Rigorous surveillance is necessary for high confidence in end-of-outbreak declarations for Ebola and other infectious diseases

Abstract

The World Health Organization considers an Ebola outbreak to have ended once 42 days have passed since the last possible exposure to a confirmed case. Benefits of a quick end-of-outbreak declaration, such as reductions in trade/travel restrictions, must be balanced against the chance of flare-ups from undetected residual cases. We show how epidemiological modelling can be used to estimate the surveillance level required for decision-makers to be confident that an outbreak is over. Results from a simple model characterizing an Ebola outbreak suggest that a surveillance sensitivity (i.e. case reporting percentage) of 79% is necessary for 95% confidence that an outbreak is over after 42 days without symptomatic cases. With weaker surveillance, unrecognized transmission may still occur: if the surveillance sensitivity is only 40%, then 62 days must be waited for 95% certainty. By quantifying the certainty in end-of-outbreak declarations, public health decision-makers can plan and communicate more effectively. This article is part of the theme issue 'Modelling infectious disease outbreaks in humans, animals and plants: epidemic forecasting and control'. This issue is linked with the earlier theme issue 'Modelling infectious disease outbreaks in humans, animals and plants: approaches and important themes'.

Keywords: Ebola virus disease; World Health Organization; end-of-outbreak declarations; outbreak forecasting; surveillance.

Figure 1.

The confidence in end-of-outbreak declarations following the apparent end of an Ebola outbreak. (a) Schematic showing the sequence of events in Liberia at the end of the 2013–2016 Ebola epidemic, in which the outbreak was incorrectly declared over three times. (b) Schematic of the compartmental model used in our analyses. (c) The number of hidden cases (E or C) remaining in simulated Ebola outbreaks at the first timepoint at which the number of symptomatic cases (I) reaches zero. (d) The confidence in end-of-outbreak declarations (i.e. the probability that no undetected infections (E or C) remain in the population), for different time periods after removal of the ‘final’ symptomatic case (blue) at the ends of major outbreaks (outbreaks in which more than 20 individuals are ever infected). The current WHO guideline period of 42 days leads to a confidence of 0.84 that the outbreak is over (red), whereas periods of 62 or 88 days correspond to confidences of 0.95 (light green) and 0.99 (dark green), respectively. (e) The confidence in end-of-outbreak declarations made 42 days after removal of the ‘final’ symptomatic case, for different values of the surveillance sensitivity (blue). For an end-of-outbreak confidence of 0.95 after 42 days without symptomatic cases, a surveillance sensitivity of 79% is required (light green). The results in panels (c) and (d) were obtained using 10 000 simulations of the model, and the results in panel (e) were obtained using 10 000 simulations of the model for each possible value of the surveillance sensitivity.