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SB04: Who is using the Corona-Warn-App, who is warned - and how fast?

CWA Team, on December 2, 2021, from German version (last updated October 15, 2021)
  1. First things first: a thank you
  2. The most important findings in a nutshell: numbers, data and facts
  3. Privacy-preserving Analytics
    1. The purpose of data donation
    2. Data collected
  4. Data about the participants and their devices
    1. Number of donations over time (by OS)
    2. Number of donations over time (by OS version)
    3. Number of donations over time (by CWA version)
    4. Participant distribution by age group
    5. Geographical distribution
    6. Differences between urban and rural districts
  5. Receipt of a warning
    1. Warnings and the people who received them
    2. Warned individuals over time
    3. Individuals warned during check-ins
  6. Test results
    1. Delay between test registration and receipt of a test result
    2. Delay between test registration and receipt of a warning
    3. Delay between risk exposure and test registration
    4. Delay between risk exposure and receipt of a warning
    5. Association between risk assessment and infection
    6. The percentage of users who donate data
  7. What to expect in the future

1 First things first: a thank you

We would like to begin by thanking the Corona-Warn-App’s users, especially those who donate their data to us every day. These donations provide evidence of the app’s effectiveness, help us improve its functionality and deliver important information about the course of the spread of COVID-19 in Germany. Furthermore, they supply the figures for epidemiological indicators that could not be depicted in the same quantity or quality without them. Therefore, this article is also aimed at settling a small part of the debt that we owe the people who use the app. Once again, we’d like to thank the app’s users for providing us with their data.

2 The most important findings in a nutshell: numbers, data and facts

  • Around 12 million app users donate their data every day (this has resulted in 9,638,099,766 records).
  • The percentage of data donors is about 49%.
  • There are differences between the number of app users in urban and rural districts, and between eastern and western Germany.
  • On average, 5 people who have been exposed to an increased risk and 11 people who have been exposed to a low-level risk are warned for each positive test result registered in the app.
  • Each check-in results in warnings about increased risk being sent out to around 23 people.
  • On average, PCR test results are delivered 21 hours after test registration (half of the test results are received within 12 hours).
  • On average, tests are registered in the app 3.6 days after a user has received a warning (half of tests are registered within 1.8 days).
  • On average, users receive warnings 3.7 days after having been exposed to a risk (half of warnings were received after about 3 days).
  • About one in five people who underwent a PCR test and who were warned about an increased risk tested positive. In contrast, less than half as many app users who did not receive a warning and still underwent a PCR test tested positive. Therefore, the CWA is warning the right people.

In the following, we focus on the purpose of data donation and describe the data that are being collected.

3 Privacy-Preserving Analytics

Privacy-preserving analytics or PPA enables CWA users to provide data held on their smartphone to an evaluating entity without having to reveal their identity.

Since 5 March 2021, CWA users have been able to donate operational data on a daily basis in an anonymous, yet authenticated, manner. Part of this process involves checking that the device sending the data and its CWA installation are genuine (for detailed documentation, see github.com/corona-warn-app/cwa-ppa-server).

CWA data donation should not be confused with Corona data donation. Corona data donation involves around half a million users donating data on body temperature, resting heart rate and other vital information collected using fitness bracelets.

3.1 The purpose of data donation

Data donation is aimed at gaining a better understanding of the app’s processes and usage. In turn, this enables its functions and user-friendliness to be continuously improved. As the app is based on a decentralised approach, a lot of important data about the CWA’s functionality is only available on end devices. Therefore, data donation is the only way of gaining the relevant data.

CWA data donation helps provide an understanding of:

  1. the events that occur in the app and how often they take place,
  2. when and the sequence in which these events occur and the interval (delay) between them,
  3. the selections that users make in response to particular events,
  4. where and when users cancel their input,
  5. and whether cancellation is due to the technical properties of a particular device or aspects such as demography (e.g. age group, residential area).

This information enables:

  1. risk assessment algorithms to be improved,
  2. parameters to be configured so as to ensure events occur at reasonable intervals,
  3. user navigation to be optimised,
  4. targeted communication measures to be targeted even more appropriately.

In addition, the data collected can be used to evaluate the temporal and spatial distribution of certain events in order to provide information about the course of the pandemic in real time and to identify local peculiarities. In turn, this enables targeted measures to be put in place at an early stage so as to counteract unfavourable developments.

3.2 Data collected

The following data are collected (see github.com/corona-warn-app/cwa-ppa-server/../PPAC):

3.2.1 Technical metadata

All data sets contain the following information:

  • Date of donation (submitted_at),
  • Authorisation flags to prevent abuse (android_ppac_basic_integrity, android_ppac_cts_profile_match, android_ppac_evaluation_type_basic, android_ppac_evaluation_type_hardware_backed).

3.2.2 Metadata from users

Users can provide the following optional information:

  • Federal state and district (federal_state, administrative_unit),
  • Age group (<30, 30-59, 60+) (age_group).

1,862,487,469 records contain this information (as of 7 October 2021).

3.2.3 Metadata from devices

The following data about end devices is available:

  • CWA Version (cwa_version_major, cwa_version_minor, cwa_version_patch),
  • Configuration token (app_config_etag),
  • OS Version (ios_version_major, ios_version_minor, ios_version_patch or android_api_level).

1,862,486,604 records contain this information (as of 7 October 2021).

3.2.4 People who received a warning (exposure risk)

The following data can be collected for proximity tracing (BLE, ENF) and presence tracing (event check-ins) (pt_..):

  • Risk level (red or green warnings); are there any differences between the current data and those that were available yesterday? (risk_level, risk_level_changed),
  • If a person has been exposed to a risk, the date when the warning was displayed on the user’s screen (see illustration). Difference between today and yesterday (most_recent_date_at_risk_level, most_recent_date_changed),
  • User metadata,
  • Technical metadata.

1,852,088,985 records contain this information (as of 7 October 2021).

Indicates exposure to an increased risk

Indicates exposure to an increased risk.

3.2.5 Test results

The following data are collected about all tests that are registered in the app, either when a result comes available or after a specific period (currently 7 days):

  • Test result (test_result) (positive, negative, indeterminate, pending) [A],
  • Hours since test registration (hours_since_test_registration),
  • Risk level (displayed) at test registration (risk_level_at_test_registration) [B],
  • days_since_most_recent_date_at_risk_level_at_test_registration [B] = Delay between the last time a user was exposed to a risk (most recent warning) and test registration (in days),
  • hours_since_high_risk_warning_at_test_registration [B] = Delay between warning and test registration (in hours),
  • User metadata,
  • Technical metadata.

The data are:

  • [A] available separately for PCR and rapid antigen tests (RAT) (values of test_result),
  • [B] and for proximity tracing (BLE, ENF) and presence tracing (event check-ins) (pt_..).

4,313,299 records contain this information (as of 7 October 2021).

3.2.6 Key submission

The following data are transmitted about the submission of diagnosis keys either as soon as a key is shared or after a specified period (currently 36 hours):

  • [B] for both proximity tracing (BLE, ENF) and presence tracing (event check-ins) (pt_..)
  • Key submitted with user metadata
    • Key submitted? (submitted)
    • Key submitted after running through the symptoms flow chart? (submitted_after_symptom_flow)
    • Key submitted with a teleTAN? (submitted_with_teletan)
    • Key submitted after a rapid antigen test? (submitted_after_rapid_antigen_test)
    • Hours since receiving test results (hours_since_reception_of_test_result)
    • Hours since test registration (hours_since_test_registration)
    • days_since_most_recent_date_at_risk_level_at_test_registration [B] = delay between the last time the user was exposed to a risk (most recent warning) and test registration (in days)
    • hours_since_high_risk_warning_at_test_registration [B] = Delay between warning and test registration (in hours)
    • User metadata,
    • Technical metadata.
  • Key submission with metadata from end devices
    • Key submitted? (submitted)
    • Advanced consent granted? (advanced_consent_given)
    • Key submitted in the background? (submitted_in_background)
    • Key submitted after running through the symptoms flow chart? (submitted_after_symptom_flow)
    • Key submitted after cancelling the key submission process? (submitted_after_cancel)
    • Last event displayed as part of the key submission process(last_submission_flow_screen)
      • Test result (2)
      • Warning others (3)
      • Symptoms (4)
      • Onset of symptoms (5)
    • Device metadata,
    • Technical metadata.

251,161 records contain this information (as of 7 October 2021).

3.2.7 Exposure Windows and Scan Instances

The following information is available from data recorded locally using the Google or Apple Exposure Notification Framework (ENF; see Google Exposure Notifications API, Apple Exposure Notifications API):

1,036,601,141 records contain information about exposure windows and 3,019,619,946 contain information about scan instances (as of 7 October 2021).

4 Data about the participants and their devices

In the next section, we take a closer look at the data donors themselves (or, more specifically, at the devices they use). Since 7 October 2021, data has been donated around 1.86 billion times. During the past few weeks, more than 11 million records have been donated every day.

4.1 Number of donations over time (by OS)

The following chart demonstrates trends over time and the types of operating system used. Out of the 12,159,958 records donated on 7 October 2021, 6,101,821 were received from devices using Apple’s iOS and 6,058,137 from Android-based devices.

Fluctuations in the number of daily donations particularly result from measures and restrictions put in place to secure the network against DDoS attacks . This mainly affects devices running Android, because the authenticity of these devices is reviewed to ensure data protection laws are upheld (for more detailed information, see the PPA documentation).

Number of donations (total and by operating system)

Figure 1: Number of donations (total and by operating system).

4.2 Number of donations over time (by OS version)

A breakdown of the data by operating system demonstrates the typical homogeneity associated with iOS and the heterogeneity associated with Android-based devices.

Number of donations (by operating system version)

Figure 2: Number of donations (by operating system version).

4.3 Number of donations over time (by CWA version)

The following graph depicts the data by CWA version. The graph demonstrates a clear exponential switch to a new CWA version.

Number of donations (by CWA version)

Figure 3: Number of donations (by CWA version).

4.4 Participant distribution by age group

The following graph depicts the distribution of the participants by age group. 34% of participants provided an indication of their age. Of these, 18.3% stated that they were under 30, 62% that they were aged between 30 and 59 and 19.7% that they were 60 or older. These figures are within expected ranges (see EDUS participants by age and sex).

Number of CWA donations (by age group)

Figure 4: Number of CWA donations (by age group).

4.5 Geographical distribution

The following graph depicts the geographical distribution of the participants and uses data from 31 August 2021. On 31 August 2021, 12,109,252 records were donated, and 3,991,351 of them (33%) include information about the user’s district.