Speed of testing most critical factor in success of contact tracing to slow Covid-19 spread, study shows
The speed of testing is the most critical factor in the success of contact tracing strategies to slow the transmission of Covid-19. If coronavirus 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 coronavirus, warn researchers in a new study. The authors say improving access to Covid-19 testing, combined with digital that minimizes tracing delays, will be key to the success of a contact tracing approach to reduce coronavirus spread.
In the best-case scenario, with zero delays and at least 80% of contacts traced, the reproduction number (number of people who will be infected by a single patient) is reduced from 1.2 to around 0.8, and 80% of onward transmission per person diagnosed could be prevented, shows analysis. For conventional contact tracing to work, test results need to be delivered within a day of an individual developing symptoms, experts emphasize.
“This study reinforces findings from other modeling studies, showing that contact tracing can be an effective intervention to prevent the spread of the SARS-CoV-2 virus (which causes Covid-19), 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,” says Professor Mirjam Kretzschmar, one of the lead authors of the study, from Utrecht University, the Netherlands, in the analysis published in The Lancet Public Health. “This will help policymakers understand where best to prioritize resources to maximize 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,” says Kretzschmar.
The analysis comes even as the Rockefeller Foundation released a report stating that the US will need to ramp up testing dramatically to 30 million tests a week by October. The country will need at least another $75 billion in federal funding to ensure diagnostic and screening testing is free and available to all Americans, especially low-income families, minorities, and highly vulnerable essential workers, says the analysis.
“America faces an impending disaster. The extraordinary scale of the Covid-19 crisis is evident in the growing deaths and economic losses the pandemic has wrought in every state. This terrifying tragedy was not and is not inevitable. Testing is the only way out of our present disaster, and it will remain the case until a vaccine or effective therapeutics are widely available,” says the foundation’s report. It adds, “We called for rapidly expanded diagnostic testing capacity from 1 million tests per week to 3 million tests per week by June, and to 30 million tests per week by October. Today we’re at 4.5 million tests per week, but unfortunately, it’s taking far too long to get to 30 million tests per week. We need to urgently fix clinical diagnostic testing and accelerate the introduction of faster, cheaper, point-of-care screening tests to prepare for the next flu season.”
The analysis
Contact tracing involves tracking down all of the people who have been in contact with an infected individual so they can be isolated to prevent further spread of the virus. This approach is an established public health measure recommended by the World Health Organisation (WHO) as a potential exit strategy to enable the alleviation of Covid-19 lockdown measures.
Conventional contact tracing methods involve a public health professional contacting the infected person and asking them to recall everyone they have been in contact with over a defined period before the onset of symptoms. 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 rate of transmission of the virus, known as 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.
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 reproduction number and the fraction of onward transmission cases that can be prevented 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, they explain.
In the best-case scenario, the model predicts that contact tracing could reduce the number of people a person with Covid-19 passes the virus on to from 1.2 to 0.8. For this to work, at least 80% of eligible people must be tested, there must be no delays in testing after the onset of symptoms and at least 80% of contacts must be identified on the same day as the test results are received. If testing is delayed by two days, keeping the reproduction number below 1 would require contacts to be traced within a day and at least 80% of contacts must be identified, the model predicts.
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 reproduction number below 1 if people with coronavirus 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 the 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. “The proportion of onward transmissions per index case that can be prevented depends on testing and tracing delays, and given a 0-day tracing delay, ranges from up to 79.9% with a 0-day testing delay to 41.8% with a 3-day testing delay and 4.9% with a 7-day testing delay,” they emphasize.
“Testing infrastructure is, therefore, the most critical factor for the success of a contact tracing system. This means that as many infectious people as possible need to be tested, and policymakers might consider lowering the eligibility threshold for access to testing. This will lead to a large proportion of negative test results, however, and future studies should focus on identifying the optimal balance between the proportion of negative tests and the effectiveness of contact tracing,” says one of the lead authors of the study, Professor Marc Bonten, Utrecht University.
Writing in a linked comment, Professor Louise Ivers from Harvard Medical School, and Professor Daniel J Weitzner from Massachusetts Institute of Technology, who were not involved in the study, say crucial questions remain to be investigated. This includes an assessment of how well smartphones measure proximity, and a better understanding of how mobile apps will integrate with overall contact tracing programs needs to be investigated. The authors also call for further research to understand what factors will encourage users to trust the privacy and security properties of mobile apps. “Understanding the potential impact of apps as part of a comprehensive integrated approach requires more evaluation of their use in real life and multidisciplinary engagement of technologists, epidemiologists, public health experts, and the public,” they write in the commentary.