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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.
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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.
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