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Interrupt chains of infection digitally with the Corona-Warn-App

Corona-Warn-App is an important component of pandemic control

Corona-Warn-App. Source: Bundesregierung

A key element in the fight against any pandemic is to interrupt the chains of infection. The Corona-Warn-App can make an important contribution to this and thereby support the health authorities in tracing contacts.

The health authorities use information from individuals who tested positive for SARS-CoV-2 to identify those with whom they were in contact before in order to contain the spread of the pathogen.

The Corona-Warn-App is an important supplement because it helps to map exposures and supports the public health authorities. Encounters with strangers in public spaces are also recorded and potentially exposed individuals are identified faster, because this happens automatically in the Corona-Warn-App.

Contact tracing by the health authorities remains necessary, e.g. to identify or inform people who do not use the app or do not have a smartphone. However, contact tracing and notification via the App does not replace the reporting channels specified by the German Infection Protection Act.

Every user of the Corona-Warn-App makes an important contribution to the containment of the pandemic. We thank the citizens for their interest and participation!

The App is published by the Robert Koch Institute on behalf of the German government.

Important note: Your data is safe. The Corona-Warn-App meets the highest standards of data protection and works with very little data. However, this means that only a limited amount of data is available for evaluation.

To download the Corona-Warn-App

Corona-Warn-App for iOS
Corona-Warn-App for Android

Updates and functionalities

All important information about the development of the Corona-Warn-App is available on https://www.coronawarn.app/en/blog/.

Why the Corona-Warn-App is important

The corona pandemic has not yet been overcome, despite the recent declining number of cases and increasing vaccination rate. The Corona-Warn-App helps to contain the pandemic by interrupting chains of infection.

Other effective measures include:
the daily wearing of masks,
practicing hand hygiene,
observing coughing and sneezing etiquette,
observing the principle of social distancing,
ventilate enclosed spaces,
as well as supplementary guidelines and recommended actions for medical personnel.

Status displays in the Corona-Warn-App

The Corona-Warn-App has two different status indicators: “low risk” (green) and “increased risk” (red).

The status display "low risk" either indicates that the app has not identified any encounters with another corona-positive person documented in the app, or that the relevant threshold values have not been exceeded.

If the status display "increased risk" appears on the smartphone, you have had at least one encounter within the last 14 days with a person who has now tested positive. You will be asked to reduce personal contacts and to contact your doctor's practice or the public health department. They will then decide whether a PCR test is to be performed.

Evaluation of the Corona-Warn-App

In order to investigate the effectiveness of the Corona-Warn-App, the Robert Koch Institute has been leading a scientific evaluation since March 2021. The evaluation relies on the voluntary participation of app users to explore the effectiveness of the Corona-Warn-App .

The aim of the evaluation is to assess the contribution of the app for interrupting infection chains. This evaluation includes the following:

  1. the Event-Driven User Survey (EDUS) among users of the Corona-Warn-App. Users can take part in a survey after they have received a red warning.
  2. Privacy Preserving Analytics. Users are asked to voluntarily donate their data to analyse app usage while their privacy remains protected.

The Event-Driven User Survey has recently been completed. Users are however still able to voluntarily donate their date.

The privacy of the users who participate in the evaluation is preserved at all times. More detailed information can be found in the following overview: Evaluation of the Corona-Warn-App: How user privacy remains protected (PDF, 167 KB, File does not meet accessibility standards.)

More details on the evaluation can be found here.

Public Dashboard for the Corona-Warn-App

For more than a year already, the project team (Robert Koch Institute, SAP, Deutsche Telekom) has been publishing key figures for the Corona-Warn-App - an important contribution to transparency, which is essential to all open-source projects.

The weekly overview of key figures is now being replaced by a public, interactive dashboard. Key figures are updated on a daily basis. In its initial version the dashboard includes the following figures: downloads, test registration, test results, sharing behavior as well as low-risk and high-risk alerts. Data can be downloaded in csv-format. The dashboard can be reached via https://www.coronawarn.app/de/analysis. It is available in German only.

Details on the dashboard’s sources as well as its use can be found here: https://www.coronawarn.app/en/blog/2021-10-04-key-figures-dashboard/.

Answers to frequently asked questions about the evaluation of the Corona-Warn-App, epidemiological questions as well as questions about the Public health service / Laboratory

Epidemiological questions (28.4.2021)

How the risk factors are calculated

During the development of the app, the Robert Koch Institute contributed its scientific expertise on how encounters, where there was a risk of infection, are categorized via the app and what measures users of the app are advised to take in case they have had an encounter with a person who has tested positive for SARS-CoV-2.

For an encounter to be assessed as a possible exposure by the Corona-Warn-App, it must have been epidemiologically relevant. This means, there must have been a risk of infection. The risk assessment in the app is based on a complex model, in the development of which various factors have been incorporated.

The risk of transmission is assessed in a four-step procedure. This includes the evaluation of:

  • How long ago the user met a corona-positive person,
  • How long the contact lasted,
  • How close the persons came to each other and
  • The transmission risk of the corona-positive person.

The result is a risk score which is displayed to the user as a message. If this "risk score" exceeds a threshold, the user automatically receives a warning on the screen.

You can read more details here: https://github.com/corona-warn-app/cwa-documentation/blob/master/cwa-risk-assessment.md

Date: 28.04.2021

How the risk calculation has changed with the switch to the more advanced Apple/Google interface

The Corona-Warn-App is based on the so-called Exposure Notification Framework (ENF) from Apple and Google. With version 1.9 of the Corona-Warn-App, it was converted to the further developed interface (version 2 of the Exposure Notification Framework, ENF).

Specifically, the following aspects have changed: In the case of several encounters with increased risk (red), users are now shown the date on which the last encounter with increased risk took place.

In addition, the epidemiological risk assessment has changed: Now, several - individually considered - encounters with low risk (green) can lead to an increased risk (red): Depending on the infectivity, 15-30 minutes of low-risk encounters (green) become an increased-risk encounter (red).

The display also changes. Users are now shown the number of days on which risk encounters have taken place, but no longer the number of encounters.

With the change to ENF 2.0, a single encounter is no longer decisive for the risk assessment, but the sum of the risk minutes per day. Thus, even several risk encounters of short duration can lead to an increased risk.

Further details on the risk calculation: https://github.com/corona-warn-app/cwa-documentation/blob/master/images/risk_calculation/risk_calculation_enf_v2_overview.pdf

Date: 28.04.2021

Why only the contacts of the last 14 days are recorded and possibly warned

The incubation period is the time from infection to the onset of the disease. On average, it is 5 to 6 days (range 1 to 14 days).
The serial interval defines the average interval from the onset of illness of an infectious case to the onset of illness of a case infected by this case. The Robert Koch Institute estimates the median serial interval for SARS-CoV-2 at four days, which is supported by various studies. Moreover, a considerable proportion probably become infected 1 to 2 days before the onset of symptoms of the infected person. Therefore, to cover all eventualities in the app, the range of 14 days was used.

Date: 28.04.2021

Information on how many people have been informed about an increased risk

There is no data on how many people were informed about a possible exposure with the help of the Corona-Warn-App, as the app is based on a decentralized approach.

All user data is encrypted and stored exclusively on the users’ smartphone. Neither the Robert Koch Institute, as the publisher, nor third parties have access to these data. There is no information on how many people have been informed about a possible exposure with the help of the Corona-Warn-App.

If a person using the app tests positive for the coronavirus, they can voluntarily inform other users. Then the random codes of the infected person are made available to all those who use the Corona-Warn-App.

If you have installed the app, the app will then check if you have met the person who tested positive for Corona. This check only takes place on your smartphone. If the exposure logging is positive, the app will show you - and only you - an exposure notification ("increased risk").

Date: 26.11.2020

Questions about the Public health service / Laboratory (28.4.2021)

What information does the user of the app receive that he or she can pass on to the public health authority?

The following specific information is associated with the exposure notification "Increased Risk":

  • Number of exposures
  • Time since the last exposure (in days)
  • Number of days with exposure logging (max. 14 days)
  • Time of the last retrieval of positive identifiers (day/time)

Only the users themselves see this information in the app. A transfer to the public health authority is voluntary.

Date: 10.09.2020

How can the public health authority verify that the caller/visitor has an "increased risk" on his app?

The callers are asked to describe the exposure notification in the app in order to check the plausibility of receiving the notification. Furthermore, there is currently no way for health authorities to confirm beyond doubt that the caller has received an "increased risk" notification via the app.

If a user contacts the public health authorities, it is recommended to clarify the contact situation in a conversation - analogous to the usual procedure. For this purpose the details of the exposure notification from the app can be used, such as the number of exposures and the time since the last exposure. If possible, this is used as a basis for categorization as a Category I or II contact and appropriate measures are taken.

If the information is not sufficient for categorization, the following procedure is recommended:

  • Offer PCR testing for SARS-CoV-2
  • Indicate that a negative test does not rule out infection
  • emphatically point out AHA rules AHA-Regeln [in German only]
  • Make a recommendation to stay at home and reduce contacts if possible.
  • In special cases, the public health authority can order a separation at home or a ban on activities.

Date: 10.09.2020

How should the Corona-Warn-App be used by the staff of health care facilities or by the staff during activities like patient transport?

1. Treatment of patients tested positive for SARS-CoV-2 with adequate protective equipment* (prior verification via the Corona-Warn-App)

When working with people who have already been verified as a positive case via the Corona-Warn-App, no further codes are exchanged by the App and thus no new warning messages are generated. An increased risk is only given for encounters for the last 14 days before verification. After that, the code exchange function is deactivated via the app of the verified case. Thus, when working with verified positive cases, the staff is not notified of an increased risk and the app can still be used with full functionality.
(e.g. work on a station with verified COVID-19 cases)

* In the direct care of patients with confirmed or probable COVID-19 infection, at least FFP2 masks must be worn in accordance with occupational safety regulations.

2. Treatment of patients tested positive for SARS-CoV-2 without adequate protective equipment* (subsequent verification via the Corona-Warn-App)

When working with persons who can be verified as positive cases via the Corona-Warn-App afterwards, warnings are generated for contact persons retroactively for 14 days until the time of verification. According to this, when working with positive cases that can be verified retrospectively, the personnel will be notified of a exposure and possibly also an increased risk. Since the selective deactivation of the warning message for certain contacts is not possible, it is advisable in this case to deactivate the Bluetooth function during the treatment of Corona-positive tested persons, since otherwise a warning message with increased risk is the result. The warning message cannot be subsequently deactivated and cannot be differentiated from another encounter without a PPE.
(e.g. work on a station with suspected COVID-19 infection)

* In the direct care of patients with confirmed or probable COVID-19 infection, at least FFP2 masks must be worn in accordance with occupational safety regulations.

3. Treatment of patients with unexplained SARS-CoV-2 infection status by persons with adequate protective equipment*

When treating patients with an unexplained infection status with adequate protective equipment, it is advisable to deactivate the Bluetooth function. If the treating person can be verified later as a positive case, the treating personnel will otherwise be notified of an increased risk. Since, as mentioned in point 2, it is not possible to deactivate the warning message afterwards and it is no longer possible to differentiate between contact with persons with and without PPE, the app should not be used. This is the only way to send truly increased risk messages specifically through the app.
(e.g. work on a station with suspected cases of COVID-19 infection)

* In the care of patients who are not a probable or confirmed case, a medical mouth and nose protector is adequate protective equipment, provided that no aerosol generating activities are performed.

4. Treatment of patients with unexplained SARS-CoV-2 infection status by persons without adequate protective equipment*

When treating patients with an unexplained infection status without adequate protective equipment, the app can be used without any functional restrictions. If the treating person can be verified later as a positive case, the treating personnel will be notified of an increased risk. This warning message is then to be interpreted as an adequate risk message.

*In the care of patients who are not a probable or confirmed case, a medical mouth and nose protector is adequate protective equipment, provided that no aerosol generating activities are performed.

Date: 28.04.2021

Questions about the evaluation of the Corona-Warn-App (09.7.2021)

What the aim of the evaluation is

The evaluation examines the effectiveness and usefulness of the Corona-Warn-App.
The evidence on the effectiveness of the Corona-Warn-App - that users of the Corona-Warn-App infect fewer other people - has two main aspects:

  • Firstly, people who are informed in time about a potential infection can reduce their contacts more quickly and thus prevent further transmission.
  • Secondly, the Corona-Warn-App can be used to inform not only friends and relatives, but also people who are not known to the warning persons, about a corresponding risk.The aim of the evaluation is to explore the effectiveness of the Corona-Warn-App. To this end, various aspects will be examined more closely - including the extent to which users who have received a red risk notification have changed their behaviour as a result.

Date: 09.07.2021

How the evaluation is implemented

In order to investigate the effectiveness of the Corona-Warn-App, the Robert Koch Institute has been leading a scientific evaluation since March 2021. The evaluation relies on the voluntary participation of app users to explore the effectiveness of the Corona-Warn-App .

The aim of the evaluation is to assess the contribution of the app for interrupting infection chains. This evaluation includes the following:

  1. the Event-Driven User Survey (EDUS) among users of the Corona-Warn-App. Users can take part in a survey after they have received a red warning.
  2. Privacy Preserving Analytics. Users are asked to voluntarily donate their data to analyse app usage while their privacy remains protected.

The Event-Driven User Survey has recently been completed. Users are however still able to voluntarily donate their date.

User participation in the survey and data donation for the evaluation of the app is and was voluntary. The highest data protection requirements apply to the evaluation using data from the app. The privacy of users who participate in the evaluation is protected at all times.

Date: 09.07.2021

How users of the Corona-Warn-App can participate in the evaluation

The Event-Driven User Survey has recently been completed. Users are however still able to voluntarily donate their date.

Users have the possibility to voluntarily and continuously provide technical usage data of their Corona-Warn-App for scientific evaluation without revealing their identity. This is possible via the so-called Privacy Preserving Analytics (PPA) - i.e. the privacy-preserving analysis of technical usage data. This can be activated in the Corona-Warn-App via the "Data Donation" function.

Date: 09.07.2021

Where the results of the evaluation are published

The Robert Koch Institute will publish the results on its topic page for the Corona-Warn-App. In addition, the findings will of course be prepared for scientific discourse and published accordingly.

Date: 28.04.2021

What happens to the results of the evaluation

The insights gained from the evaluation are used for the further development of the Corona-Warn-App. On the one hand, they should show potential for improvement and, on the other hand, also serve as proof for the app's effectiveness.

Date: 28.04.2021

How the user’s privacy is protected

A verification procedure and technical measures ensure that users who participate in the evaluation cannot be identified. This applies both to the online survey and to the analysis of the technical usage data. In any case, the privacy of the participating users remains protected.

In addition, these protective measures prevent misuse and ensure data quality.

At the same time, this procedure ensures that only users who have actually installed the Corona-Warn-App participate in the evaluation and that they can only take part in the survey once.

More detailed information about privacy protection during evaluation can be found in this overview: Evaluation of the Corona-Warn-App: How user privacy remains protected (PDF, 167 KB, File does not meet accessibility standards.)

Date: 28.04.2021

What the data donation for the analysis of technical usage data is all about

In addition to the event-driven surveys, a data protection-compliant option was also created for users to voluntarily and continuously provide technical usage data of their Corona-Warn-App for scientific evaluation without revealing their identity.

With the help of so-called Privacy Preserving Analytics (PPA) - i.e. a privacy-preserving analysis of technical usage data - the processes and usage of the CWA can be better understood and the app can thus be continuously improved in its function and user-friendliness. Specifically, the following aspects can be examined with the help of the analysis of technical usage data:
Which events (e.g. display of an increased risk, registration of a test, display of a test result, key submission to warn others) occur in the app and with what frequency, when these events occur, how and with what time interval they follow each other,
what choices users make in the app as a result, where and when users cancel processes within the app, with which demographic data (e.g. age group, if specified) this is associated.

Date: 28.04.2021

Answers to questions about general questions and technical questions as well as questions about the test result, event registration, integration of rapid test can be found here.

Date: 06.05.2022