May 1, 2009

As the Swine Flu Grows a Pandemic Seems Inevitable

update written by Avram L. Nemetz, M.D. Medical Director

Yesterday, the day after the WHO advanced its pandemic warning level to Stage 5, the visible public debate among flu authorities shifted abruptly to mitigating an apparently "inevitable" pandemic.

When a new pandemic virus emerges, it's like turning on a light switch: The event is done. A new influenza virus is here-the H1N1 strain first seen in Mexico-and nothing can make it stop. By its nature it is designed to spread throughout the human population, it is distinct from any previous virus to which we have had exposure, and none of us has any pre-existing immunity to this novel strain.

None of what is happening is surprising to those who have studied flu pandemics, any more than a big earthquake is "surprising" to a seismologist. Events have moved quickly, but this is precisely how the flu virus has operated throughout its recorded history, which we can track back with precision for at least three centuries. You never know exactly when one is going to happen-so in a sense it's always surprising-but once an event is underway there are predictable consequences that can be followed almost mathematically.

Beware of statistical models that are reassuring in the short term. The last paragraph in the linked NY Times article states:

Some experts are cautiously optimistic. A computer simulation of this outbreak released Wednesday by a team from Northwestern University projected a worst-case scenario, meaning no measures have been taken to combat the spread. It predicted a mere 1,700 cases in the United States four weeks from now.

This is reassuring only if you do not think about what happens in the four weeks after that. A spread from two initial cases to 1700 indicates a doubling in the number of cases every two-and-a-half days. (This may be faster than some pandemics have developed.) If you follow the model out further, in 53 days you have 108,000 cases, and in 88 days you have 13,600,000 cases of influenza. This timing is consistent with the model from Los Alamos, NM cited in the previous update.

The timing of the models would suggest rare cases of flu now, scattered cases of flu until June, but by July and August, possibly significant disruption of life and commerce nationwide.

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Interesting tidbits in the news speak to the already widespread nature of the virus. Remember, it is just one week since its existence was first reported to the public.

The White House issued a health advisory when an aide who had traveled to Mexico with President Obama contracted flu-like symptoms after the trip, and appears to have spread the illness to three members of his family.

Demonstrating the least rational international response to a possible pandemic to date, Egypt has announced and begun carrying out the culling of all the nations' 300,000 pigs. Farmers are resisting. This is occurring despite the fact that Egypt has no known cases of swine flu, and the H1N1 virus we are concerned about is not spread by pigs. It is a respiratory virus spread from person-to-person.

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The next predictable step in the sequence of events will come shortly. The CDC has a Pandemic Severity Index in which pandemics are rated like hurricanes, from Category 1 to Category 5.

The 1918 Spanish Flu pandemic was Category 5, while our most recent pandemic, the1968 Hong Kong Flu, was Category 1. The difference is based on the case fatality ratio-a death rate of 2.0% or higher is at the top, while a rate of 0.1% or lower is at the bottom of the scale. If we have a pandemic on our hands, let's hope it's a mild one.