Earlier this week, we published a new perspective on Ebola in the Lancet medical journal. As the devastating outbreak continues to spread in West Africa, it may be silently immunizing large numbers of people who never fall ill or infect others, yet become protected from future infection. If this is true, it would have significant ramifications for outbreak projections -- how many more infections and deaths will we see in the coming weeks and months -- and it would allow us to improve Ebola control strategies.
What do we know about silent Ebola infection?
When a person is exposed to a virus, like Ebola, their immune system produces antibodies that linger in their blood long after the infection subsides. Antibody blood tests can therefore be used to deduce past infections. Following several prior Ebola outbreaks in Central Africa, such tests showed that large numbers of people who remained healthy during an outbreak had antibodies to the virus. In one study, 71 percent of individuals with positive Ebola antibody tests had not gotten sick; in another, 46 percent of close contacts of infectious Ebola patients who remained healthy tested positive for Ebola antibodies.
The latter study also found minute concentrations of Ebola virus in these individuals' blood, suggesting that their antibodies could not be explained by their exposure to dead virus, but that rather they had truly been infected by live virus. Could silent Ebola infections be contagious? Given that Ebola typically spreads through contact with bodily fluids of very sick individuals, who have exceedingly high viral counts, it is very unlikely that silent (asymptomatic) cases can spread the virus with the low levels found in their blood.
But the question remains, are these people immune to future Ebola infection? Since survivors of full-blown Ebola disease seem to become immune to re-infection, we hope that the answer is yes.
How many people are silently infected?
We don't know exactly, but our rough approximations tell us that as many as 50 percent of infections are silent. So for every case of severe Ebola, there may be another person who is infected without ever knowing it. Our best evidence for this comes from the 1996 outbreak in Gabon, after which researchers monitored 24 contacts of known Ebola cases that never became sick, and found 11 of these individuals (46 percent) were silently infected.
Why should we care whether silent Ebola infections are common?
Epidemics are fueled by susceptible people. The more there are, the bigger an epidemic can become. Immunization of any kind -- via vaccine or natural infection -- makes people resistant and thereby slows transmission. If silent Ebola infections actually protect against future re-infection, then Ebola is acting as its own vaccine, leaving a large wake of uninfectable people in its path. Importantly, this wake is likely to include healthcare workers who frequently contact patients and are at considerable risk for future exposure. If so, Ebola is simultaneously killing some individuals while protecting others within the population subgroup at highest priority for interventions, such as future vaccines.
Widespread acquired immunity would therefore have three important implications. First, outbreak forecasts that do not consider this phenomenon will overestimate the future extent of the outbreak. Second, naturally acquired immunity will amplify the effects of disease control measures, including vaccination. Third, if we can reliably detect immune individuals, then they can safely take on risky health care tasks and thereby prevent spread to non-immune caregivers.
What should be done about silent Ebola infections?
We urge the public health community to consider the pros and cons of an immediate on-the-ground investigation of silent Ebola infections. They are probably common and non-contagious, and the key remaining issues are whether silent infections provide immunity and whether immune individuals can be readily identified. These questions can only be answered in the midst of an outbreak, like the one ongoing in West Africa, when we can directly observe whether silently infected individuals become re-infected.
On the pro side, knowing more about silent infections will improve outbreak projections and potentially lead to life-saving advances in Ebola control measures. On the con side, given the limited resources and dire situation in West Africa, public health agencies must prioritize treating patients and preventing further infection via case isolation and contact tracing. Moreover, surveillance for silent infection requires regular blood draws from seemingly healthy contacts of Ebola patients, which, given the risks of Ebola exposure, must be done with utmost caution.