A Fast and Efficient Contact Tracing Could Stop Covid-19

A few months after the beginning of the COVID-19 pandemic, there are still plenty of questions that scientists are trying to answer: can people build immunity to the virus? Is a vaccine possible? How long will the pandemic last?

One thing, however, is undoubtedly clear: time is a key factor in the containment of the virus and has a crucial impact on the extent of its effects on human health. When it comes to detecting new cases of infection, a few days, or even a few hours, can make a huge difference in terms of direct impact on people’s health and lives, on the possibility of taking proper care of those who get sick, and on the chance that every nation has to contain the spread of the virus.

The importance of time was also the starting point of the online colloquium given by Dr. Luca Ferretti last Friday 24 April, to a virtual audience of more than 300 people. The colloquium, titled 'Epidemic Control of COVID-19 Through Instantaneous Contact Tracing: The Case for a Mobile App-based Solution', was coordinated jointly with the Scuola Internazionale Superiore di Studi Avanzati (SISSA), where Ferretti completed his PhD.

The talk was built on epidemiological considerations, and on the fact that extensively performing tests on people is very time-consuming: in Italy, test results can take up to a week before they are ready. Ferretti suggests then that an effective solution could be digital, through mobile applications for widespread contact tracing.

Ferretti has a background in theoretical particle physics, but his research interests range into the study of population genomics, dynamics of pathogens and virus-host interactions. He is currently a researcher at the Big Data Institute of Oxford University, UK, where his work sits at the intersection of epidemiology, genetics and evolution.

Ferretti and his team recently published a paper in Science magazine that pioneers the search for a contact-tracing app to tackle the epidemic control of the virus and prevent further transmission. In the past weeks, the option of mobile apps for contact tracing has been broadly discussed and debated by both the public the scientific community, especially for their implications in terms of the preservation of people’s privacy and the security of data.

In Ferretti’s opinion however, the main question that scientists should keep in mind when thinking of a contact-tracing app is how big a role it would play in the control of the epidemic, and how it should be developed and used to be effective. Therefore, he focused his talk on the scientific aspects underlying this technology, and the reasons why it could be a valid solution to stop or control the spread of the virus.

Ferretti started by summarizing the key aspects and parameters that play a fundamental role in the spread of the infection, to try and understand, from an epidemiological point of view, under which conditions the pandemic could be controlled. “The problem of this infection,” said Ferretti, “is the fact that it overwhelms the intensive care departments in hospitals, and entire national health systems, making it not always possible for everybody to receive the cures they need. The key factor is then acting on the epidemic as strongly as possible and as fast as possible.”

According to Ferretti, to be able to monitor the growth of the virus and its rate, it is crucial to distinguish between the different ways in which the virus is transmitted. The SARS-CoV-2, the “coronavirus”, is mainly transmitted through droplets of saliva between people who are in close contact or proximity. Transmission can happen if an infected person is symptomatic (has symptoms of the disease), pre-symptomatic, or asymptomatic (has very mild or no symptoms). There is also a contribution from environmental sources of infection, like remains of droplets on surfaces or in ventilation systems, but this contribution, although difficult to assess, is estimated to not be significant to2 the overall pandemic. Moreover, current studies show that asymptomatic cases, even if they are thought to represent a very large number on the total of the overall infected population, seem to give a rather little contribution to the overall pandemic as well.

Considering the two remaining sources of transmission, that is symptomatic and pre-symptomatic individuals, it has been noted that the latter contribute to the transmission of the virus in about 30-45% of the cases. This means that the virus is more likely to spread because people who don’t show symptoms are less likely to isolate themselves and practice social distancing. Further studies showed moreover that just the portion of transmission of the virus through pre-symptomatic cases could be enough to sustain the pandemic.

This is the point where contact tracing comes into play: by tracking people that have been in close contact with an infected individual, the method could alert users of their potential risk and ask them to isolate themselves, even if they don’t show any symptoms yet.

In this way, a more effective contact tracing protocol would lead to a bigger impact on the overall infection growth. “The good news is that with little delays in the contact tracing process there is a quite good chance of success in keeping a decently sized epidemic under control,” said Ferretti. “But the bad news is that contact tracing takes time and the bigger the delay in the detection of potential contacts the smaller the chance to control it.”

Traditional contact tracing done manually is very time-consuming and the average time necessary to complete a case is three days. According to Ferretti’s results, a 72-hour delay makes it essentially impossible to control the pandemic. This key result highlights the importance of a mobile contact tracing app, that could complete a contract tracing case in an average time of only four hours.

Traditional tools against the current pandemic - such as a strict lockdown - are in some cases insufficient and have high social and economic costs, while medical tools such as extended screening and testing, or the development of vaccines, are very slow processes that are also not easy to scale up to a global dimension. “That is why isolation paired with contact tracing is a key tool to stop the pandemic, but only with high efficiency and short response times,” concluded Ferretti, “and this is only possible with digital technologies that are fast and far more easily scalable, two key issues for a valid solution.”

Ferretti and his group proposed an app that uses Bluetooth technology on mobile phones to track down and keep record of people who have been in proximity with each other. When an individual shows symptoms of COVID-19, or is tested positive to the virus, the app can very easily and instantaneously send a signal to all the people who have been detected in proximity of the infected person in the previous days. In this way, all those people can be aware that they are potentially at risk and can isolate themselves for the period of time advised by the health authorities in order to stop the chain of contagion.

Of course, this is not an easy solution, as it presents many challenges to a wide application in every part of the world. For example, it requires that more than half of the population use it, and it needs maintenance for the software and the technology behind the app. Moreover, there are clearly ethical issues that need to be addressed by building trust with the population and adopting a transparent approach, especially in the software design and in the treatment of data.

A contact-tracing app approach is therefore a very promising tool to stop the COVID-19 pandemic, but it is not a stand-alone solution, according to Ferretti. While public support, trust and compliance are critical, this technology must work in strict connection and cooperation with public health strategies of tracking, surveillance and widespread rapid diagnostic testing.

A recording of the colloquium is available on ICTP's YouTube channel.

---- Marina Menga