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.
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.