Highly effective contact tracing and isolation may be viable containment strategies for a coronavirus disease 2019 [COVID-19] outbreak largely depend on the number of secondary infections generated by each new patient and the percentage of transmission that occurs before symptom onset, which are currently unknown variables for COVID-19. This is according to results of a study published in The Lancet Global Health.
The investigators of this study developed a stochastic transmission model to quantitatively assess if isolation and contact tracing are effective methods to control onward transmission of COVID-19. The model explored a range of epidemiologic scenarios representing different transmission characteristics: the number of initial patients; reproduction number (the number of secondary patients produced by each individual; delay between symptom onset and isolation (short, long); probability that contacts were traced; percentage of transmission before symptom onset; and the percentage of subclinical infections. In the model, isolation was assumed to prevent further transmission. Outbreaks were controlled if no new infections were reported 12 to 16 weeks after initial patients or before 5000 cumulative patients. Feasibility of outbreak control was reported as the weekly maximum number of patients undergoing contact tracing and isolation for each successful scenario.
The probability of controlling an outbreak was higher at all levels of contact tracing when the reproduction number was 1.5. However, when the reproduction number was 2.5, 80% of contacts needed to be traced and isolated to achieve control in 90% of the simulated outbreaks. Contact tracing and rapid isolation were associated with preventing transmission, which effectively decreased the reproduction number and improved the number of scenarios in which outbreak control was likely.
The number of initial patients played a large role in the probability of achieving control: a scenario starting with 5 initial patients had a >50% chance of being controlled within 3 months, and 40% of the outbreaks being controlled with no contact tracing at all.
The largest variable in determining whether an outbreak was controllable was the delay between symptom onset and isolation: at 80% of contacts traced, the probability of achieving control decreased from 89% when calculated with a short delay to 31% calculated with a long delay.
Transmission occurring before symptom onset at the rates of 15% and 30% made it more difficult to achieve outbreak control. In many of the scenarios run by the investigators, between 25 and 100 symptomatic cases occurred within one week of the peak of the simulate outbreak; all of these patients and their contacts would need to be isolated. With the assumption of 20 contacts per symptomatic patient, this means that for 100 cases, a total of 2000 contacts would been to be traced to achieve control.
In several plausible scenarios, the investigators suggested that contact tracing and patient isolation alone would be insufficient to control outbreaks. However, rapid and effective contact tracing can reduce the initial number of patients, which would make the outbreak easier to control overall. Long delays between symptom onset to isolation, lower levels of contact tracing, and a higher proportion of transmission before symptom onset are all factors that decrease the probability of achieving outbreak control.
Hellewell J, Abbott S, Bosse NI, et al. Feasibility of controlling COVID-19 outbreaks by isolation of cases and contacts [published online February 28, 2020]. Lancet Glob Health. doi:10.1016/S2214-109X(20)30074-7