A sharp drop in new Ebola infections in West Africa is prompting scientists to wonder whether the virus may be immunizing some people at the same time as killing their neighbors.

So-called "asymptomatic" Ebola cases - in which someone is exposed to the virus and develops antibodies but does not get sick or suffer symptoms - are disputed among scientists, with some saying their existence is little more than a pipe dream.

Yet if, as some studies suggest, such cases do occur in epidemics of the deadly disease, they may be a key to ending outbreaks quickly by protecting those who can keep the infection at bay.

"We wonder whether 'herd immunity' is secretly coming up - when you get a critical mass of people who are protected, because if they are asymptomatic they are then immune," Philippe Maughan, senior operations administrator for the humanitarian branch of the European Commission, said. "The virus may be bumping into people it can't infect any more."

'Zero by February'

Latest World Health Organization data show new cases of infection in West Africa's unprecedented Ebola epidemic dropping dramatically in Guinea, Sierra Leone and particularly in Liberia.

Liberia on Monday announced the closure of an Ebola treatment facility that was at the center of the virus' worst outbreak in history. "We think we can make it to zero by the end of February at latest," Liberian Commerce Minister Axel Addy said in Geneva. "We're quite optimistic."

Most experts are sure the main driver is better control measures reducing direct contact with contagious patients and corpses, but there may also be other factors at work.

So-called herd immunity is a feature of many infectious diseases and can, in some cases, dampen an outbreak if enough people get asymptomatic, or "subclinical" cases and acquire protective antibodies. After a while, the virus - flu, measles, polio - cannot find nonimmune people to be its hosts.

But some specialists with wide experience with disease outbreaks are skeptical about whether this phenomenon happens in Ebola, or whether it could affect an epidemic.

"There is some suggestion there may be cases that are less severe ... and there may even be some that are asymptomatic," said David Heymann, an infectious disease expert and head of global health security at Chatham House, a London-based think tank.

Steve Bellan, of the University of Texas in the United States, argues that asymptomatic people could help with disease control tasks like caring for patients and conducting burials, reducing the number of nonimmune people exposed in these risky jobs.

Bellan points to two studies in particular. One, conducted after an Ebola outbreak in Gabon in 1997, found that 71 percent of "seropositive" people - those with traces of the Ebola virus in their blood - did not have the disease. The other, published in April 2002, found 46 percent of asymptomatic close contacts of patients with Ebola were seropositive.

Reuters - AFP