Scientists wonder if herd immunity responsable for drop in new Ebola infections in West Africa

[Pathfinder]

Scientists ask if Ebola immunizes as well as kills

BY KATE KELLAND AND EMMA FARGE
LONDON/DAKAR Tue Jan 27, 2015 1:46pm GMT

(Reuters) - A recent sharp drop in new Ebola infections in West Africa is prompting scientists to wonder whether the virus may be silently immunizing some people at the same time as brutally killing their neighbors.
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"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, told Reuters. "The virus may be bumping into people it can't infect any more."
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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 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.

"But herd immunity is just the wrong term. There could be household immunity developing, but even that is only hypothesis."
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http://uk.reuters.com/article/2015/01/27/us-health-ebola-asymptomatic-idUKKBN0L00R220150127

 

[Emily]

I don't see what there is to 'wonder' about. Herd immunity probably isn't the right term for a virus as difficult to transmit as ebola is, but it seems clear that a natural mechanism akin to ring vaccination has been taking place. They need to do serum analysis prior to vaccination trials, otherwise the vaccines will look more effective than they really are.

[h=4]Ebola control: effect of asymptomatic infection and acquired immunity[/h] Bellan, Steve E et al.
The Lancet , Volume 384 , Issue 9953 , 1499 - 1500
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Evidence suggests that many Ebola infections are asymptomatic,[SUP]1[/SUP][SUP],[/SUP] [SUP]2[/SUP] a factor overlooked by recent outbreak summaries and projections.[SUP]3[/SUP] Particularly, results from one post-Ebola outbreak serosurvey[SUP]1[/SUP] showed that 71% of seropositive individuals did not have the disease; another study[SUP]2[/SUP] reported that 46% of asymptomatic close contacts of patients with Ebola were seropositive. Although asymptomatic infections are unlikely to be infectious,[SUP]2[/SUP] they might confer protective immunity and thus have important epidemiological consequences.
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Widespread asymptomatic immunity would likewise have implications for Ebola control measures and should be considered when planning intervention strategies. For instance, should a safe and effective vaccine become available, the vaccination coverage needed for elimination will depend on pre-existing immunity in the population (appendix). Immunity resulting from asymptomatic infections should reduce the intervention effort needed to interrupt transmission but might also complicate the design and interpretation of vaccine trials. Trials and interventions are likely to target exactly those high-risk populations most likely to have been asymptomatically immunised. Thus, for assessment of vaccines and other countermeasures, baseline serum should be collected to improve both estimates of intervention effectiveness and our understanding of asymptomatic immunity. Additionally, assessment of intervention measures should account for the contribution of asymptomatic immunity in curbing epidemic spread...

http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(14)61839-0/fulltext​