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.

Discrepancies in the World Health Organization’s Count of MERS Cases in Saudi Arabia

With Saudi Arabia reporting more than 80% of all Middle East Respiratory Syndrome (MERS) cases to date, detailed information about the individual cases from Saudi Arabia is critical to understanding the nature and spread of this novel disease. There are some discrepancies among the MERS cases reported by World Health Organization (WHO) from Saudi Arabia. Of course, WHO is constrained by the quality of data provided by its various member states.

Generally, WHO provides detailed information on the initial cases of a novel disease outbreak in its online publication, Disease Outbreak News (DON). The first WHO report of a MERS [novel coronavirus] infection was published on September 23, 2012. Between September 2012 and April 16, 2014, WHO reported details on 228 MERS cases with varying levels of details provided by the reporting member states. Of the 228 cases reported by WHO though that date, 181 were individually reported cases from Saudi Arabia. The DON of April 14, 2014 (15 reported cases from Jeddah and Riyadh) was the last Saudi Arabian case-by-case report from WHO. After that date, WHO only provided aggregate case totals from Saudi Arabia. These aggregate totals were embedded in 6 DON reports between May 7, 2014 and June 13, 2014 as noted in the table below.

On June 13, 2014, WHO provided a summary of these aggregated cases. According to WHO, 402 cases were summarized. However, a tally of the WHO totals from the previous DONs indicates that only 401 cases were actually aggregated. A comparison of the WHO total with media reports from the Saudi Arabia Ministry of Health indicates that 404 MERS cases were publicly reported by the Ministry of Health.

For the period between April 11, 2014 and May 4, 2014, WHO reports 229 cases from Saudi Arabia, although the Saudi Arabia Ministry of Health only publicly reported 228 cases during that period. One possible explanation is that the 51 one-year-old individual from Riyadh reported on April 9, 2014 by  the Saudi Arabia Ministry of Health (link FluTrackers case # 227), not previously enumerated by WHO, was added to the aggregate total (see: link). Also,  between May 5, 2014 and May 9, 2014 Saudi Arabia Ministry of Health reported 62 MERS cases. During this same period WHO only reported 58 cases in aggregate (see: link). It is possible that WHO failed to incorporate four cases into its cumulative tally for this 5-day period in Saudi Arabia.

With the reporting of specific case details of two individuals from Saudi Arabia in DON on June 16, 2014,  WHO has now apparently returned to reporting individual case details from Saudi Arabia. Better individual case details may again be flowing from Saudi Arabia to WHO.

Depending on the quality and accuracy of the data provided by Saudi Arabia, a discrepancy of 3 or 4 cases among more than 500 reported cases falls within a reasonable error factor. Such differences will not appreciably affect interpretations or speculations drawn from such a large sample of cases.

Of more serious concern is a report on June 3, 2014  by the Saudi Arabia Ministry of Health indicating retrospective reporting of more than 100 additional MERS cases in Saudi Arabia (and acknowledged by WHO - link) going back to May 2013 (See: The Saudi Arabia Ministry of Health Quietly Announces an Additional 113 Cases of MERS). This means that from May 2013 through April 2014, about 15% of all MERS cases in Saudi Arabia were unreported. A 15% error factor is unacceptable when public health officials and the rest of the world are trying to understand the nature of a novel infectious disease with a high fatality rate.