# 3702
The preoccupation (some might say `obsession’) of a great many people in the scientific world and Flublogia is in coming up with a reasonable CFR (Case Fatality Ratio) for this novel H1N1 pandemic.
And as I’ll demonstrate, it isn’t as easy as you’d think . . . and perhaps, not as important as well.
The CFR is the percentage of those infected that die either directly or indirectly due to the virus. If 1000 people get infected, and 1 dies, then the CFR is .1% . . . which is roughly the mortality rate we see with seasonal flu. If 10 people die out of 1,000, then the CFR is 1.0%.
In 1918, during the worst pandemic in recent history, 25 people out of every 1,000 who were infected in the US died (it was higher in some countries, and lower in others). The CFR was roughly 2.5%.
Early on, the CFR of the novel H1N1 virus was calculated to have been about .4% or about 4 times higher than seasonal flu. Today that number is in doubt, although it is still within the high end of some estimates.
But the truth is . . . nobody really knows.
We hear assurances that the novel H1N1 virus is `no more deadly than seasonal flu’, and when you just look at the raw numbers, that may be true.
But numbers don’t tell the whole story.
Seasonal flu, which is estimated to claim 36,000 American lives each year, normally takes 90% of its toll among the elderly. Mostly among those over the age of 80. Seasonal influenza is, for lack of a better term, a `harvester’ of the aged and infirmed.
For centuries pneumonia, which often comes as a consequence of influenza, has been called `The Old Man’s Friend’, simply because it usually takes the elderly quickly, and without much struggle.
Pandemic flu, in comparison, often strikes a younger demographic. In the case of novel H1N1, most of the infections and 90% of the deaths we’ve seen have been UNDER 65.
This is the opposite of what we would normally expect from seasonal influenza, and much harder for a society to accept. We are talking relatively young adults and children dying from a disease that normally kills the elderly.
One could easily argue that the death of a child is 10 times (or perhaps a 1000 times) more tragic, and costly to society, than the death of an 90-year-old in a nursing home.
Each year, we lose between 50 and 100 children to seasonal influenza. This year, up until the arrival of the novel H1N1 virus, we had lost 68 children to seasonal flu. An average year.
As you can see, this year, we have a double-spike of pediatric influenza deaths. Our normal winter spike, and one this summer.
Since May, we’ve lost an additional 43 children who have died as a result of the novel H1N1 virus. A new `counting year’ begins at the end of September, and sadly, this metric may be our best indicator of the impact of the novel virus on our society.
Pediatric influenza deaths must be reported to the CDC, and so it is one of the few areas where we get an actual, and reasonably accurate, count of deaths.
The rest of the deaths we see reported – among adults – are assumed to be `the tip of the iceberg’ and not truly representative of the real number. That’s why the CDC estimates flu deaths every year, because we aren’t set up to monitor and report them.
While this age shift to younger adults and children is extremely worrisome, the good news is this cohort is generally stronger, and in better overall health than those in their 80s and 90s, and therefore are less likely to die as a result of influenza infection – seasonal or pandemic.
That should have a bit of a dampening effect on the overall CFR, as opposed to a virus that attacked across all age groups.
It appears that those over the age of 55 – 65 have some degree of immunity to this virus. No one knows if that trend will last, but for now, they make up a small percentage of those sickened.
The fact that this novel H1N1 virus has (thus far) largely spared the older generation may greatly reduce the total number of deaths, but the age-shifting of the deaths we do see will make its impact far greater than if it were taking mostly elderly victims.
For several months in the blog (see here, here, and here) I’ve been discussing the possibility that we may see a High Morbidity-Low Mortality pandemic.
One that makes a lot of people sick, but claims relatively few, albeit younger, lives.
While it is too soon to know with certainty – and this virus could mutate and pick up virulence at any time – right now that is what we appear to be seeing.
I say `appear’, because the tracking and reporting of influenza deaths in this country, and around the world, leaves much to be desired. Still, despite those limitations, I think we’d have strong anecdotal evidence if this virus were claiming a lot of lives.
The 122 MRS surveillance system, which tracks deaths in 122 American cities due to pneumonia and influenza, isn’t showing any huge spike in deaths this summer. In fact, we’ve remained below the epidemic threshold for most of the past 4 months.
Are we missing some influenza-related deaths, even in these cities?
Undoubtedly. Every day in this country, roughly 6,500 people die from one cause or another. Most will not have an autopsy, and fewer still will be given a post-mortem test for influenza.
Most will be `attended’ deaths, where the victim’s doctor will sign the death certificate (usually without seeing the deceased), and will put down whatever chronic disease they were being treated for.
The death certificate will read Coronary Artery Disease, COPD, or perhaps Myocardial Infarction – and very rarely will influenza even be considered as a contributing factor. Which is why the CDC estimates flu deaths each year, instead of trying to count death certificates marked with `influenza’ listed as the cause of death.
And as many holes as there are in our surveillance systems, the surveillance net from most other countries is even more porous.
But despite these shortcomings in global surveillance and reporting, the number of hospitalizations and the number of pediatric deaths – while elevated – remain reasonably low for a pandemic, even in the Southern Hemisphere.
And so it is hard to make a good case for a CFR much higher than that of seasonal flu. And that is good news. The bad news is, those that are dying are generally younger, healthier member of society.
Skeptics will no doubt use the low CFR to say, `See . . .this isn’t much of a pandemic . . . a bunch of worry over nothing.’ But that misses the point.
If we see the same number of deaths this year from H1N1 that we do in a normal year, and if even half are under 65, that would represent a huge tragedy and loss for our society.
I can’t estimate with any accuracy what the CFR of this virus is today (although I believe it to be relatively low), and I certainly can’t predict what it will be this winter and spring.
While tracking the total number of deaths is important, we lack the other important number to solve this equation – the total number of people infected.
Between asymptomatic, mild, and untested cases, we’ve no idea what that number really is.
And without that number, we have nothing to divide the total number of deaths into, to derive the CFR.
My hope remains that we will be lucky enough to see nothing worse than a Category 1 pandemic.
But a CAT 1 pandemic is nothing to be trifled with.
And lest we forget, a CAT 1 pandemic in the developed world is likely to be a CAT 2 pandemic in lesser developed nations.
While we obsess over the numbers, and try to quantify the importance and severity of this pandemic, the truth is the numbers don’t tell the whole story.
That would require equating the death of a 10 year-old child or a 30 year-old pregnant woman, to that of a 90-year old nursing home resident.
And while it may sound insensitive, from a societal standpoint, they are worlds apart in terms of impact.
None of this, I’m sure, will dissuade anyone from trying to calculate the CFR of this pandemic. I’m not sure it’s even possible - given the variability of a virus over time and geographic regions – but I do understand the the need to come up with a number.
Whatever that number is, as long as the age-shift in victims remains pronounced, we shouldn’t try to compare it to the CFR of seasonal flu. It’s really apples and oranges.
When we are talking about a virus that claims mostly young adults and children, we are talking about something else entirely.
