Anyone who has talked to me recently will know that I'm going on a lot about swine flu right now. That is because I think its important in a way that most of what hits the headlines is not. I'll be addressing issues with infection and death figures in this post. I will probably come back to the topic in subsequent posts.
How many people are/were infected?
The WHO collates figures on numbers infected right? No! The WHO collates the number of tests for swine flu that have come back positive. Wikipedia also collates these figures (and does so faster than the WHO). At the time I'm writing this the figure stands at 80,633.
I don't know of any credible estimate of world wide infections at all. However, the CDC recently estimated that 1,000,000 people have been made ill in the USA. The difference between these two figures is large but it can be explained as follows: Once the number of cases rises beyond about 1,000 countries stop testing people they believe have the disease because the testing facilities are insufficient. The more people get infected the worse a guide these numbers will become (and they're probably poor to start with). The USA stopped testing most people for swine flu after a few thousand cases. But they can still estimate how many people have become ill by how many people turn up with flu like symptoms adjusted for how many of them on average test positive (or other similar statistical techniques).
The WHO numbers (and those reported by national governments) have been endlessly reported, picked over and analyzed. However, these figures are a poor way to track the disease (except for most other ways that are currently available). I will explain two of the problems with the figures so you get the idea:
1) The reporting procedures of countries may differ. Some countries will test almost no one (such as many African countries) and some may test everyone they can in the first few weeks and then start monitoring the disease in other ways (like the USA and Australia).
2) The chance of a case being noticed will be higher in a person for whom the disease is worse so more infections may be detected amongst groups who are worse affected.
At the end of the day these figures are really lower bounds and probably only give a very rough guide to the early stages of the disease in a country. Better estimates of the numbers infected should begin to become available but the best estimates may well emerge many months or years after the first wave of the pandemic is over.
Who is getting infected?
This is relatively easy to answer. Most people for are infected are young, in the Southern hemisphere or in North America. The geographic distribution will change and the age distribution could change too. Unlike the seasonal flu few old people are getting infected (and very few are dying)
Who is dying?
The highest deaths are between the ages 15 and 50. The USA has (as of now) a mean age of death of 37. Mexico had a mean age of death in the 20s towards the end of April (I don't have more up to date figures). I have found good information on who is at greater risk: Pregnant women, asthmatics, diabetics and those with heart disease. All increase the risk significantly. However, between 1/3 and 1/2 of all deaths are in fit healthy people.
How many people have died from swine flu?
The WHO is keeping a running tally of essentially all deaths from swine flu, right? No, wrong again! The WHOs confirmed death figures cannot even be used as an estimate of this number. This might seem absurd (surely its easy to notice that someone has died?) but I'll explain why this is:
People die all the time. There are many many deaths every day and a significant number die in a way that looks superficially similar to a flu caused death. Many people who have seasonal flu die from opportunistic secondary infections or complications that are essentially caused by the flu itself. By the time these people die the flu symptoms and the virus may have gone. Because we are in the early stages of this pandemic the deaths are swamped by deaths that look superficially similar. This makes it hard to detect deaths in people not already known to have swine flu.
As an example of this difficulty consider the deaths from flu in the USA. The number of death certificates issued each year which list flu as a contributing factor is about 1,800. However, it is estimated that flu is a significant factor in the deaths of 36,000 people in the USA each year. That is a difference of about 20 times. The second figure can be calculated by varying methods (and the numbers change but not enough to invalidate my point) which rely on statistical sampling or modeling.
So the number of deaths from the swine flu could easily be 20 times as great as the WHO confirmed figure. However, we shouldn't assume that the problems with estimation are the same for seasonal flu as for swine flu. We may:
*Miss fewer deaths because more young people are dying from swine flu. Younger people are less likely to randomly die so they are more likely to be detected above the background noise. The strong media attention has surely increased the chance that people who are in a serious condition will be tested for swine flu. On the other hand the availability of tests is much more limited for swine flu than for seasonal flu.
*Miss more deaths because at this early stage its easier for deaths to be drowned out by the noise and economic pressures may cause countries to mislead the international community about the deaths they've had. Sadly this second point is probably significant.
Finally it should be noted that you cannot calculate a fatality rate by dividing the confirmed deaths by the estimated infections (as many news sites have). That will just give you a hopelessly inaccurate figure.
Its not just me saying this either. For instance the CDC is on record saying "Only counting deaths where influenza was included on a death certificate would be a gross underestimation of influenza’s true impact.".