By JOHN RICHARD SCHROCK
Some state governors working toward “opening up” their state from various levels of quarantine are basing their actions on contact tracing to isolate the newly infected and slow the rate of transmission. However, a study just released in the May 8 issue of the journal Science indicates that using contact tracers to interview new patients and track down their recent contacts will be inadequate and only the use of cell phone apps can slow the epidemic.
In “Quantifying SARS-CoV-2 Transmission Suggests Epidemic Control with Digital Contact Tracing,” nine researchers at Oxford University in England calculated the rates of transmission.
The prior SARS epidemic had a much higher fatality rate, but did not have asymptomatic carriers. That made it easy to identify individuals who were infectious and isolate them. The COVID-19 virus is different. There are asymptomatic carriers. And those who become sick may not present any symptoms of infection until after Day 5, but be infectious in Days 3 and 4. And these earlier contacts will still feel healthy and lack symptoms, but are already spreading the infection to others. This time lag makes the manual process of classical contact tracing—as well as a patient’s uncertainty of all the people who came near them—an impossible task.
The Oxford team used precise data published from the epidemic in China that found the incubation time averaged 5.2 days and the epidemic doubling time was 5.0 days.
The important calculation is the rate of contagion, or R0 which indicates how many persons get the infection from a carrier. In the earliest stages of the epidemic in China, R0 was approximately 2.0 or one infected person infected two more. They found that “The contributions to R0 included 46% from presymptomatic individuals (before showing symptoms, 38% from symptomatic individuals, 10% from asymptomatic individuals (who will never show symptoms, and 6% from environmentally mediated transmission via contamination.” They estimated that “...presymptomatic transmissions alone are almost sufficient to sustain epidemic growth.”
To reduce the epidemic, R0 would have to be brought below 1.0, a level where one infected person would infect less than one other person. However, manual contact tracing takes time and results in an average of a 3-day delay in notification. That does not bring the rate of contagion down. However, their study also showed that “Immediate notification through a contact-tracing mobile phone app could, however, be sufficient to stop the epidemic if used by a sufficiently high proportion of the population.”
They conclude that “The use of a contact-tracing app that builds a memory of proximity contacts and immediately notifies contacts of positive cases would be sufficient to stop the epidemic if used by enough people, in particular when combined with other measures such as physical distancing.”
While the Oxford team’s calculations were mathematical, they correspond closely with the successful actions taken by China, South Korea and Taiwan. Their cell phones are less expensive and nearly everyone has one. Their citizens have rapidly downloaded apps that were promptly developed to provide contact-tracing and enforced by a variety of social controls that might or might not be accepted in Western countries.
Today, if I landed in Beijing, my cell phone would be loaded with an app that designates my status: red places me in quarantine, yellow limits my movement and green allows me to travel widely. Similar apps have been the key to the successes in South Korea and Taiwan. But the West is a different culture.
The Oxford team’s proposal for instant digital contact tracing works as follows: “Proximity events between two phones running the app are recorded. Upon an individual’s COVID-19 diagnosis, contacts are instantly, automatically, and anonymously notified of their risk and asked to self-isolate.”
But we will all need cell phones, and we will all need to use the app.
However, the Oxford team’s model, as well as the Asian evidence that this concept works, indicate that manual contact tracing alone will not slow the further spread of this epidemic.
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John Richard Schrock has trained biology teachers for more than 30 years in Kansas. He also has lectured at 27 universities in 20 trips to China. He holds the distinction of “Faculty Emeritus” at Emporia State University.