Implied mortality rates – graphed

14 06 2009

I’ve been debating whether to do a graph of the change in Swine Flu’s “implied mortality rate” over time. I’ve decided to go ahead, but I need to be clear what it is – and what it isn’t.

What do I mean by an “implied mortality rate”? All I mean is the total number of deaths reported due to Swine Flu, divided by the total number of confirmed cases. So, for example, the latest WHO figures (12 June, update 48) report 145 deaths and 29,669 confirmed cases. 145 divided by 29,669 is 0.0049. So the implied mortality rate is 0.49%.

What does a mortality rate of 0.49% mean? It means that for every 10,000 people who get Swine Flu, you would expect 49 of them to die.

There is a very important caveat here. This rate says nothing about your chances of dying if you catch Swine Flu. Why? Firstly, your chances will be influenced by your circumstances. In particular, how your body responds to getting the flu, whether you have other medical issues, and what standard of health care you have access to.

Secondly, and more importantly, I suspect that a mortality rate of 0.49% is massively over-stated. In other words, I suspect the actual mortality rate is far lower. Why? Well, I assume that not every case of Swine Flu gets reported (for example, I assume a lot of people don’t get tested – take what’s happening in Victoria right now, as one example, where the Gov’t has basically said to GPs “don’t test unless serious”). But, my guess is that pretty much every death caused by Swine Flu is getting reported. So, we have a large numerator (deaths) on a denominator that is smaller than what it is in reality. I’ve heard some commentators say that we are under-reporting cases (not deaths) of Swine Flu by a factor of 10. If that is the case, the real mortality rate is more like 0.05%. That is, for every 10,000 people who catch Swine Flu, five, not 50, die.

Anyway, what I’ve done is graphed the WHO figures back to April, when they started to report. I’ve graphed deaths divided by cases and turned that into a percentage. I get the following graph:

implied fatality rate full range

 You can see that we get a sudden spike up. This was when we got confirmed deaths out of Mexico in late April. Initially, we had no confirmed deaths, just some confirmed cases (so a rate of 0%). Then we got deaths confirmed, but with only a small number of cases actually diagnosed – hence a big spike to almost 10%. But look what’s happened since – as we’ve kept on finding new cases, the number of deaths has not kept pace proportionately. Hence the implied mortality rate has declined rapidly.

In many ways I really don’t like the above graph – because there is no way in the world that the actual mortality rate was ever anywhere near 10%. I think you need a good lot of confirmed cases reported before you can start getting actual reliable statistics. So, what I’ve done is graphed only from 14th May, when the total number of cases was almost 6,500 and the number of deaths was 65. That is, an implied mortality rate of 1%. That graph is far more interesting, because it lets you see how rapidly this rate continues to decline, even as the virus spreads. In fact over the last month that rate has halved – and does not look like slowing down. Here it is:

implied fatality rate

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6 responses

14 06 2009
GSE

I want to stress that I am not interested in conspiracy theories, wear a tin foil hat or have ever been clinically paranoid but… (you knew there was a but coming, didn’t you) …
Taking into consideration what can only be described as a border-line flippancy about SF by the Victorian and Federal Governments, what are the chances that future deaths relating to SF will be filed under heart condition, pneumonia etc. and therefore not reported to the public?
At the end of the day it is the bureaucracy which makes decisions on not reporting new cases (equals to keeping stupid public in the dark), limiting testing, not providing GPs with basic tests etc. and they will stop at nothing to keep their and ours heads in the sand.

14 06 2009
aussieflublogger

But – I think you can rest assured in the bureaucracy’s inability to implement anything well – even a cover up! I can’t imagine it wouldn’t leak if they tried a stunt like that…

14 06 2009
GSE

Tomorrow I am leaving for Europe and been checking for their flu reporting and status.
I’ve found a Polish website (influenza.pl) which has plenty of usefull links as well as regular updates on cases across the world.
According to them (quoting WHO) in Australia on the 12th there were 1307 cases, on the 13th 1441.
You may want to have a look here: http://www.pzh.gov.pl/oldpage/epimeld/grypa/aindex.htm

The graph shows influenza infections over the last few years.

cheers

15 06 2009
aussieflublogger

Thanks mate. Have a good trip! The Polish site looks good – it seems to give the historical context (ie what’s “normal”) that my stuff is lacking. On the numbers – WHO tends to lag in its reporting – but ends up using the Health and Ageing government data that I use. Cheers. Nick.

20 06 2009
Implied mortality rate continues to fall – updated graph « Swine Flu in Australia

[…] Implied mortality rate continues to fall – updated graph 20 06 2009 As we get more data in, the implied mortality rate of this virus continues to fall. Since I last did this graph (as at 7 days ago) the implied fatality rate has fallen 0.08% to 0.41%. That’s over a 16% reduction in a week. For a full discussion on what I mean by “implied mortality rates” refer here. […]

11 07 2009
Mortality rate stops falling « Swine Flu in Australia

[…] Mortality rate stops falling 11 07 2009 It had to happen, but it’s interesting nonetheless. The “implied mortality rate” that I calculate from the WHO data has stopped its decline and seems to have levelled out at about 0.45% over the last two weeks. Remember, this mortality rate is simply the deaths due to swine flu divided by the confirmed cases. I use the WHO global data. For a full explanation of the implied mortality rate and why I think the real rate is probably a lot lower, see this post here. […]

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