If current trends continue, they say the future number of cases in Liberia and Sierra Leone could exceed 550,000 by January. That estimate is based on reported cases. If under-reporting is taken into account, their calculations suggest that figure is likely to be nearer 1.4 million.
While the Centers for Disease Control and Prevention (CDC) say they cannot guarantee the accuracy of their estimates, they are clear about their key messages: "if conditions remain unchanged, the situation will rapidly become much worse," and they serve as "a warning and a call to action."
The CDC report their calculations - and how they arrived at them with a new model - in their latest Morbidity and Mortality Weekly Report (MMWR). The report details the CDC estimates for future Ebola cases in Liberia and Sierra Leone, where the epidemic is uncontained and out of control.
Number of cases doubling every few weeks
The report notes that on March 22, 2014, there were 49 reported cases of Ebola virus disease (usually referred to as Ebola) in Guinea, where the current outbreak in West Africa began.
According to the CDC, there will be up to 21,000 cases of Ebola in Liberia and Sierra Leone by the end of September.
By the end of August, the outbreak had spread to neighbouring Liberia and Sierra Leone, and the total number of probable, confirmed and suspected cases had risen to 3,685.
The CDC estimate that by the end of September, there will be around 8,000 Ebola cases - or as many as 21,000 cases if corrections for under-reporting are included - in Liberia and Sierra Leone.
And if current trends continue, "without additional interventions or changes in community behavior," these figures will rise to around 550,000 cases - or 1.4 million including under-reported cases.
The estimates assume the current conditions of disease transmission will remain unchanged. The main driver of the estimates is that in Liberia, the number of cases is doubling about every 15-20 days, and in Sierra Leone and Guinea, they are doubling around every 30-40 days.
EbolaResponse modeling tool takes into account disease stages and types of isolation
For their calculations, the CDC constructed a new modelling tool called EbolaResponse, comprising an Excel spreadsheet containing a set of formulae and assumptions.
The model allows researchers to estimate the daily movement of patients within disease stages (i.e. susceptible, incubation, infectious, recovery or death) using probabilities for three different types of isolation.
The three different types of isolation are: hospitalization (such as in an Ebola treatment unit or medical care facility), home with effective isolation (a home or community setting where there is reduced risk of disease spread, including safe burial where needed), and home with no effective isolation.
Ideally, Ebola treatment units and appropriate medical care facilities have infection control procedures that prevent disease spread, but the CDC note this is not always the case. So the model assumes there is an average daily risk that transmission will occur - however this is fewer than one person infected per infectious patient.
The model includes information from previous Ebola outbreaks - for example, periods of infectivity, time between exposure and illness. There is no evidence that the virus is spreading differently from previous outbreaks.
To estimate the number of under-reported cases, the report uses a factor of 2.5. This was calculated using estimates of beds-in-use from the model and comparing them to expert opinions of actual beds-in-use on a given day (August 28th). The difference is a potential under-reporting correction factor (1.5 more beds were being used than the model estimated.)
'Cost of delay will be devastating'
The CDC say we know how to control and stop the epidemic. The model suggests this will require about 70% of people infected with Ebola be cared for in Ebola treatment units, or if these are full, they should be cared for at home or in a community setting, where risk of disease is reduced and burials are conducted safely.
Every month of delay in reaching this 70% target will increase the number of cases and deaths, and the need for more beds and resources. "The cost of delay will be devastating," say the CDC.
If we are to avoid the catastrophic scenario the model projects for January, effective interventions need to occur quickly, says the federal agency. This includes "appropriate disease control methods, communication, changes in community behavior, and adequate resources (such as staff, beds, equipment, supplies)."
The CDC model estimates the number of beds that would be needed in medical facilities and Ebola treatment units, but it is not designed to give details of resources such as enhanced protection measures to minimize disease spread.
In a recent Science editorial, Peter Piot, who in 1976 co-discovered the Ebola virus in Zaire (now Democratic Republic of Congo) while working at the Institute of Tropical Medicine in Antwerp, Belgium, says that the current Ebola crisis requires a "rapid response at a massive global scale."
Now a professor at the London School of Hygiene & Tropical Medicine, Prof. Piot says the epidemic in West Africa is the result of a "perfect storm" that combines dysfunctional health services, low trust in Western medicine and governments, denials that the Ebola virus exists, and unhygienic burial practices.
Meanwhile, the UN Security Council has resolved that the Ebola outbreak in Africa constitutes a threat to international peace and security, and calls on member states to respond urgently to the crisis.
The crisis is so urgent that an expert panel of the World Health Organization (WHO) concluded in August that it would be ethical to evaluate unregistered investigational treatments for Ebola virus disease in people.
To this end, the aid agency MSF/Doctors without Borders, recently reported that Ebola treatment trials are to be fast-tracked in West Africa for the first time.