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:
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.
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 a severe 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 human-to-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.
- 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.
Copyright © 2007 Regents of the University of Minnesota. Originally published on
the CIDRAP Business Source website, February 22, 2007. Reproduced with permission.