A simplified overview


In the following we will assume that Contract Tracing devices are smartphones, as this is also the assumption in most developments currently underway; in the near future, however, other devices might be used in combination with or instead of smartphones.


What is Contact Tracing

Contact Tracing is a tool to monitor and record proximity between users that has taken place; this can be used to inform users of risks of possible infection transfer in the recent past; for COVID-19 the recent past is currently estimated as a period of up to 2 weeks.


How does Contact Tracing work

  • Each user is given a unique identifier in the form of an anonymous Contact Tracing token.
  • When two persons, with the Contact Tracing applications active on their devices, are in each other’s proximity for at least a time T and within a distance D (currently T = 15 minutes / D = 1 meter), they exchange their anonymous Contact Tracing tokens
  • When a user is tested positive, e.g. after suspecting COVID-19 symptoms, the user:
    • uploads a list of anonymous Contact Tracing tokens received within a timeframe in which infection could have been transferred
  • Each user receives regularly lists of anonymous Contact Tracing tokens of other users tested positive; when a match is found with anonymous Contact Tracing tokens stored, the user:
    • uploads a list of anonymous Contact Tracing tokens received within a timeframe in which infection could have been transferred
    • request to be tested


How is the privacy of the users protected

  • Each user is known only by a unique and anonymous Contact Tracing token
  • Contact Tracing information, i.e. the received anonymous Contact Tracing tokens, are stored on the users’ devices locally
  • The matching of risks for possible transfers of infection takes place on the users’ devices

This means that the only traceable items are users requesting to be tested . . .


What is needed to effectively mitigate COVID-19 spreading with Contact Tracing

  • There needs to be sufficient testing capacity able to test and give test results timely
  • Since Contact Tracing results in significant numbers of potential infected cases to be tested, this means that the number of new infections needs to be relatively small, so that the potential infected cases to be tested stay within the available testing capacity


What are limitations of Contact Tracing for COVID-19?

  • Current Contact Tracing systems only monitor possible person-to-person infection transfer (proposals are under discussion to add person-object-person to Contact Tracing systems)
  • With the long incubation time of COVID-19, a significant delay may be found between a person tested positive and possible infection transfer that have been recorded in the Contact Tracing system; with increasing time, this results in an increasing number of possible infected cases as well as more tests to be performed
  • Not all smartphones may be able to support Contact Tracing
  • The strict anonymity makes it difficult to pinpoint ‘hotspots’ of infection transfer


Additional details


To be successful Contact Tracing needs to meet the following criteria

  • making a significant contribution to tracing-testing-action to mitigate COVID-19 spreading
  • implementable
  • able to be deployed on a wide range of devices already in use
  • efficient in use of local resources (battery, memory, processing power) and telecom resources
  • guaranteeing as much as possible privacy of users in line also with the GDPR
  • being accepted by the health authorities in the respective European countries

and last but not least

  • being widely accepted by the general public


What other eHealth / IoT tools would be needed to mitigate COVID-19 spreading

  • The extension of Contact Tracing to person-object-person by including proximity to objects
  • Testing of objects, e.g. by testing cleaning- or waste-water, would complement Contact Tracing, and would help find positive cases much earlier than is waiting for COVID-19 symptoms.  


Why is Bluetooth used

  • Bluetooth (BT) communication
  • takes place directly between BT supporting devices,
    • guaranteeing proximity, and
    • allowing distance between the devices to be estimated
    • works inside buildings, metro, etc.
    • has a limited range, avoiding attempted communication with a large number of devices
    • is widely available on smartphones

other technical options are less favoured

  • GPS does not work in buildings, metro, etc.
  • GPS-A is not sufficiently precise to be able to estimate the distance between two devices
  • Wi-Fi could be used, but this
    • would conflict with other uses of Wi-Fi, and
    • its much larger range would unnecessarily involve many devices


Will the Contact Tracing system be usable on all mobile phones?

That may not be the case, as telephones may be required to be

-          smartphones with

-          a recent version of a supported operating environment, currently Android and iOS, and

-          sufficient resources (memory, processing power, battery capacity,) and

-          supporting a compatible Bluetooth version and protocol stack, and

-          supporting access to ‘the internet’ via cellular data services such as 2G, 3G, 4G, or via Wi-Fi

This may indicate that, at least initially, only relatively recent smartphones would be able to be used with the Contact Tracing system


Standardization of Contact Tracing systems is undertaken in ETSI, by the newly formed ISG E4P, basing itself on work of the ETSI eHEALTH group, and supported by other groups in and outside ETSI.


ETSI                     the European Telecommunications Standards Institute 


ISG E4P               Industry Specification Group: Europe for Privacy-Preserving Pandemic Protection

ETSI ISG E4P  Press Release