Photo Credit NIAID
# 8527
Whenever a novel virus emerges and begins to spread among humans, one of the first questions asked is `just how deadly is it?’
It is, surprisingly, an incredibly difficult question to answer. Not only just in the beginning of an outbreak such as we are now, but often, even years or decades after an outbreak has ended.
The question seems simple enough: Of those that are infected, what percentage will succumb to the disease? Or, as epidemiologist’s call it, the CFR (Case Fatality Ratio).
Invariably, the earliest reports of any novel illness carry an exaggerated CFR, because only the sickest of the sick seek treatment, and those are the ones most apt to die. The early reports from Mexico in the spring of 2009 suggested a much higher CFR for the novel H1N1 virus than we ended up seeing globally.
Five years after the start that pandemic, we still don’t have a really good handle on its CFR, and if you go back to the pandemics of 1918, 1957, and 1968 the best you will find are estimates. And those estimates often vary considerably between researchers.
The worst pandemic in modern times was the Great Pandemic of 1918, which killed somewhere between 40 million and 100 million people. In 2006, in a Lancet journal (doi:10.1016/S0140- 6736(06) 69895-4) article cited as much as a 30-fold difference in mortality rates around the world:
Christopher JL Murray , Alan D Lopez , Brian Chin , Dennis Feehan , Kenneth H Hill
Excess mortality ranged from 0·2% in Denmark to 4·4% in India. Since there was some under-registration of mortality in India, total pandemic mortality could have been even higher.
The CFR in the United States was estimated at roughly 2.5%, a far cry from the astronomical fatality rate estimates we’ve seen cited for the H5N1 virus (60%), H7N1 virus (30%), and even SARS (10%) – yet high enough to practically paralyze a nation and claim over 600,000 American lives.
While the 1918 pandemic claimed roughly 1 in 40 Americans who fell ill, our most recent pandemic (2009) was a couple of orders of magnitude less deadly. ( See CIDRAP NEWS CDC estimate of global H1N1 pandemic deaths: 284,000).
And even with our yearly seasonal influenza epidemics there is considerable academic debate over the true burden of the disease. After more than a decade of promoting the `flu kills roughly 36,000 Americans each year’ meme, the CDC refined their estimates in 2010 ( see MMWR: Estimates Of Yearly Seasonal Influenza Deaths).
For deaths with underlying pneumonia and influenza causes (the most narrow definition of flu-related fatalities used) the models estimated a yearly average of 6,309 (range: 961 in 1986--87 to 14,715 in 2003--04) influenza-associated deaths.
Using a broader criteria (underlying respiratory and circulatory causes including pneumonia and influenza causes) the models estimated an annual average of 23,607 (range: 3,349 in 1986--87 to 48,614 in 2003--04) influenza-associated deaths.
Despite the 12-fold difference in deaths between the 1986-87 and 2003-04 seasons, the operative word here remains `estimated’. Numbers are extrapolated from a surveillance subset of the nation, and there is no good definition for what constitutes a `flu-related’ death.
So, if we can’t say with any precision how deadly seasonal flu is after decades of scrutiny, what can we say about the MERS Coronavirus?
Well, today the CFR of MERS – based on a relatively small number of known cases - appears to run somewhere between 30% to 40% of symptomatic patients. But that number drops when you add in those who are infected but display no symptoms.
The problem is, we are only now starting to get an idea of how large the number of asymptomatic cases may be, or how many `mild’ cases have gone undiagnosed. The larger that cohort, the lower the CFR.
Complicating matters, it is also likely that some number of people have died due to complications of MERS infection, but were never diagnosed with the virus. Whether that number is large, or relatively small, is still a matter of conjecture.
We also don’t know much about the attack rate of this virus. That is: What percentage of people who are exposed will contract the virus? During the first two years, based on the limited number of positive tests among contacts of known cases, the attack rate doesn’t appear to be very high. And why some people contract the virus, while others don’t, is still unknown.
But those numbers could change should the virus better adapt to human hosts.
The bottom line is, while it is far too soon to put any decent number on the CFR of MERS, the good news is that not everyone who is exposed will even contract the virus. And among those that do, some will be asymptomatic, some will experience only mild symptoms, while some portion will see moderate to severe (or even life-threatening) illness.
Although some possible risk factors (including obesity, kidney disease, diabetes, COPD) have already been identified, we don’t know at this point why some people experience mild illness while other’s condition quickly deteriorates.
Ultimately I expect we will probably find the CFR of MERS to be considerably lower than today’s numbers suggest. But putting that in perspective, even a 10-fold reduction would still put us in the low single digits - and a 100-fold reduction would be still be deadlier than any influenza virus since 1918.
While admittedly not a nightmare-scenario out of a Stephen King novel - were MERS to began to spread globally and with a significant attack rate – the impact of a CFR even as low as .1% would be considerable.
Credit - HHS Interim Pre-Pandemic Planning Guidence: Community Strategy For Pandemic Influenza Mitigation In the United States.
Thus far we are no where near that point yet with MERS, and with luck, never will be.
But the fact remains that another pandemic will come sometime in the future, and its impact could be as moderate as 2009 or far worse than 1918. Making enhanced surveillance, and ongoing pandemic preparedness, essential steps if we hope to mitigate its effects and to protect the public.
For more on pandemic planning, and preparedness, you may wish to revisit:
NPM13: Pandemic Planning Assumptions
The Pandemic Preparedness Messaging Dilemma
Pandemic Planning For Business
