Speed Of Testing Critical In Success Of Contact Tracing Strategies To Slow COVID-19 Transmission – Analysis


What is the current status of COVID-19 pandemic?  As on October 3, 2020 17:35 GMT, there were 34,995,770 cases and 1,035,553 deaths due to the disease worldwide. The impact of the disease has overwhelmed many countries in many fields. In countries where the number of confirmed COVID -19 cases is declining authorities are gradually lifting the lockdown measures. Specialists consider that even when physical distancing measures are continued, we need other public health measures to control the epidemic.  They realize contact tracing, in combination with the quarantine and potential testing of contacts, a key component in a phase when countries gradually lift lockdown measures. 

The Lancet Public Health journal (July 16, 2020), published a timely mathematical study which models the effectiveness of conventional and app-based strategies on community transmission of the virus. The study showed that speed of contact tracing strategies is most critical to slow down COVID-19 transmission.

The analytical study further revealed the following (verbatim): 

  • Even if all contacts are successfully traced, a delay of three days or more between symptom onset and testing will not reduce onward transmission of the virus sufficiently to control further spread.
  • In the best-case scenario, with zero delays and at least 80% of contacts traced, the R number is reduced from 1.2 to around 0.8, and 80% of onward transmission per person diagnosed could be prevented
  • For conventional contact tracing to work, test results need to be delivered within a day of an individual developing symptoms
  • Mobile apps can speed up contact tracing and keep R below 1, even if only 20% of the population use them

[R number of an infection can be thought of as the expected number of cases directly generated by one case in a population where all individuals are susceptible to infection.]

“The researchers say improving access to COVID-19 testing, combined with digital that minimise tracing delays, will be key to the success of a contact tracing approach to reduce spread of the virus.”  A press release from the Journal asserted.

“If COVID-19 testing is delayed by three days or more after a person develops symptoms, even the most efficient contact tracing strategy cannot reduce onward transmission of the virus.”  The researchers cautioned 

The study 

The researchers led by Professor Mirjam Kretzschmar , from the University of Utrecht, the Netherlands searched PubMed, bioRxiv, and medRxiv for articles published in English from Jan 1 to June 20, 2020, with the following keywords: (“2019-nCoV” OR “novel coronavirus” OR “COVID-19” OR “SARS-CoV-2”) AND “contact tracing” AND “model*”.  They found that population-level modelling studies of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have suggested that isolation and tracing alone might not be sufficient to control outbreaks and additional measures might be required. 

However, few studies have focused on the effects of lifting individual measures once the first wave of the epidemic has been controlled. Lifting measures must be accompanied by contact tracing strategies to keep the effective reproduction number below 1. If that is not done, the adverse impact will be severe. [In many countries including India, once authorities relax the lock down measures, it is business as usual!] 

The researchers noted that no one has done a detailed analysis, with special emphasis on the effects of time delays in testing of index patients and tracing of contacts. 

The time-honoured   method to slow down infection involves, testing, tracking and treating. The researchers diligently broke down various steps.

They assessed the impact of timeliness and completeness in various steps of a contact tracing strategy using a mathematical model. They explicitly assigned values to time delays between time of infection and symptom onset, and between symptom onset, diagnosis by testing, and isolation (testing delay). The model also included tracing of close contacts (egs. household members) and casual contacts, followed by testing regardless of symptoms and isolation if testing positive, with different tracing delays and coverages.  They computed effective reproduction numbers of a contact tracing strategy (RCTS) for a population with physical distancing measures and various scenarios for isolation of index cases and tracing and quarantine of their contact.

Conventional contact tracing methods

In conventional contact tracing methods, a public health professional contacts the infected person and asks them to recall everyone they have been in contact with over a defined period before the onset of symptoms. Recently, several countries have introduced mobile apps to speed up this process, by automatically alerting people who have been in proximity to the infected person using data from their mobile device. According to experts, to be successful, contact tracing measures must keep the reproduction or R number, below 1. This means that, on average, the number of individuals who will be infected by a single infected person must be less than one.

 “This study reinforces findings from other modelling studies, showing that contact tracing can be an effective intervention to prevent spread of the SARS-CoV-2 virus, but only if the proportion of contacts traced is high and the process is fast. Our study builds on this to show, in detail, what role each step in the process plays in making this approach successful. This will help policy makers understand where best to prioritise resources to maximise the chances of success. For example, we found that mobile apps can speed up the process of tracking down people who are potentially infected, but if testing is delayed by three days or more even these technologies can’t stop transmission of the virus.” Professor Mirjam Kretzschmar clarified.

Contact tracing involves tracking down all of the people who have been in contact with the infected individual so they can be isolated to prevent further spread of the virus. The World Health Organisation (WHO) recommended this approach as an established public health measure and as a potential exit strategy to alleviate COVID-19 lockdown measures.

In the new study, the researchers used a mathematical model that reflects the various steps and delays in the contact tracing process. This enabled them to quantify how such delays affect the R number and the fraction of onward transmission cases that they can prevent for each diagnosed person.


The model assumes that around 40% of virus transmission occurs before a person develops symptoms. In the absence of any strategies to mitigate the spread of the virus, each infected person will transmit the virus to an average of 2.5 people. Introducing physical distancing alone, assuming that close contacts are reduced by 40% and casual contacts by 70%, will reduce the reproduction number to 1.2.

The model assumes that conventional contact tracing takes a minimum of three days and is less efficient at tracking down contacts than mobile app technologies, which are assumed to be instantaneous.

The findings predict that conventional contact tracing will only work to keep the R number below 1 if people with COVID-19 receive a positive test result on the same day they develop symptoms of the virus.

Contact tracing based on mobile app technology can accommodate a delay in testing of up to 2 days and keep the R number below 1, as long as at least 80% of contacts are tracked down. In this case, the number of people infected from those contacts would be reduced by half.

Once testing is delayed by three days or more, even a perfect system that [would] trace 100% of contacts with no delays cannot bring the R number below 1, according to the model.

Overall, the study found that reducing the time between a person developing symptoms and receiving a positive test result is the most important factor for improving contact tracing effectiveness.

Limitations of the model

The authors note that their model does not take into account the age structure of the population. This might influence the proportion of asymptomatic cases, as these are more common in younger people and children, and might also influence mobile app usage. The model also does not account for infections acquired in hospitals and other healthcare settings, such as care homes.

Writing in a linked Comment Article, Professor Louise Ivers and Daniel J. Weitzner, who were not involved in the study, highlight four crucial questions that remain to be investigated:  Firstly, we need to assess how well smart phones measure proximity. Secondly, better understanding of how mobile apps will integrate with overall contact tracing programmes needs to be investigated. The Comment authors also call for further research to understand what factors will encourage users to trust the privacy and security properties of mobile apps. Finally, they highlight the potential for conventional and digital contact tracing strategies to perpetuate health disparities, and further evaluation is needed to prevent this.

Expert reactions

Dr Daniel Lawson, Lecturer in Statistical Science, School of Mathematics, University of Bristol, stated that this study explores some harsh mathematical truths about contact tracing. 

“Simply put, if the tracing system is not fast enough then it becomes useless, as onward transmission has already occurred when contacts are isolated,” he cautioned.

Dr David Bonsall, senior scientist and clinician at Oxford University’s Nuffield Department of Medicine (and epidemiological advisor on the NHSx app programme & test and trace programme) argued that  contact tracing efforts need to be integrated with other important public health measures, including social distancing, shielding of vulnerable groups, and face masks.  

“If combined with rapid contact tracing they are capable of lowering the R value below 1, preventing and containing local outbreaks.  However, contact tracing must notify people before they infect others; high rates of pre-symptomatic transmission requires that the total delay, from symptom onset to contact-tracing, does not exceed 48 hours” he cautioned.

“In our simulations, twice as many infections were prevented by a digital system that notified contacts instantly, when people requested a test, compared to a system that delayed tracing by 48 hours after testing.  We agree that manual tracing efforts also should aim to return test results within 24 hours, and trace people rapidly thereafter.” He added

He revealed that in the UK, the majority of community testing relies on the postal service to send out and collect testing kits from peoples’ homes; this might take 3-5 days from request to result.  

“This testing strategy will not be fast enough to stop many onward infections, and may fail to prevent second waves.” He asserted

Admittedly, contact tracing is only one of the several measures to control spread of the disease. Track, Test. Treat: Hand washing, wearing mask, obey guidelines for social distancing. All are equally important; a deficiency in any one will neutralize the efforts. Even in States where contract tracing is done properly, all is not well!

Dr. K S Parthasarathy

Dr. K S Parthasarathy is former Secretary, Atomic Energy Regulatory Board and a former Raja Ramanna Fellow in the Strategic Planning Group, Department of Atomic Energy, Mumbai. Dr. K S Parthasarathy may be contacted at [email protected]

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