David wrote:
"The need for clear and sound information about bird flu is obvious if such reactions are to be avoided. Government officials clearly have a responsibility to ensure that this takes place. But in an era of widespread distrust of public institutions, this is no longer sufficient. Equally, if not more, important, is the role of journalists and the media.
"The task is made both more important and more difficult when official organisations seek, for reasons of their own, to place a 'spin' on the information they present. Last year, for example, we criticised the behaviour of governments in Asia that were restricting the information they divulged about bird flu outbreaks — sometimes even denying that outbreaks had occurred (see Bird flu: the communication challenge).
"More recently a new culprit has emerged, namely the temptation by international agencies, perhaps keen to squeeze extra funding from reluctant donors, to overstate the size of potential problems they are likely to face.
"Last month, for example, the World Health Organization issued a hurried correction after its top official responsible for handling the bird flu crisis, David Nabarro, told the media that the diseases could cause 'between five and 150 million deaths', comparing the challenge to that of a combination of climate change and HIV/AIDS. The following day, the agency clarified the statement to say that its estimate of the number of people who could die was 'between two million and 7.4 million'."
What we really need, and don't have, is an estimate of the probability distribution of pandemic sizes!
The National Vaccine Program Office notes, "History suggests that influenza pandemics have probably happened during at least the last four centuries. During the 20th century, three pandemics and several "pandemic scares" occurred." I have seen one estimate that there have been ten flu pandemics in the last 300 years.
The BBC reports:
- Spanish Flu 1918-9 - Killed up to 40million. (Others suggest that the number may have been 50 million to 100 million; one of the problems is that no one really knows the numbers of people killed by these pandemics, and the farther in the past, the less certain the data.)
- Asian Flu 1957-8 - About 1million died from the flu.
- Hong Kong Flu 1968-9 - Similar death toll to the Asian Flu.
Some of the "false alarms" according to HHS:
- 1976: Swine Flu Scare (See my previous posting.)
- 1977: Russian Flu Scare (An epidemic caused by a type of flu that had disappeared for 20 years. People under the age of 20 had no immunity to the A/H1N1 virus but older people did. Thus the worst of the epidemic was confined to young people, and the event was not seen as a true pandemic.)
- 1997-1999: Avian Flu Scare (at least a few hundred people became infected with the avian A/H5N1 flu virus in Hong Kong in 1997 and in 1999, another novel avian flu virus – A/H9N2 – was found that caused illnesses in two children in Hong Kong. It is the avian flu that is of most concern at this time.)
The World Health Organization states, considering normal flu outbreaks (that is, the annual flu outbreaks caused by already circulating viruses for which there is preexisting immunity or resistance in a significant portion of the population):
"In annual influenza epidemics 5-15% of the population are affected with upper respiratory tract infections. Hospitalization and deaths mainly occur in high-risk groups (elderly, chronically ill). Although difficult to assess, these annual epidemics are thought to result in between three and five million cases of severe illness and between 250,000 and 500,000 deaths every year around the world."
Thus, we can expect a new pandemic to occur about once every 30 years, and it is more than 35 years since the last one, but we can not predict with any precision which year such a pandemic will strike.
We can also predict false alarms, in which infections occur which are perceived by public health officials to constitute important warning signs of the possibility of a pandemic, but which are not followed by the feared pandemic.
When an epidemic does emerge, the number of people infected will depend in part on the ease with which the virus is transmitted from person to person, as well as the effectiveness of public health measures taken. The number of severe cases of the disease will depend on the number of people infected, the level of cross immunity in the population (if any), and the virulence of the virus. So too, the number of deaths will depend on the lethality of the virus in those infected, and the effectiveness of the medical services in treating the very ill.
Put it another way. Perhaps 97 of the last 100 years saw "normal" flu epidemics, which might cost 250,000 to 500,000 deaths per year. Two of the hundred years saw pandemics that cost perhaps one million deaths. One year saw a pandemic that cost many tens of millions of deaths per year. On the other hand, over the last half century on the five or six occassions on which public health officials thought a pandemic likely, pandemics actually occurred twice.
Will a pandemic occur in 2006 or 2007? No one knows. Under any circumstance, flu is a major problem worldwide. Any pandemic is terrible, and it seems to me that the public health officials are telling us that one is more likely in the next couple of years than usual. Pandemics causing tens of millions of death are quite rare -- only one in the last hundred years. However, as Hurricane Katrina has shown us, one-in-a-hundred-years events do happen, and they can be so damaging that preparation is the wiser course.
Predicting the course of an epidemic shares something with predicting the course of a hurricane. During the event itself, as data accumulates, predictions become more and more accurate. Trying to predict a couple of years in advance the exact course of either a hurricane or a pandemic is a futile exercise. At best, for long term predictions one can only begin to guess at what would be a once a century event. But it is important to use the best current information to guess how likely the events in the next couple of years are to be the once-in-a-century variety.
The publication of a prediction of a pandemic by WHO to a global audience is different than the publication of a prediction by an epidemiologist in a professional journal. WHO is of course concerned with the accuracy of the prediction. It is also concerned that it not cause panic by too draconian a prediction, but that its prediction is sufficiently grave as to alert the proper authorities to take appropriate actions. It does not surprise me that WHO might waffle on the prediction. I think it unfair for David to imply it might be doing so for ulterior motives.
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