Dynamic and control of Ebola virus
The ongoing outbreak of Ebola in West Africa is unprecedented in scale both geographically as in number of cases. Up to 19 October, there have been more than 9 900 confirmed, probable and suspected cases and over 4 800 deaths reported in the affected countries.
The latest data report an increase of 720 cases and 322 deaths in one week between 14 and 19 October [1]. In the Lancet Infectious Diseases, Joseph Lewnard and colleagues [2] use a mathematical model to assess the effectiveness of expanding the currently available interventions for controlling the outbreak in Montserrado, one of the most affected areas. The interventions considered are upscaling of Ebola treatment centres, increasing case ascertainment through contact tracing, and intensified allocation of household protective kits.
The authors estimate the basic reproduction number in Montserrado to be 2.49 (95% CI 2.38-2.60). This value defines the expected number of EVD cases caused by an individual infected with Ebola virus in an otherwise susceptible population without any public health or clinical interventions. The basic reproduction number calculated in this article, as with any mathematical model, relies heavily on the underlying assumptions regarding certain parameters (e.g. incubation time, infection time, mortality and underreporting). Basing themselves on an estimated reproduction number of 2.49, the authors project that by 15 December 2014, 170 996 total reported and unreported Ebola cases, and 90 122 deaths will be reported as a result of the Ebola outbreak in the region of Montserrado alone if no further expansion of control measures will be in place.
Using the full capacity of a mathematical model, the authors demonstrate the urgent need for further expansion of existing control measures in this region. The simulations, modelling three existing intervention strategies, demonstrate the need for a further increase of these interventions to reduce the impact on the number Ebola cases and deaths. Thus, many more EVD treatment centres than have been planned are needed to avert substantial numbers of EVD cases and deaths. The effectiveness of new EVD treatment centres can be maximised with concurrent acceleration of case ascertainment and the allocation of protective kits.
These interventions will further reduce EVD cases and deaths and augment control probability but are insufficient as stand-alone interventions.
In addition, a timely response is needed to prevent further escalation of the spread of the disease as even small delays, according to the model, will have a negative impact on the number of lives that can be saved. Four thousand eight hundred additional beds at treatment centres in combination with five-fold increased case ascertainment can avert 77 312 cases by 15 December. However, this exceeds largely the currently efforts to scale up the interventions. The authors estimate that 137 432 (range 129 736 – 145 874) Ebola cases could have been avoided if all three intervention strategies would have been in place before 15 October 2014. If only in place on 15 November 2014, the number of Ebola cases avoided would be 53 957 (range 46 963 – 60 490).
ECDC comment:
These projections should be regarded as indicative of possible trends and not as exact predictions, as they rely heavily on the assumptions used by the modellers. However, in line with other publications, this model demonstrates the severity of the outbreak and the escalating situation in the short term. The strength of this paper also comes from the focus on one specific region, Montserrado, and the acknowledgement that the dynamic of the disease outbreak is different among the regions. It addresses the importance that health care interventions need to be tailored and targeted to each region specifically. Finally, the paper shows that simple efforts increasing intervention strategies already in place can have a huge impact in reducing the number of Ebola case in the short run. To reverse the current escalation of the outbreak, there is a critical need to increase the extent of intervention far beyond what is currently planned and to ensure interventions are implemented in the shortest possible timeframe.