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Ebola Outbreak – Half of Secondary Transmissions Need to be Prevented

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Ebola Outbreak
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“This is likely the greatest peacetime challenge that the United Nations and its agencies have ever faced. None of us experienced in containing outbreaks has ever seen, in our lifetimes, an emergency on this scale, with this degree of suffering, and with this magnitude of cascading consequences.”

This statement given by Dr Margaret Chan, Director-General of the World Health Organization on the 18th of September 2014 in her address to an emergency session of the UN Security Council should have sent shudders down the spine of every “two legs good” human making their way on this merry planet. These are grand words reflecting nothing less than a unprecedented situation on the ground in West Africa. One that is recognised by nearly every scientific authority in the world as one that could spiral further out of exponential control without immediate large scale intervention on behalf of technologically advanced and developed nations.

Amid the dire projections however is the some much needed hope. Many an opinion piece or news report will jump on a headline stating that without any further assistance the epidemic will continue to exponentially grow leaving numbers in worst a case scenario in the hundreds of thousands by years end. What they fail to effectively explore or even to mention is the other side of the mathematical coin which is the rate of growth is highly dependent on this potential assistance.

Basic Reproduction Number

This largely orbits around one metric named the Ro or “Basic reproduction number”. This number relates to the number of people “on average” one infected person will infect over the course of their infectious period. In the the language of epidemiology an Ro number greater than 1 ( Ro>1) means that an epidemic can continue to grow where as an Ro number less than 1 (Ro<1) will result in an unsustainable epidemic and will disappear over time. Some infectious diseases have a low Ro and some have very high R0. Ebola in this sense is low on the scale with almost all the common infectious diseases we are familiar with having an Ro number significantly higher. Whilst Ebola has averaged an Ro of between 1-4, HIV has averaged 2-5, Polio and Small Pox 5-7 and measles a massive 12-18.

Last week the journal Eurosurveillance, which is the peer-reviewed publication of the European Centre for Disease Prevention and Control published a mathematical assesment based on case reports on the speed in which the epidemic is growing. Their assessment gave a basic reproduction number (Ro) “broadly ranging from one to two”. This was “consistent with published estimates from prior outbreaks in Central Africa”. Their hyperthetical scenario based on continued and recent trends resulted in “an additional 77,181 to 277,124 cases by the end of 2014.” This figure along with others estimates (some higher and some lower) have formed the back bone of headlines around the word giving rise to a fear factor associated with an exponential growth that almost has no parallel. But this estimate presupposes that help will not come.

Ro numbers vary not only with the infectious potential of any given epidemic but also with the response. The conclusion of the Eurosurveillance estimates give the current Ebola outbreak an Ro number of less than two therefor “indicate that the outbreak could be brought under control if more than half of secondary transmissions per primary case are prevented.”

This has been achievable in the past with small scale outbreaks of the Ebola virus and with a significantly scaled up approach the hope is that it will work on the medium scale. The major risk is that this is not only larger than any previous outbreak but now largely within an urban and transnational setting. This combined with a potential dislocation of law and order exacerbates the problem significantly and renders previously relied on models broken.

Scaled International Response Begins

The unanimous adoption last week of a key resolution by the 193-Member UN General Assembly, that followed a historic meeting of the Security Council, which declared the Ebola outbreak in West Africa a threat to peace and security, shows that the international community is now taking the threat seriously.  Advanced teams  from UNMEER the United Nations Mission for Ebola Emergency Response arrive in early the coming week and the United States Military have already deployed C-17 aircraft carrying military handling equipment to Monrovia and have began establishing command headquarters.

The first shipment of the ramped-up United States military response to Ebola arrives in Liberia, Sept. 18, 2014. Source: US Defence Dept.
The first shipment of the ramped-up United States military response to Ebola arrives in Liberia, Sept. 18, 2014. Source: US Defence Dept.

West Africans according to all sources share customs and socio-political circumstances that result in all things from fear of aid workers to putting their faith in witch doctors in hope of survival. Yet the developed world has dragged its feet to an extent that only now have we brought to bear an assistance package that is too late for some and at risk being to little for the rest (an unknown quantity). It is worth mentioning that some 1-2% of the net worth of any one of the worlds richest 10 individuals would immediately double the monetary effort being pledged in what has the potential to be a global catastrophe.

Infectious diseases give no thought to ethics, human rights or the political persuasions of mankind. Yes there may be close to 7 billion of us on the planet yet we are only one of 8.7 million species estimated to inhabit mother earth. But even this pales in “blue dot” comparison to the 1-4 billion species that have come and gone over the past 3.5 billion years. All but a few of which have gone extinct through disease, genetic obsolescence, over-predation or any number of other factors.

Ebola is real and like the signs popping up everywhere across West Africa reminding them of this fact the developed world is also only now coming to terms with this.

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