in the case of COVID-19.

COVID-19 Contact Tracing

In what follows, we argue first that the epidemic is too widespread for contact tracing to limit disease spread; second, that errors in PCR tests substantially raise the human costs of contact tracing and render it less effective; and finally, that contact tracing creates strong incentives among the public to mislead public health authorities and avoid voluntary testing.

COVID-19 infection is already too widespread in the United States, and many other countries, for contact tracing to work effectively. Seroprevalence studies of COVID-19 antibodies provide definitive evidence. The presence of a specific antibody in any given individual provides strong evidence of prior infection by SARS-CoV-2. To date, scientists around the world have conducted over 50 population-level seroprevalence studies that all point to the same qualitative conclusion.4 The COVID-19 epidemic is between 5 and 400 times more widespread than case reports, which detect active infections, might indicate. For example, although case reports in the United States suggest there were just over 1 million identified cases by late April 2020,5 a recent study by the Centers for Disease Control and Prevention found evidence that there were between 6 and 24 times that many infections.6

The seroprevalence evidence establishes a crucial fact: a large fraction—as much as half, or more—of all people infected by SARS-CoV-2 never develop symptoms at all, or at worst, develop mild cold-like symptoms.7 While people who have no symptoms are less likely to spread the disease than people who expectorate virus-filled droplets, there is good evidence that the asymptomatic infected can spread the disease.8 Since there are so many COVID-19 patients with no symptoms, it should be no surprise that the asymptomatic infected, in aggregate, are responsible for many new cases....