The epidemiological impact of the NHS COVID-19 app

Wymant, Chris; Ferretti, Luca; Tsallis, Daphne; Charalambides, Marcos; Abeler-Dörner, Lucie; Bonsall, David; Hinch, Robert; Kendall, Michaela; Milsom, Luke; Ayres, Matthew; Holmes, Chris; Briers, Mark; Fraser, Christophe

doi:10.1038/s41586-021-03606-z

Cited by 243 publications

(252 citation statements)

References 21 publications

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“…More broadly, contact tracing could play an important role in limiting transmission of SARS-CoV-2 and other pathogens [49]. However, we found that its efficacy depends on participation in seeking testing immediately following symptom onset, quick return of test results, and sufficient contact tracing capacity if case numbers surge.…”

Section: Discussionmentioning

confidence: 80%

“…Closing businesses has had devastating impacts on employment and economies, as well as cascading impacts on society. TTIS has far smaller economic and societal costs, but its efficacy in controlling epidemics is not fully understood, and some studies suggest that it is insufficient to keep R t below 1 in PLOS COMPUTATIONAL BIOLOGY many settings, especially without widespread testing and digital contact tracing [13,14,22,49]. We examined how the efficacy of contact tracing decreases with increasing case burden.…”

Section: Discussionmentioning

confidence: 99%

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Contact tracing efficiency, transmission heterogeneity, and accelerating COVID-19 epidemics

Gardner

Kilpatrick

2021

PLoS Comput Biol

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Simultaneously controlling COVID-19 epidemics and limiting economic and societal impacts presents a difficult challenge, especially with limited public health budgets. Testing, contact tracing, and isolating/quarantining is a key strategy that has been used to reduce transmission of SARS-CoV-2, the virus that causes COVID-19 and other pathogens. However, manual contact tracing is a time-consuming process and as case numbers increase a smaller fraction of cases’ contacts can be traced, leading to additional virus spread. Delays between symptom onset and being tested (and receiving results), and a low fraction of symptomatic cases being tested and traced can also reduce the impact of contact tracing on transmission. We examined the relationship between increasing cases and delays and the pathogen reproductive number Rt, and the implications for infection dynamics using deterministic and stochastic compartmental models of SARS-CoV-2. We found that Rt increased sigmoidally with the number of cases due to decreasing contact tracing efficacy. This relationship results in accelerating epidemics because Rt initially increases, rather than declines, as infections increase. Shifting contact tracers from locations with high and low case burdens relative to capacity to locations with intermediate case burdens maximizes their impact in reducing Rt (but minimizing total infections may be more complicated). Contact tracing efficacy decreased sharply with increasing delays between symptom onset and tracing and with lower fraction of symptomatic infections being tested. Finally, testing and tracing reductions in Rt can sometimes greatly delay epidemics due to the highly heterogeneous transmission dynamics of SARS-CoV-2. These results demonstrate the importance of having an expandable or mobile team of contact tracers that can be used to control surges in cases. They also highlight the synergistic value of high capacity, easy access testing and rapid turn-around of testing results, and outreach efforts to encourage symptomatic cases to be tested immediately after symptom onset.

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Section: Discussionmentioning

confidence: 80%

Section: Discussionmentioning

confidence: 99%

Contact tracing efficiency, transmission heterogeneity, and accelerating COVID-19 epidemics

Gardner

Kilpatrick

2021

PLoS Comput Biol

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“…EN technology represents a potentially disruptive public health strategy for pandemic control. Recent modelling and simulation evaluations have demonstrated the epidemiological value of EN tools in improving contact identification, particularly of contacts unknown to the index case, and slowing secondary transmission of infection [11][12][13][14] The preliminary evaluation of WA Notify described here seeks to answer the question: To what extent did WA Notify avert new COVID-19 cases in Washington during the first four months of its use?…”

Section: Introductionmentioning

confidence: 99%

Early Epidemiological Evidence of Public Health Value of WA Notify, a Smartphone-based Exposure Notification Tool: Modeling COVID-19 Cases Averted in Washington State

Segal

Zhang

Karras

et al. 2021

Preprint

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Background Secure and anonymous smartphone-based exposure notification tools are recently developed public health interventions that aim to reduce COVID-19 transmission and supplement traditional case investigation and contact tracing systems. We assessed the impact of Washington State's exposure notification tool, WA Notify, in mitigating the spread of COVID-19 during its first four months of implementation. Methods Due to the constraints of privacy-preservation and anonymized data, aggregate metrics and disparate data sources were utilized to estimate the number of COVID-19 cases averted based on a modelling approach adapted from Wymant et al (2021) using the following parameters: number of notifications generated; the probability that a notified individual goes on to become a case; expected fraction of transmissions preventable by strict quarantine after notification; actual adherence to quarantine; and expected size of the full transmission chain if a contact had not been notified. Results The model was run on a range of secondary attack rates (5.1%-13.706%) and quarantine effectiveness (53% and 64%). Assuming a 12.085% secondary attack rate and 53% quarantine effectiveness, the model shows that 6240 cases were averted statewide during the first four months of its implementation. Based on an estimated COVID-19 case fatality of 1.4%, WA Notify saved 30-120 lives during the study period. Conclusions These findings demonstrate the potential value of exposure notification tools as a novel public health intervention to help mitigate the spread of COVID-19 in the U.S. As new variants emerge and non-essential travel bans are lifted, exposure notification tools may continue to play a valuable role in limiting the spread of COVID-19.

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“…participation rates, delays in quarantining and notification, the amount of traced contacts, high pre-/asymptomatic transmission rates, or missing bidirectional tracing [15,18,19,23]. Recently, it was reported that a 30% participation rate might have lead to a 15% reduction in cases in the UK during the last quarter of 2020, while for France, a modeling analysis suggested an 8% reduction in peak prevalence (on top of household isolation, which was found to provide a base reduction of 27%) [22,28]. For empirically recorded temporally resolved contact networks, a DigCT efficacy of < ∼ 5% and ≈ 10% have been found based on modeling, respectively, for a participation rate of a = 30% [20].…”

Section: Introductionmentioning

confidence: 99%

“…So far, it remains essentially elusive, however, to which extent DigCT applications may have contributed to the reduction of COVID-19 outbreak sizes. In particular, the empirical results discussed above have not been used to quantify outbreak reduction in real-world settings, apart from the UK [28]. Also, drastic nonpharmaceutical interventions (NPIs) usually referred to as "lockdown" measures most likely changed both the population's contact structure as well as its mixing properties, hence the question arises how these changes, and contact structure in general, might influence the success of DigCT applications.…”

Section: Introductionmentioning

confidence: 99%

Digital contact tracing contributes little to COVID-19 outbreak containment

Burdinski

Brockmann

Maier

2021

Preprint

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Digital contact tracing applications have been introduced in many countries to aid in the containment of COVID-19 outbreaks. Initially, enthusiasm was high regarding their implementation as a non-pharmaceutical intervention (NPI). Yet, no country was able to prevent larger outbreaks without falling back to harsher NPIs, and the total effect of digital contact tracing remains elusive. Based on the results of empirical studies and modeling efforts, we show that digital contact tracing apps might have prevented cases on the order of single-digit percentages up until now, at best. We show that this poor impact can be attributed to a combination of low participation rates, a non-flexible reliance on symptom-based testing, low engagement of participants, and delays between testing and test result upload. We find that contact tracing does not change the epidemic threshold and exclusively prevents more cases during the supercritical phase of an epidemic, making it unfit as a tool to prevent outbreaks. Locally clustered contact structures may increase the intervention’s efficacy, but only if the number of contacts per individual is homogeneously distributed, a condition usually not found in contact networks. Our results suggest that policy makers cannot rely on digital contact tracing to contain outbreaks of COVID-19 or similar diseases.

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The epidemiological impact of the NHS COVID-19 app

Cited by 243 publications

References 21 publications

Contact tracing efficiency, transmission heterogeneity, and accelerating COVID-19 epidemics

Contact tracing efficiency, transmission heterogeneity, and accelerating COVID-19 epidemics

Early Epidemiological Evidence of Public Health Value of WA Notify, a Smartphone-based Exposure Notification Tool: Modeling COVID-19 Cases Averted in Washington State

Digital contact tracing contributes little to COVID-19 outbreak containment

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