Thursday 11 July 2013

New H7N9 numbers and Case Fatality Rate/Ratio/Risk [UPDATEDx2]

The latest Influenza A(H7N9) virus numbers are out and we can see the deaths have ticked up. We have no public information on these or many of the past releases either. With 132 cases (presumably this still does not include the Taiwan case or the asymptomatic Beijing boy, so I maintain 134) with 43 deaths.

My data (which suffer from the public reporting process; I do not mean to imply that WHO or other experts do not have these details) suggest we are missing:   

  • Case details (which cases) on 13 deaths (30% of fatalities)
  • 5 dates of onset 
  • Date of fatality in 1 known case 
  • 56 dates of hospitalisation 
  • 38 dates of discharge 
I am going to use a new term on VDU, to avoid "Case Fatality Ratio". Mine is being called the Proportion of Fatal Cases (PFC)[1]...just by me mind you!

The PFC is a percentage calculated as the currently known number of fatalities divided by the number of total lab-confirmed cases including fatalities, regardless of whether surviving cases are inpatients (hospitalized) or outpatients.

At writing, the current H7N9 PFC was 32.1%

The PFC is just a number - it's not meant to imply that it includes every case that ever happened (it never could) and does not account for those cases who will die directly or indirectly as a result of their infection later on, but who may be alive at the time of calculation. 

The PFC is a snapshot to be used before an outbreak is done and dusted. It is meant as a guide to what is happening right now using the data we can get our hands on. Sometimes that's lots of data (as it was with H7N9 - but has not been for a while now) or very limited data (as it is with MERS-CoV cases).

Case Fatality Ratio/Rate/Risk (CFR) makes use of the number of recovered cases in its denominator.[2] So it's important to know survivor numbers. As suggested above, this requires that all the people who will recover from their infection, have recovered (and been discharged) from their infection. 

Using the CFR early in an emerging virus/disease outbreak, when what usually brings in outbreak to our attention is death, is great for selling papers, but not helpful realistic in a bigger picture sense. 

The CFR is most useful at the end of an epidemic/pandemic, but not so much when data-in-hand is poor during the early days of many outbreak. 

Of course, some will take a PFC and multiply it by the world's population as an estimate of how many are going to die if the virus reaches pandemic levels. That's not helpful or accurate. Just accept it as that snapshot of what's happening now.

  1. J. P. Dudley and I. M. Mackay. Age-Specific and Sex-Specific Morbidity and Mortality from Avian Influenza A(H7N9). J. Clin. Virol. 2013. Sept.

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