As the number of coronavirus cases in India increases, authorities in different states are relying on contact tracing, a concept in epidemiology (the study of the distribution and determinants of health-related states or events) that involves tracing the number of people an infected person comes in contact with. The idea behind contact tracing is to stop the outbreak by breaking the transmission chains. But what exactly does it involve and how effective is it?
What is contact tracing?
Contact tracing is not a novel concept and has been used as a method to track cases of the Ebola virus in Africa. It is one of the methods of detecting an outbreak and the number of infected people.
In 2014, when the first Ebola cases began to be reported in Sierra Leone’s Kambria district, a contact tracing mechanism was devised, wherein the tracers were responsible for monitoring the contacts of confirmed Ebola cases daily. According to the Centre for Disease Control (CDC), the system in the district was able to identify 13 Ebola cases, which would have been overlooked otherwise.
According to WHO, contact tracing can be broken down into three steps:
a) Contact identification. This involves identifying the contacts of the infected person by asking about the person’s activities and those of people around them since the onset of illness. In the case of the first positive COVID-19 patient from Chandigarh for instance, a chain of 119 people was traced directly or indirectly to the patient;
b) Contact listing. This means listing all those people who came in contact with the infected person. “Efforts should be made to identify every listed contact and to inform them of their contact status, what it means, the actions that will follow, and the importance of receiving early care if they develop symptoms,” WHO says. In some areas across India, authorities are releasing lists of those who are quarantined and are identifying their houses by putting quarantine posters in front of their houses;
c) Contact follow-up. which involves regular follow-ups with all the contacts to monitor for symptoms and test for signs of infection.
How can contact tracing help in controlling the coronavirus outbreak?
In a detailed Twitter thread, Marcel Salathé, Professor at the École polytechnique fédérale de Lausanne, has said that since everyone has many contacts, contact tracing is useful when there are only a few cases. “At this point, in many countries, we have so many cases that everyone would be contacted. This is essentially the lockdown — everybody isolates.”
Research published in the journal Lancet in February on the feasibility of controlling the COVID-19 outbreak by using isolation of cases and contacts said that in order to control 90 per cent of the outbreaks, 80 per cent of contacts needed to be traced and isolated.
However, while a fifth of the world’s population is currently isolated and under lockdown, it may not be feasible to trace contacts of all the infected patients given the scale of the current coronavirus outbreak, with over 4 lakh infected people.
Further, the study notes that in some scenarios, isolation alone would be unlikely to control the outbreak within a period of three months. “Case isolation was more effective when there was little transmission before symptom onset and when the delay from symptom onset to isolation was short.”
The conclusion may be that while contact tracing is an important step during a disease outbreak, it is insufficient alone in controlling it, requiring other interventions. “Rapid and effective contact tracing can reduce the initial number of cases, which would make the outbreak easier to control overall. Effective contact tracing and isolation could contribute to reducing the overall size of an outbreak or bringing it under control over a longer time period,” the study says.