Testing indicators to monitor the COVID-19 pandemic

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Two articles in The Lancet Infectious Diseases called for improved COVID-19 testing capability after demonstrating good diagnostic performance for RT-PCR testing of self-collected nasal and pharyngeal swabs and nasal swabs
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  • Tsang NNY
  • So HC
  • Ng KY
  • bj hood
  • Leung GM
  • IP DKM
Diagnostic performance of different sampling approaches for SARS-CoV-2 RT-PCR assays: a systematic review and meta-analysis.

and good sensitivity for rapid diagnostic testing of antigens.

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  • Boom Y
  • Fai KN
  • Nicolas B
  • et al.
Performance and operational feasibility of rapid diagnostic tests of antigens and antibodies for COVID-19 in symptomatic and asymptomatic patients in Cameroon: a clinical, prospective, diagnostic study.

These important developments will help control the pandemic, but the impact of changes in testing on surveillance data must be anticipated, i.e. the ability to monitor the epidemiology of COVID-19.

Since the start of the pandemic, testing and contact tracing have been the primary measures used to control the spread of SARS-CoV-2.
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Countries are testing “war” tactics against COVID-19.

Initially, the testing capacity was low, so the test data was essential for the interpretation of the COVID-19 case notification rate (i.e. the number of cases per 100,000 population) because the number of reported cases tended to reflect only the number of tests performed. This has held true with increasing testing capacity, which has made it possible to test mild or asymptomatic cases. The format of these indicators varied from country to country, but the objectives were similar. For EU and European Economic Area (EEA) countries, the European Center for Disease Prevention and Control (ECDC) calculates both the test rate (number of tests for SARS infection- CoV-2 per 100,000 inhabitants carried out the previous week) and the positivity of the tests (percentage of positive tests among all tests for SARS-CoV-2 infection carried out the previous week), which, among others, are also used by the European Council to coordinate the restriction of free movement in response to the COVID-19 pandemic.

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Council of the European Union
Council Recommendation (EU) 2020/1475 of 13 October 2020 on a coordinated approach to restricting free movement in response to the COVID-19 pandemic (text of EEA relevance).

The rate of weekly tests at EU and EEA country level has increased linearly since March 2020 and exceeded 4,000 tests per 100,000 population in May 2021. This trend has continued despite fluctuations in notification rate.

Although the positivity of the test is easy to calculate, its interpretation, and in particular its course over time, can be difficult when the numerators and denominators vary for factors unrelated to epidemiology (for example, case definition or strategies test). For example, the EU case definition for COVID-19 that initially relied on detection of SARS-CoV-2 nucleic acid (RT-PCR) included detection of SARS-CoV- antigen. 2 in a clinical sample since the end of 2020, which is less sensitive than RT-PCR. The introduction of target groups less likely to be symptomatic for the test, such as school-aged children, could also affect the positivity of the test.

The emergence and deployment of new tests can impact test metrics whether or not these metrics are listed in the lab criteria for case definitions. In ECDC’s initial guidance on the use of rapid antigenic tests for COVID-19 (prior to inclusion in the EU case definition), the inclusion of rapid antigenic diagnostic tests was recommended during the calculation of test rates and test positivity.
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ECDC
Options for using rapid antigenic tests for COVID-19 in the EU / EEA and UK. November 19, 2020.

However, the guidelines also stated that “positive confirmatory PCR tests or rapid diagnostic tests for recurrent antigens in the same individual should not be included in these counts.” Defining rapid and frequent antigenic diagnostic tests can be difficult in settings where asymptomatic individuals need to be tested regularly, such as schools. In EU and EEA countries, rapid diagnostic tests for antigens are now widely available, although it is difficult to estimate their impact as information on laboratory methods is not available for 75 % of cases reported to ECDC. In Slovakia, around 55% of the cases reported in 2021 were diagnosed by rapid antigenic diagnostic tests.

Now that self-tests are more and more available, test data could become even less reliable. Regarding the use of self-tests for COVID-19, the ECDC describes their possible impact on COVID-19 surveillance under different scenarios given the systematic use of confirmatory laboratory tests, communication the results of the self-tests and the number of tests distributed.
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ECDC
Considerations for Using Self-Tests for COVID-19 in the EU / EEA. March 17, 2021.

In the absence of these components, there is a risk that the test indicators will be biased in an unpredictable way. Negative self-tests may not be reported and positive self-tests may not be confirmed by laboratory testing.

Test data are rarely used in routine surveillance of infectious diseases. In most cases, the laboratory information collected is limited to the laboratory methods used to determine cases, but negative tests are not collected. Some countries like the UK now offer repeat testing to their entire population, inviting people to report their results online or over the phone.
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UK Department of Health and Welfare
Twice a week rapid tests will be available for everyone in England.

Assuming that all results are reported, these data could still be biased towards specific groups or locations, and sampling strategies would remain important for surveillance purposes.

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  • Vandenberg O
  • Martine D
  • Rochas O
  • van Belkum A
  • Kozlakidis Z
Considerations for COVID-19 diagnostic testing.

In response to vaccine deployment and the continued emergence of new variants, these strategies should be combined with sequencing strategies to ensure detection and surveillance of variants of concern.

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ECDC
Guidance for representative and targeted genomic surveillance of SARS-CoV-2. May 3, 2021.

We do not declare any competing interests.

The references

  1. 1.
    • Tsang NNY
    • So HC
    • Ng KY
    • bj hood
    • Leung GM
    • IP DKM

    Diagnostic performance of different sampling approaches for SARS-CoV-2 RT-PCR assays: a systematic review and meta-analysis.

    Lancet Infect Dis. 2021; ()

  2. 2.
    • Boom Y
    • Fai KN
    • Nicolas B
    • et al.

    Performance and operational feasibility of rapid diagnostic tests of antigens and antibodies for COVID-19 in symptomatic and asymptomatic patients in Cameroon: a clinical, prospective, diagnostic study.

    Lancet Infect Dis. 2021; 21: 1089-1096

  3. 3.

    Countries are testing “war” tactics against COVID-19.

    Science. 2020; 367: 1287-1288

  4. 4.
    • Council of the European Union

    Council Recommendation (EU) 2020/1475 of 13 October 2020 on a coordinated approach to restricting free movement in response to the COVID-19 pandemic (text of EEA relevance).

  5. 5.

    Options for using rapid antigenic tests for COVID-19 in the EU / EEA and UK. November 19, 2020.

    European Center for Disease Prevention and Control,
    Stockholm2020

  6. 6.

    Considerations for Using Self-Tests for COVID-19 in the EU / EEA. March 17, 2021.

    European Center for Disease Prevention and Control,
    Stockholm2021

  7. 7.
    • UK Department of Health and Welfare

    Twice a week rapid tests will be available for everyone in England.

  8. 8.
    • Vandenberg O
    • Martine D
    • Rochas O
    • van Belkum A
    • Kozlakidis Z

    Considerations for COVID-19 diagnostic testing.

    Nat Rev Microbiol. 2021; 19: 171-183

  9. 9.

    Guidance for representative and targeted genomic surveillance of SARS-CoV-2. May 3, 2021.

    European Center for Disease Prevention and Control,
    Stockholm2021


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