TORONTO – The case count in West Africa’s unprecedented Ebola outbreak could grow by tens or even hundreds of thousands of cases before the end of this year, a new study suggests.
The work, published Thursday in the online journal Eurosurveillance, said that if growth continues at its current pace, a worst-case scenario would see an additional 77,181 to 277,124 cases by the end of 2014.
The authors suggested it is unlikely the worst-case scenario would come to pass because containment efforts are being scaled up in response to the crisis. But in an interview, one of the authors admitted that if the current numbers are wrong and many more cases have been missed in recent weeks, their estimate could potentially be too low.
“If the cases are roughly uniformly distributed over time, then our estimates shouldn’t change much,” said Gerardo Chowell of Arizona State University in Tempe.
However, if most of the missed cases had more recently occurred, “this would be very bad news.”
An editorial that accompanied the study warned that the world cannot hope that this Ebola outbreak will burn itself out. It was written by Adam Kucharski and Peter Piot of the London School of Hygiene and Tropical Medicine. Piot is one of the discoverers of the Ebola virus.
They called the epidemic an international crisis and said it demands a commensurate response.
“It is true that outbreaks of acute infections will generally decline once a large number of people have been infected because there are no longer enough susceptible individuals to sustain transmission,” the authors noted.
“But … given the vast populations in affected areas and the disease’s high fatality rate, this is clearly not an acceptable scenario.”
They suggested that unless the containment efforts start to gain ground, the number of cases could double every two weeks.
The most recent numbers from the World Health Organization suggest there have been at least 4,269 cases of the disease and 2,288 deaths in this outbreak, which is believed to have started late last year. Those numbers are higher than the total of all known Ebola cases and Ebola deaths since the first known outbreak in 1976.
The agency has said it believes its numbers are underestimates, but because the outbreak is so large and the situation in hot zones so chaotic it does not have a good handle on how far off the numbers are.
Because of those problems, an infectious diseases expert at the University of Minnesota warned against seeing the numbers in this study as a true projection.
Michael Osterholm said he wasn’t trying to suggest the case numbers aren’t big or that they won’t get much bigger, but just that at this point it is difficult to quantify how large the outbreak might get.
“All we can say is there are lots and lots of cases, and more coming. And there are lots and lots of deaths, and more coming,” said Osterholm, who is director of the Center for Infectious Diseases Research and Policy.
“And I don’t think we can put any real precision around this at this point. All we can say for our planning is that it’s a very, very big number. And we just have to take that into consideration.”
The work is what is known as a modelling study. Chowell and a colleague, H. Nishiura from the University of Tokyo, calculated what is called the reproductive number of the outbreak, the number of new infections that stem from an average case. When a disease has a reproductive rate of less than one, it cannot sustain itself and will eventually stop spreading.
In the case of this outbreak, they estimated the reproductive rate was between one and two. In Sierra Leone and Liberia, the harder hit of the countries, the reproductive number was calculated to be between 1.4 and 1.7. The worst-case scenario of 77,181 to 277,124 was calculated using those figures.
Chowell, who was reached in Japan, said in a telephone interview that as of Aug. 26 there was no evidence the containment efforts were bringing down the reproductive number. That would occur if new cases were isolated as soon as the people became sick, eliminating the risk they would infect others. People with Ebola are only believed to be contagious when they have symptoms.
Chowell said he and Nishiura will continue to track the numbers to see if the increased efforts are making an impact on the outbreak.