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Covid Control put to the test

Contact tracing and low case numbers make an important contribution to curbing a second wave of the corona epidemic

Göttingen, September 18, 2020

/MPI, PH/ BN, Duppé/

The Covid 19 pandemic can be contained only with a set of concerted measures. This is the result of model calculations recently published by a team from the Max Planck Institute for Dynamics and Self-Organization in Göttingen and the University of Göttingen. According to these calculations, health authorities can significantly limit the spread of Sars-CoV-2 by testing, contact tracing and isolation. However, this alone is not enough to prevent a second wave of the epidemic, as the researchers' calculations show. They have defined conditions under which the number of Covid-19 patients in Germany will remain under control.

In containing the Covid 19 epidemic, all of us, the individual contributions, as well as the public health authorities, are crucial. They can break chains of infection by first of all tracing the contacts of people with a positive corona test. The contact persons are then isolated for as long as necessary. That is, either until it has been proven that they do not carry the virus, or until they are no longer infectious, should they have become infected. Although the TTI (test, trace and isolate) procedure does not work perfectly, the health authorities are making a significant contribution to reducing the reproduction rate R, the average number of people infected with the virus, to below 1. Beyond this tipping point, Sars-CoV-2 spreads even faster than exponentially, i.e. very quickly in an uncontrollable manner, and a second wave of the epidemic unfolds.

The figure shows different propagation velocities: The model of the Göttingen researchers

A second tipping point: when health authorities’ capacities are overburdened

A team led by Viola Priesemann, research group leader at the Max Planck Institute for Dynamics and Self-Organization, has now used models of theoretical epidemiology to determine how effectively public health authorities can reduce reproduction rates in practice. The researchers calculated how Sars-CoV-2 spreads in the population using different approaches to testing and contact tracing.

They have demonstrated, for instance, that the capacity limit of the health authorities represents a second important tipping point at which the epidemic can get out of control. "The more contacts the health authorities miss because they are overburdened, the more these undetected carriers of the virus infect other people, and containment becomes increasingly difficult," says Viola Priesemann. An exact figure for the number of new infections per day does not emerge from the calculations. "This depends very much on the cooperation of the population, as well as on the number of social contacts," says Sebastian Contreras, PhD student at the Max Planck Institute and first author of the current study. "However, our study shows that it is advisable to keep a safety margin from the capacity limits of the health authorities in terms of case numbers and to be vigilant even if case numbers increase slowly".

Besides the capacity limit, the success of the health authorities depends on another condition. According to the calculations of the Göttingen team, they can only contain Covid-19 through testing and contact tracing if carriers of the virus, as long as they are undetected, infect at most two other people. The reproduction number R must not exceed 2 in the majority of the population who is unaware of their current infection status. Only then can public health authorities add to reduce the total reproduction count, which is ultimately the decisive factor and is calculated by the RKI, below 1 by identifying and isolating corona carriers.

Most people who know that they have caught the virus isolate themselves. Therefore, it is mainly those carriers spreading the pathogen who do not know about it, for example because they have no symptoms or have not been tested yet as they have only mild, unspecific symptoms. Without any precautionary measures, the undetected virus carriers are estimated to infect between three and four more people, as was the case at the beginning of March, although there is also a small seasonal effect here.

In order to reduce the number of average infections in this group to which we all belong, below two, we need to reduce the risk of infection by 20 to 50 percent. To do this, we should, firstly, carefully follow the AHA rules: Keep your distance, maintain hygiene and wear everyday masks. On the other hand, we can also reduce the risk of infection by restricting the number of distinct physical contacts. In this context, the Göttingen researchers advise against large-scale events: "Large events can facilitate super-spreading events. So far, neither their origin nor their dynamics are fully understood, but we have already been able to show their destabilizing effect in our study," says Johannes Zierenberg, PhD physicist and co-author of the study. Come autumn, general precautions will become even more important when weather conditions favor the virus again.

Infection chains can be broken more effectively

Mitigating the precautions for everyone would only be possible if the triad of testing, contact tracing and isolation became more effective. For example, people with a positive test result would be able to comply with the quarantine even more consistently. In addition, the health authorities have set screws for a more effective identification of corona carriers. For example, they could try to keep the time between infection and detection to an absolute minimum. However, weekly tests for all those without suspicion are not practical - the 1.4 million tests available each week are not enough for 80 million people.

The health authorities could also do everything in their power to locate as many contacts of infected persons as possible, in the shortest possible time. "In our model, we assume that contact tracing and isolation are not perfect. For example, we assume that the health authorities currently identify about two thirds of contacts of carriers of the virus, especially those with a high risk of infection, i.e. contacts in the family, at work or with friends," says Viola Priesemann. Tracing and isolating additional volatile contacts, for example in supermarket restaurants or on the subway, can quickly drive the effort into immeasurable proportions. According to the Göttingen calculations, it is therefore not advisable to trim a step in the interruption of infection chains to the absolute optimum.

"Our calculations show that it is more effective to improve all or several of these factors a little bit with TTI," says Viola Priesemann. "It is particularly effective if we get a little better in as many places as possible: If people who suspect an infection quickly reduce their contacts, if they conscientiously adhere to the quarantine and if the contacts of carriers are quickly identified and also quarantined as a precaution. Then we can allow ourselves much more freedom in other areas without losing control over the spread of the infection".


Dr. Viola Priesemann via
Max Planck Institute for Dynamics and Self-Organization, Göttingen

Original publication

Sebastian Contreras , Jonas Dehning, Matthias Loidolt, F. Paul Spitzner, Jorge H. Urrea-Quintero, Sebastian B. Mohr, Michael Wilczek, Johannes Zierenberg, Michael Wibral und Viola Priesemann. "The challenges of containing SARS-CoV-2 via test-trace-and-isolate."


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