Testing, self-isolation and contact tracing with physical distancing measures, could control the COVID-19 pandemic
In the absence of a vaccine or highly effective treatments for COVID-19, combining isolation and intensive contact tracing with physical distancing measures – such as limits on daily social or workplace contacts – might be the most effective and efficient way to achieve and maintain epidemic control, according to a new research published in The Lancet Infectious Diseases journal.
Using social-contact data on more than 40,000 individuals from the BBC Pandemic database to simulate SARS-CoV-2 transmission in different settings and under different combinations of control measures, the researchers estimate that a high incidence of COVID-19 would require a considerable number of individuals to be quarantined to control infection. For example, a scenario in which 5,000 new symptomatic cases were diagnosed each day would likely require 150,000-200,000 contacts to be quarantined every day if no physical distancing was in place.
The study is the first time researchers have used social contact data to quantify the potential impact of control measures on reducing individual-level transmission of SARS-CoV-2 in specific settings.Â
Researchers modelled the rate at which the virus is transmitted, known as the reproductive number (R), or the average number of people each individual with the virus is likely to infect at a given moment under different strategies. To keep the COVID-19 epidemic declining, R needs to be less than 1.
Successful strategies will likely include intensive testing and contact tracing supplemented with moderate forms of physical distancing
In the model, the secondary attack rate (the probability that a close contact of a confirmed case will be infected) was assumed to be 20% among household contacts and 6% among other contacts. The researchers calculated that, had no control measures been implemented, R would be 2.6 – meaning that one infected person would infect, on average, 2-3 more people.
The model suggested that mass testing alone, with 5% of the population undergoing random testing each week (i.e. 460,000 tests per day in UK), would lower R to just 2.5, because so many infections would either be missed or detected too late.
Compared with no control measures, self-isolation of symptomatic cases (at home) alone reduced transmission by an estimated 29% (lowering R to 1.8); whilst combining self-isolation, household quarantine, and tracing strategies could potentially lower transmission by as much as 47% (R 1.4) when using app-based contact tracing (assuming the app is adopted by 53% of the population), and by 64% with manual tracing of all contacts (R 0.94).
Limiting daily contacts outside home, school, and work to four people (e.g. by restricting mass gatherings) along with manual tracing of acquaintances only (i.e. people they have met before) and app-based tracing, would have the greatest impact, reducing disease spread by 66%, and lowering R to 0.87. However, they note that the effectiveness of manual contact tracing strategies is highly dependent on how many contacts are successfully traced.
“Our findings reinforce the growing body of evidence which suggests that we can’t rely on one single public health measure to achieve epidemic control”, said Dr Adam Kucharski from the London School of Hygiene & Tropical Medicine, UK. “Successful strategies will likely include intensive testing and contact tracing supplemented with moderate forms of physical distancing, such as limiting the size of social gatherings and remote working, which can both reduce transmission and the number of contacts that need to be traced.”
He added: “The huge scale of testing and contact tracing that is needed to reduce COVID-19 from spreading is resource intensive, and new app-based tracing, if adopted widely alongside traditional contact tracing, could enhance the effectiveness of identifying contacts, particularly those that would otherwise be missed.”
