Mortalities from a Flu Pandemic Hard to Predict

[Guest guest]

Mortalities from a Flu Pandemic Hard to Predict by Jon Hamilton

When public officials talk about bird flu, they often quote a scary

statistic: Half of all the people known to be infected with the virus

have died. But scientists say that figure has little bearing on

what's likely to happen in an actual pandemic.

In fact, flu experts have pretty much ignored the 50-percent figure

when estimating how many people might die in a bird-flu pandemic.

That's because such a high mortality rate goes against all of our

experience with flu viruses, says Dr. Fauci, director of the

National Institute of Allergy and Infectious Diseases.

" We have never in our wildest dreams seen that in our history, where

you have something that spreads rapidly throughout the world and

kills 50 percent of the people, " Fauci says. " Even with the infamous

1918 pandemic, we didn't even come close to a 50-percent mortality.

It was more like 1.5 to 2 percent. "

Of course, that was high enough to kill tens of millions of people

worldwide.

Fauci says one reason flu doesn't kill more people is that even a

strain as lethal as the current bird flu usually gets weaker as it

spreads.

" It is highly, highly likely that it will decrease its mortality and

its virulence for humans, because from an evolutionary standpoint, it

makes no sense for viruses to kill all their hosts, " Fauci says.

That would amount to viral suicide.

Another problem with the 50-percent figure is that it includes only

the people who got so sick they were actually tested for bird flu.

" There probably are milder cases of illness, as well as people who

have no symptoms whatsoever, " says Dr. Tim Uyeki, a medical

epidemiologist at the Centers for Disease Control and Prevention.

Uyeki says death rates from viruses like West Nile and SARS seemed

very high at first. But they began to fall once doctors began finding

the people who got milder cases.

And Uyeki says there is already some evidence that the current bird-

flu virus, known as H5N1, doesn't make everyone it infects severely

ill.

When H5N1 surfaced in Hong Kong in 1997, it appeared to kill about a

third of the people it infected. But Uyeki says a study of poultry

workers there told a different story.

" Among market poultry workers, " he says, " about 10 percent had

antibodies to H5N1 virus, suggesting that they had been infected. But

these were people who never were identified as severe cases, had

never been hospitalized. "

So the actual death rate in Hong Kong may have been much lower than

it seemed.

But if 50-percent mortality is too high, it's hard to know what the

right number is.

One reason is that the H5N1 virus isn't acting the way most viruses

do, says Dr. Frederick Hayden of the University of Virginia.

" This virus so far has not shown any diminution in virulence for

birds or for mammalian hosts, " Hayden says. " If anything, we're

seeing evidence of increased pathogenicity. "

So scientists are making educated guesses. Fauci says the

public, and policy makers, need numbers in order to make plans.

He says they want to be told there's " a 5-percent chance of this or a

10-percent chance of that, or a 2-percent mortality is a worst-case

scenario. "

But Fauci adds, " The one thing that you learn with infectious

diseases, particularly with influenza -- because influenza is a

particularly bad actor in this regard -- [is that] it is really quite

unpredictable. "

Fauci says public health officials are preparing for the worst. But

in this case, that means a mortality rate closer to 2 percent than 50

percent.

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