Walking the Pale Horse

"...when he had opened the fourth seal, I heard the voice of the fourth beast say, Come and see. And I looked, and behold a pale horse: and his name that sat on him was Death, and Hell followed with him. ~Revelations 6:7-8

The Swine Flu appears to be just another flu, nothing special.

First, the "Attack Rate" is the number of people who get sick as measured against the total number of people exposed. Preliminary data from cases in the United States show a swine flu virus attack rate in the 25% to 30% range, which is consistent with seasonal influenza, says Anne Schuchat, MD, interim deputy director for the CDC's science and public health program.

Second, the "Basic Reproductive Number" is a measure infectiousness or how many secondary cases of flu a typical patient will cause in a population with no immunity to the pathogen. For highly contagious measles, the basic reproductive number is above 15; for smallpox, above 5. For ordinary influenza, the basic reproductive number ranges from 1.5 to 3.0.

"According to the preliminary models, the reproductive number that we have in the Mexico City metropolitan area is 1.5," said Miguel Ángel Lezana, director of the National Center for Epidemiology and Disease Control. "It's a number fairly low, and that's good news."

Scientists still debate the infectiousness of the virus causing severe acute respiratory syndrome, or SARS. It generally receives a basic reproductive number of 3. The most recent estimates of the infectiousness of the 1918 influenza pandemic range from 1.8 to 2.0.

With the low reproductive number of Swine Flu, quick school closings and other isolationist actions should be very effective in slowing the spread, delaying a pandemic until a developed vaccine vanquishes it.

Finally, the "Case-Fatality Rate" is the fraction of infected people who die.

For the Swine Flu, it is still unknown, but appears to be shrinking as new data comes in. Initial data from Mexico calculated at 7.5%, a horrendous figure, but normally only one-in-ten cases of flu are reported, meaning it would really be only 0.75%. For the 1918 influenza, it was 2-2.5% for the United States as a whole, but in military camps and on troop ships, the rate rose to a brutal 7-10%, and in some Inuit villages, it soared to 70%. Ebola outbreaks have been 60-90% depending on strain.

The other two flu pandemics of the 20th century, however, were far milder. The Asian influenza of 1957-58 had a fatality rate of 0.2-0.5%, and the rate during the Hong Kong influenza of 1968-69 was even lower, about 0.1%, close to seasonal flu. The adjusted Mexican rate at 0.75% would still be over 7-times normal flu rates, but rates of other reporting agencies indicate it will continue to drop.

Although too soon to know for sure, the entire affair seems more an exercise than the release of The Pale Horseman of Revelations. As world population density increases and travel evermore quickens dispersal, a deadly pandemic is a matter of "when"--not "if." This time we're just walking the horse.