Monday, December 1, 2014

How will we know when the number of Ebola infections starts to decline?

According to the World Health Organization (WHO), three West African countries continue to experience intense transmission of Ebola. More than 16,000 cases of Ebola have been reported from Guinea, Liberia, and Sierra Leone in the past several months since the outbreak started earlier this year (link). There is some evidence that the rate of new Ebola infections in these countries is not growing as fast as previously estimated which is good news. The WHO situation report published on November 26, 2014 (link) states “Case incidence is stable in Guinea, stable or declining in Liberia, but may still be increasing in Sierra Leone”.

However, there is great uncertainty over the quality of the reporting data emanating from West Africa on this Ebola outbreak. Also, based on the fluctuating numbers of newly reported cases in each of these three countries, it is difficult to assess the increases or decreases in the incidence of cases in these three countries. Assuming that the case numbers reported in the WHO situation reports are representative the number of infections in each of these countries, the average number of new cases per day can be graphed on a timeline. Below, the average number of new cases per day is compared with the cumulative moving average of cases since the start of the outbreak within each of the three countries.

These graphs clearly show that the trajectory of the number of new cases in each country generally support the WHO statement. At this time, the number of new  daily cases  in Guinea appears to be declining towards the cumulative moving average. Recent new case counts for Liberia have fallen below the long term cumulative moving average. For Sierra Leone, the reported average number of Ebola cases is above the cumulative moving average,  This is a clear indication that rate of Ebola infections in Sierra Leone have not yet started to decline.

Eventually, declines in the number of new Ebola infections in these three countries will only be apparent when the daily average of newly reported Ebola cases drops below-and stays below-the cumulative moving average. At that time the cumulative moving average will begin to decline as well. Comparing the number of daily new cases in these countries in relation to the cumulative moving daily average will help identify when there is a downturn in the number of new Ebola infections in these countries. 

Graph Notes:

1. Data used to construct these graphs is derived from the country totals provided by WHO in the Ebola situation and data updates current through November 28, 2014 (link). The new daily cases counts includes all Ebola cases reported from the country including, confirmed, probable, and suspected cases.

2. The average number of new Ebola cases per days is computed as the total number of newly reported cases since the last report divided by the number of reporting days. The average number of new cases per day is recalculated after each WHO report.