A recent sharp drop in new Ebola infections in West Africa is prompting scientists to wonder whether the virus may be silently immunising some people at the same time as brutally killing their neighbours. Reuters Health reports that so-called "asymptomatic" Ebola cases – in which someone is exposed to the virus, develops antibodies, but doesn't get sick or suffer symptoms – are hotly 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 factor in ending outbreaks more swiftly by giving secret protection to those lucky enough to be able to bat the infection away.
"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."
Latest World Health Organisation data show new cases of infection in West Africa's unprecedented Ebola epidemic dropping dramatically in Guinea, Sierra Leone and particularly in Liberia. 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 "sub-clinical" cases and acquire protective antibodies. After a while, the virus – be it flu, measles, polio – can't find non-immune people to be its hosts. But some specialists with wide experience of disease outbreaks are highly sceptical about whether this phenomenon happens in Ebola, or whether it could affect an epidemic.
Others are more hopeful and are urging researchers in West Africa to seek out and test possible asymptomatic cases with a view to using the secrets of their silent immunity. Steve Bellan of the University of Texas argues that if scientists can reliably identify asymptomatic people, they could help with disease-control tasks like caring for patients and conducting burials, reducing the number of non-immune people exposed in these risky jobs.
With the largest Ebola epidemic on record raging through three of Africa's most under-resourced countries, scientists and medics have focussed all efforts on the sick and dying and not on testing people with no symptoms. If they do, however, and if they were to find what Bellan and some others suspect, it could prompt a reappraisal of what jolted a relatively sudden downturn in new cases.
Ian MacKay, a virologist at Australia's University of Queensland, agrees that possible sub-clinically-acquired immunity is one of many unexplored mysteries of the Ebola virus. "One thing that this particular outbreak shows is that we really don't know an awful lot about these kinds of infectious diseases," he said. "We tend to think we can answer all the questions, but this is one of those things we may end up being taught by the virus itself."
[link url="http://www.reuters.com/article/2015/01/27/health-ebola-asymptomatic-idUSL6N0V22TR20150127"]Full Reuters Health report[/link]