The oft seen graphic (above) illustrates that official numbers of nearly any disease you care to mention only represent the `tip of the pyramid’, since (depending upon the illness’s severity, the accuracy of testing, and the population’s access to those tests) only a portion of those afflicted will be counted..
It helps to explain why the CDC recently revamped their Estimate Of Yearly Lyme Disease Diagnoses In The United States to 300,000 rather than the 30,000 that are officially reported each year. A ten-fold increase.
Even more dramatically, it is estimated that for every West Nile Fever case reported in this country, as many as 50 cases go unreported (see JAMA: The 2012 West Nile Encephalitis Epidemic in Dallas, Tx).
And this ambiguity occurs in a country with more-than-decent surveillance and testing.
Which is why one of the most commonly expressed `rules of thumb’ heard when flu bloggers backchannel and discuss just about any emerging virus is, ` Whatever the official count is, you can probably safely add a zero to it.’’ A somewhat more cynical version, when discussing outbreaks those regions of the world where testing is spotty or rare, is to add `two zeros’.
If that sounds extreme, well compare that `guesstimate’ to a couple of recent studies.
One, released by Hong Kong researchers over the summer, extrapolated from the roughly 130 known cases of H7N9 in China last spring, the true number was probably somewhere between 1,500 and 27,000 symptomatic cases (see see Lancet: Clinical Severity Of Human H7N9 Infection).
Or the study last spring; CID Journal: Estimates Of Human Infection From H3N2v (Jul 2011-Apr 2012), that estimated that during a period when just 13 H3N2v cases were reported, as many as 2055 (90% range, 1187–3800) illnesses may have occurred between August 2011 and April 2012.
Whether these are reasonable estimates . . . well I’ll leave that to others to decide. But it is reasonable to expect – particularly with respiratory viruses – that for every case you see, there are many others that go undiagnosed. No one should be shocked or alarmed. That’s just the way viruses work.
All of which serves as a (probably unnecessary) prelude to referring you to Dr. Ian Mackay’s VDU Blog post this morning which looks at the recent Lancet study (see Branswell: Transmission Estimates Of MERS-CoV – Lancet Infectious Disease) that suggests that for every MERS case counted, 5 to 10 go uncounted.
So without further ado, I’ll invite you to go read:
Thursday, 14 November 2013
Epidemic is a big word, and while it generally means "a rise in the number of cases above what you'd expect", you can see from the definitions below that there are many ways to spin the meaning. For the public at large, it generally means "bad scary stuff" and so it's important that we use this word sparingly.