One of the things we’d like to know about the swine flu virus is its Case Fatality Ratio (CFR, commonly called a case fatality rate, although it isn’t technically a rate but a proportion). But what is a CFR? And how is it different from a mortality rate?

The CFR is an estimate of the probability that someone with the swine flu will die of it (technically, before dying from something else or recovering). The higher the CFR, the more virulent the virus. So what’s virulence? Virulence refers to the severity of the disease the virus produces. Rabies is a virulent virus. Everybody dies from it once they start getting symptoms. Rhinovirus infection (common cold) isn’t very virulent. We use CFR here to describe how virulent this virus is.

That’s a little too simple, though, because disease, whether virulent or not, isn’t just about the virus but it’s about the virus and the host relationship. The virus uses the host to make copies of itself by hijacking the host’s cellular protein making machinery for its own purposes. If it can do this without much cost to the host, that’s fine. That’s why the some viruses are able to co-exist happily with their hosts. Many birds are infected by influenza viruses without any apparent ill effect. The very same virus, however, might kill us. That’s a different kind of host parasite relationship. Virulence isn’t something inherent in the virus but a function of a host – virus combination. That’s one reason why we can’t tell how virulent a virus is by just looking at the virus. We need to know its biology in the environment of our bodies.

One observable measure of virulence, then, is the CFR, which brings us back to the question: what is CFR? Again, in simplest terms we can estimate it by using as the numerator the number of swine flu deaths in Mexico (or the US) and divide it by the total number of cases. That proportion is an estimate of dying of swine flu. Simple. Unfortunately a little too simple, because we have great difficult ascertaining both the numerator and the denominator.

The numerator, ideally, would be everyone who died of swine flu. To do that we’d have to have a strict definition of a swine flu death, for example, someone who died of respiratory failure and had laboratory confirmed infection with the swine flu virus (don’t give me grief about the definition; it’s just for illustration). Does that cover everyone who died of swine flu? What about people who didn’t die exactly that way? What about the accuracy of the test for swine flu infection? Do we even have a tally of all the deaths? Have they all been tested, or were some buried or cremated before testing?

In a sense the numerator is the easier part. The denominator is meant to tally all the swine flu infections. Not just the hospitalized cases, not just the symptomatic cases. If you really want to know the risk once infected, you have to have them all. That could be a lot of people, too many to get specimens from and send them to Atlanta. Many people probably never see a doctor and ones that do don’t get a specimen, and of the specimens some get lost or misplaced or degrade, and the lab may make mistakes. So you are likely to miss many cases. But without laboratory confirmation you are also likely to count many people who are sick with other viruses, which are prevalent at any time of year. Think about your city during flu season. How would you go about figuring out the total number of people who had flu? Once you start thinking about it you begin to see the difficulties.

If you undercount the denominator you falsely inflate the CFR. That’s almost certainly the case when you try to estimate CFR in Mexico by dividing number of deaths, 140 (only 20 of which have been laboratory confirmed), by the number of suspect cases (1640 or whatever the number will be when you read this). But you might overcount the denominator, too. There are a lot of viruses that produce “flu-like” symptoms. That would underestimate the CFR.

So what’s a mortality rate? A swine flu mortality rate is the number of deaths divided by the population at risk of dying. The numerator is the same (number of deaths from swine flu; a time period should be specified) but the denominator is now all the people in Mexico City. CFR is a proportion, a number between zero and one. The mortality rate is a number of deaths in some convenient unit of population, say number of swine flu deaths per 100,000 people. It is a whole number between zero and 100,0000 (hopefully not, because that would mean it wiped out the whole city). These two numbers are telling us different things about the virus. A mortality rate is easier to calculate because the denominator is easier to estimate, using census information, but doesn’t reveal virulence, so CFR is what is interested in this case.

We’ll discuss other basic measures and principles as this wears on. After all, there is only so much you can say when there isn’t much new to report beyond new cases. It’s something to do until we get more scientific data. And when we get it, we’ll all be on the same page.