Fear of an avian-flu pandemic is widespread and growing. It is, of course, a very serious concern — all the more reason for reports on avian flu to be as clear and informative as possible. On the whole, this standard has been reasonably approximated, but one glaring exception has been the reporting on the mortality rate from H5N1 avian flu.
News story after news story repeats the statistic that out of 140 or so human cases of avian flu reported so far in Southeast Asia, more than half have resulted in death. The reporters then intone that the mortality rate for avian flu is more than 50 percent.
This, of course, is a terrifying figure. But before examining it, let's first look for a bit of perspective. The standard sort of influenza virus, it's believed, infects somewhere between 20 million and 60 million people in this country annually. It kills an average of 35,000, and it thus has a mortality rate that is a minuscule fraction of 1 percent. The Swine flu in the 70s killed a handful of people, more of whom may have died from the vaccine for it than from the disease itself. And the Spanish flu of 1918 to 1919 — the deadliest pandemic in modern history and also an avian flu — killed 500,000 to 700,000 people here and an estimated 20 million to 50 million people worldwide. Most assessments of its mortality rate range between 2 percent and 5 percent.
If the avian H5N1 virus mutated so that human-to-human transmission was as easy as it is with the normal flu, and if the mortality rate of more than 50 percent held, the U.S. alone would be facing tens of millions of deaths from the avian flu.
There is one glaring problem with this purported mortality rate of more than 50 percent, however: It is based on those cases that have been reported, and this leads to an almost textbook case of sample bias. You wouldn't estimate the percentage of alcoholics by focusing your research on bar patrons, nor would you estimate the percentage of sports fans by hanging around sports stadiums. Why do something analogous when estimating the avian-flu mortality percentage?
Who goes to hospitals and emergency rooms but people who are very sick? Who doesn't go to hospitals and emergency rooms but people who are not so very sick?
Given how many people in Southeast Asia deal with chickens and other poultry in a hands-on way, some unknown, but likely quite large number of them have contracted avian H5N1 flu over the last several years. Its symptoms are many times indistinguishable from those of the normal flu. Some no doubt have died from it, but the majority are likely to have recovered. The people who have recovered from the avian flu don't make it into the mortality-rate calculations, which are, as a consequence, likely to be skewed substantially upward.
Little Known About Changes and Virulence
The Centers for Disease Control and Prevention cautiously acknowledge on its Web page that it is quite possible that "the only cases currently being reported are those in the most severely ill people." Random samples -- the life blood of statistics -- are hard to obtain under the best of circumstances, and canvassing poor people in rural Southeast Asia certainly doesn't qualify.
The fact is that not only is it almost impossible to estimate how long it might take for the present H5N1 virus to mutate into one easily transmitted between humans, but it's almost impossible as well to estimate the virulence of such a virus. Sample bias, various epidemiological models, and the fact that extremely virulent viruses are less likely to spread -- because they kill a high percentage of their hosts -- all suggest that a mortality rate of more than 50 percent is much, much too high.
This, of course, does not mean complacency is in order. It does mean that everyone -- health professionals, the media and politicians -- should try very hard not to hype risks or minimize them.
Postscript on Iraqi War Dead
Another figure in the news recently has been the number of Iraqis killed in the war. President Bush mentioned last month that in addition to the more than 2,100 American soldiers killed so far in Iraq, that there were approximately 30,000 Iraqis killed. He was likely referring to the approximate figure put out by Iraq Body Count, a group of primarily British researchers who use online Western media reports to compile an extensive list of Iraqi civilians killed. The organization checks the names and associated details of those killed. It necessarily misses all those whose names don't make it into the reports, and it makes no attempt to estimate the number it misses. The group's list contains almost 30,000 names at present.
A study that appeared in the prestigious British medical journal, the Lancet, in October 2004, used statistical sampling techniques to estimate all Iraqis killed because of the war and its myriad direct and indirect effects. The figure researchers came up with at that time -- 15 months ago -- was approximately 100,000 dead, albeit with a large margin of error. The Lancet study used the same techniques that Les F. Roberts, a researcher at Johns Hopkins and lead author of the study, used to investigate mortality caused by the wars in Bosnia, the Congo and Rwanda. Although Roberts' work in those locations was unquestioned and widely cited by many, including British Prime Minister Tony Blair and former Secretary of State Colin Powell, the Lancet estimates on Iraq were unfortunately dismissed or ignored in 2004.
These last 15 months have considerably raised the American death toll, the IBC numbers, and any update that may be in the works for the Lancet's staggering 100,000 figure. In fact, if the Lancet estimates rose at a rate proportional to the IBC's numbers since October 2004 -- from about 17,000 then to about 30,000 -- the updated figure would be approximately 175,000 Iraqis dead since the war began.
Professor of mathematics at Temple University, John Allen Paulos is the author of best-selling books including "Innumeracy" and "A Mathematician Plays the Stock Market." His "Who's Counting?" column on ABCNews.com appears the first weekend of every month.