Do Isolation and Contact Tracing Work?

A mathematical modeling paper aims to determine the extent to which the isolation of infected patients and the tracing of those they may have come into contact with is effective at reducing the spread of COVID-19 outbreaks. HGHI’s Anders Gundersen with a summary of the findings.

Feasibility of controlling COVID-19 outbreaks by isolation of cases and contacts

The Lancet Global Health | Hellewell et al. | Feb. 28, 2020 To produce these estimates, a number of variables were estimated or altered to gain an understanding of how possible it is to contain the virus under a few different circumstances. These variables (along with the values used in the study to construct the various scenarios) were:
  1. R0 – The reproductive number(1.5, 2.5, 3.5) (See also our explainer on R0)
  2. The number of initial cases (5, 20, 40)
  3. Delay from symptom onset to isolation (3.43, 8.09 days)
  4. Percentage of contacts that were traced (0%, 20%, 40%, 60%, 80%, 100%)
  5. Proportion of transmission that occurred before symptom onset
  6. The proportion of asymptomatic infections (0%, 10%)

Simply put, the authors found that with a reproductive number of 1.5, containment was probable regardless of the severity of the other variables considered. With a reproductive number of 2.5 (the average of most available estimates), however, containment was much less likely unless roughly 80% of contacts were traced. Additionally, larger numbers of initial cases were shown to be much more difficult to contain. With 40 initial cases and a reproductive number of 2.5, even tracing 80% of contacts did not lead to containment.

This paper highlights the ongoing uncertainty of this outbreak. Many of these scenarios are possible and many of these variables are still unknown. What this paper does indicate is that even the most likely scenarios pose a considerable containment challenge, but contact tracing and isolation will help to reduce the overall number of cases and the chances of transmission, ultimately making the outbreak easier to manage overall.