http://www.cidrap.umn.edu/news-perspective/2007/02/severe-pandemic-not-overdue-its-not-when-if
A severe pandemic is not overdue - it's not when but if
cidrap.umn.edu/news-perspective/2007/02/severe-pandemic-not-overdue-its-not-when-if
(CIDRAP Source Weekly Briefing) – We have no grounds for confidence that a severe
pandemic is imminent. Our communications shouldn't imply otherwise.
Medical historians tell us there have been nine influenza pandemics in the past 300 years. So
one every 30 to 35 years or so, or roughly three per century, is everybody's best guess about
the future frequency of influenza pandemics.
But extrapolating from nine cases is far from a sure thing. Scientists wouldn't be all that
shocked if pandemics started coming more frequently or less frequently. And even if the
average remains three per century, it's only an average. The 21st century could still give us
just one pandemic—or five.
A semi-official list of the nine pandemics since 1700, listed by the year they started is:
1729
1732
1781
1830
1833
1889
1918
1957
1968
There is nothing cyclic about this list. The shortest gap between pandemics is 3 years; the
longest so far is 56 years. (Some authorities include 1899 and 1977 on the list as well. Adding
them doesn't improve the pattern any, though it does increase the expected frequency a bit.)
To all intents and purposes, flu pandemics are random events. So there are no grounds to
claim that a pandemic is overdue simply because there hasn't been one since 1968. A random
event cannot be "overdue." Risk-perception experts have a name for the mistaken view that
random events are patterned. They call it the gambler's fallacy—named for the tendency of
many roulette players to imagine that a number is overdue because it hasn't come up all night.
(The other gambler's fallacy is imagining that a different number is "hot" because it has come
up several times in quick succession.)
Bottom line: The probability of a flu pandemic hasn't increased because we've gone without
one for 38 years. And if we have one this year, the probability of having another (of a different
strain) the next year won't decrease. 1/3
Still, scientists would be pretty surprised if influenza pandemics simply stopped happening
altogether. So it's fair to say about the next flu pandemic that "it's not if, but when." And a lot of
people have said just that. A Google search for "pandemic" plus "not if but when" yielded
about 1,000 links (some of them from CIDRAP).
But this is not a fair thing to say about asevere flu pandemic. By most accounts, 1918 was the
mother of all flu pandemics, worse than any (or nearly any) we know about before or since.
Most survivors of the 1918 pandemic remembered losing friends or relatives to it. By contrast,
most of us who lived through the pandemics of 1957 and 1968 barely noticed.
Maybe a pandemic as bad as 1918 happens once every 300 years or so. (There was
apparently a big one in 1580, too.) Maybe 1957 and 1968 will turn out to be the exceptions,
and most future pandemics will be more like 1918. We don't know. That's why I say that, when
we're talking about a severe pandemic, something as bad as 1918 or worse, it's not when but
if.
Precisely because 1957 and 1968 were so forgettable, the claim that future pandemics are
inevitable is often heard as a claim that severe pandemics are inevitable. And that's just not
true.
Or it may be heard as a claim that the influenza strain that currently dominates news coverage,
H5N1, will inevitably launch a pandemic. That's not true either. H5N1 has been around at least
since 1997 without becoming capable of efficient human-to-human transmission. Does that
mean that it probably won't? We don't know. H5N1 has proved incredibly deadly to both poultry
(millions) and people (scores). Does that mean that if it becomes capable of efficient humanto-human
transmission, the pandemic it launches will be a severe one? We don't know that
either.
Right now is the first time we've ever been able to watch closely as a new bird flu strain either
does or doesn't lead to a human pandemic. So we can't say whether H5N1 is acting like past
bird flus that later launched bad pandemics, or past bird flus that later launched mild
pandemics, or past bird flus that never launched pandemics at all.
In recent months there have been a few news stories on the theme: "Whatever became of the
bird flu scare? How come the predicted pandemic didn't happen?" What's missing from these
very damaging stories is the crucial fact that the pandemic risk hasn't abated simply because
no pandemic has materialized so far.
Pandemic preparedness advocates blame these sorts of stories on the short attention span of
the media. But we who give the mainstream media their information—including CIDRAP—
deserve some of the blame ourselves. If reporters and the public earlier got the impression that
a severe H5N1 pandemic was imminent, they got it from us. The misimpression that the risk
was necessarily imminent and the misimpression that the risk is now past are identical twins.
The first gave rise to the second.
Two closely connected risk-communication principles are at stake here. 2/3
The first principle is to acknowledge uncertainty. An overconfident risk communicator is likely
to generate skepticism in the audience even before the truth is known. And if the truth turns
out to be different from your confident prediction, trust in you erodes. That doesn't mean you
shouldn't make predictions. It means your predictions shouldn't sound more confident than the
facts justify. Talk about H5N1 the way weather forecasters talk about a distant hurricane. It
might be headed our way—or not. It might strengthen—or weaken. We need to track it closely,
stockpile essential supplies, and make contingency plans. Will it be category 5 or category 2?
Will it hit here or go elsewhere? It's not when, but if.
The second principle deals specifically with worst-case scenarios—that is, low-probability,
high-magnitude risks. The wisdom of taking precautions depends not just on the probability of
a risk, but also on its magnitude. When a risk is awful enough, precautions make sense, even
though the risk may be unlikely and the precautions may be wasted. We don't buy fire
insurance because we're confident our house will catch fire; we buy it because a fire could be
devastating. And wise insurance salespeople don't tell us that we're overdue for a fire. They
don't claim that it's not if but when our house will burn.
I'll say more about the communication of worst-case scenarios in my next column.
An internationally renowned expert in risk communication and crisis communication, Peter
Sandman speaks and consults widely on communication aspects of pandemic preparedness.
Dr. Sandman, Deputy Editor, contributes an original column to CIDRAP Source Weekly
Briefing every other week. Most of his risk communication writing is available without charge at
the Peter Sandman Risk Communication Web Site (
http://www.psandman.com). For an index of
pandemic-related writing on the site, see
http://www.psandman.com/index-infec.htm.