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:
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: