The Ebola epidemic sweeping West Africa could infect up to 500,000 people by the end of January, according to a new estimate under development by the Centers for Disease Control and Prevention.

The report is scheduled to be released next week, but work on it is still ongoing and projections could change, said a person who is familiar with its contents but was not authorized to speak because the report is not yet public.

The CDC projection assumes no additional aid by governments and relief agencies. But the United States this week launched a $750 million effort to establish treatment facilities with 1,700 beds in Liberia, the hardest hit country. And the U.N. Security Council voted unanimously Thursday to create an emergency medical mission to respond to the outbreak, with an advance team in West Africa by the end of the month.

“CDC is working on a dynamic modeling tool that allows for recalculations of projected Ebola cases over time,” Barbara Reynolds, a spokeswoman for the agency, said in an e-mail. “CDC expects to release this interactive tool and a description of its use soon.”

The CDC projection was first reported by Bloomberg News.

The Ebola epidemic over time

The World Health Organization said last month that the outbreak could reach 20,000 cases before it is brought under control. But infectious disease experts, aid officials and global health advocates said cases are increasing so rapidly that the total number is almost certain to be much higher, especially in the worst-affected countries of Liberia, Guinea and Sierra Leone.

On Friday, the streets of Sierra Leone’s capital city of Freetown were uncharacteristically empty as the government began a controversial three-day lockdown in an effort to slow the spread of the deadly virus. Only law enforcement personnel, security officials and a handful of others deemed “essential” by the government are allowed on the streets during that time — and only with a government-issued pass, the Guardian reported. Researchers have been using different models in an attempt to predict the size of the outbreak, but the complexity and magnitude of the current epidemic poses additional challenges.

Previous outbreaks have been in rural areas and were brought under control using a relatively straightforward strategy of isolating and treating infected patients and contact tracing — finding everyone who comes into direct contact with a person, watching for signs of illness, and then isolating and treating them.

But now that the disease has spread to the large cities and urban areas, cases are doubling in about three weeks, U.N. officials said Thursday.

“One of the scary things about this outbreak is that all the general models of the past have been broken,” said John Connor, associate professor of microbiology at Boston University School of Medicine and investigator at the university’s National Emerging Infectious Diseases Laboratories.

He said there is no way to know how many people will be infected in the future, because there are too many variables. “That’s like asking whether it’s going to be sunny a month from now,” he said.

“I’m really worried that no one has a handle on everything that’s happened,” he said. “Do we know all the places where there’s been virus present?”

Millions of people in Sierra Leone are preparing for a three-day nationwide lockdown, which aims to curb the spread of Ebola and allow health workers to share information and calm fears about the virus. (Reuters)

Alessandro Vespignani, a Northeastern University physicist, has looked at the outbreak and studied the response on the ground. He has worked on a model to estimate the growth of the disease, and by the end of this month, he has said, the epidemic could get much worse — as in, “thousands and thousands of cases” worse. Vespignani and his colleagues project between 6,000 and 10,000 Ebola cases by late September.

“These predictions are like the weather forecast,” Vespignani said. “These are statistical predictions. So you have uncertainty cones.”

Vespignani’s model assumes that response to the deadly virus stays the same, that the picture of the event remains unchanged. It assumes there will not be increased medical attention and that the spread of Ebola will continue.

As of Sunday, the WHO said the virus had infected 5,357 people and killed 2,630. In addition to Guinea, Liberia and Sierra Leone, the disease has spread to Nigeria and Senegal. WHO officials said those numbers vastly underreport the true number of cases.

Laurie Garrett, senior fellow for global health at the Council on Foreign Relations, who has been speaking with health workers and officials in the region, said that the cumulative number of cases will probably reach 250,000 by Christmas unless the U.S. plan provides for immediate measures that lead to long-term escalations in supplies, logistics support, air shipments and personnel on the ground in West Africa.

Joel Achenbach and Sarah Larimer contributed to this report.