Between 550,000 and 1.4 million people in West Africa could be infected with the Ebola virus by January 20, 2015, according to a report issued on Tuesday by the US Centers for Disease Control and Prevention (CDC).

The top range of the estimate, 1.4 million, assumes that the number of cases officially cited so far, 5,864 according to the count kept by the World Health Organisation, is significantly underreported, and that it is likely that 2.5 times as many cases, or nearly 20,000, have in fact occurred.

CDC emphasised that the projections, based on an epidemiological model that takes into account how many people each Ebola patient eventually infects as well as other factors, is based on data available in August. They therefore do not account for the recently announced US government Ebola relief effort, which includes sending 3,000 members of the armed forces to the Ebola-stricken region.

"Extensive, immediate actions - such as those already started - can bring the epidemic to a tipping point to start a rapid decline in cases," CDC said in a statement.

Meanwhile, 30 more health care workers have died of Ebola in Sierra Leone than previously thought, WHO said on Tuesday, suggesting the risk to medical staff may have been understated.

A WHO update published on Monday put the number of dead healthcare workers in Sierra Leone at 61, out of a total of 96 who had fallen ill with the disease.

An update last week said 74 healthcare workers had caught Ebola in Sierra Leone as of September 14 and 31 had died.

The revised figure means almost six out of 10 health workers who caught the disease in Sierra Leone have died, rather than four out of 10 as previously thought.

WHO epidemiologist Eric Nilles said the additional deaths were found during a "retrospective evaluation" that aimed to improve the overall quality of data published by the WHO.

"WHO continues to improve the characterisation of the epidemic in Sierra Leone and as we do so we may see further increases in the proportion of health care workers affected," Nilles said.

The WHO said that as of Sept. 22, a total of 348 health care workers were known to have developed Ebola and 186 of them had died. Half of the cases were in Liberia and 67 in Guinea, which along with Sierra Leone have been worst hit by the outbreak.

In Nigeria, 11 health workers have contracted the disease and five have died. In total, Nigeria has had 8 Ebola deaths and 20 cases.

United Nations and WHO officials have said the high risk for health workers has hampered efforts to find volunteers to go to the epicentre of the outbreak in West Africa.

"We have difficulty in attracting the required numbers of people who are volunteering and feel comfortable to do so," said Antonio Viglilante, head of the UN Development Programme in Liberia.

The Ebola epidemic in West Africa has killed more than 2,800 people since it began in Guinea earlier this year. - Reuters