Sunday, May 03, 2009

Modest exponential growth for H1N1 swine flu, but probably peaked

Yesterday I indicated that the growth in lab confirmed cases of influenza A(H1N1) "swine flu" was not yet exponential. I looked at the data again when the latest data came out today and now it is clear that there was in fact exponential growth in lab confirmed cases, both CDC and WHO, on the order of 50% growth per day, 47% growth per day for the U.S. and 55% growth per day globally. For the 10-day period since April 24, 2009, I calculated that the doubling interval for growth in lab confirmed cases was about 2.25 days globally for the WHO data and about 2.29 days for the U.S. CDC data.

BUT... the most recent WHO data grew at only a 36% daily rate and the most recent U.S. CDC data grew at only a 41% rate. My hunch is that the growth rate has peaked.

Here is the WHO and CDC data for lab confirmed cases and my calculated daily growth rates and days to double the growth rate of each day:

  1. 4/24 - WHO: 25, doubled in 4 days; CDC: 8, doubled in 2 days.
  2. 4/25 - WHO: 25, no change, doubled in 3 days; CDC: 11, 38%, doubled in 1 day.
  3. 4/26 - WHO: 38, 52%, doubled in 2 days; CDC: 20, 82%, doubled in 1 day.
  4. 4/27 - WHO: 40, 5%, doubled in 1 day; CDC: 40, 100%, doubled in 2 days.
  5. 4/28 - WHO: 105, 163%, doubled in 2 days; CDC: 64, 60%, doubled in 3 days.
  6. 4/29 - WHO: 148, 41%, doubled in 2 days; CDC: 91, 42%, doubled in 4 days
  7. 4/30 - WHO: 257, 74%, doubled in 2 days; CDC: 109, 20%, doubled in 3 days.
  8. 5/01 - WHO: 367, 43%, doubled in 2 days; CDC: 141, 29%, not yet doubled.
  9. 5/02 - WHO: 658, 79%, not yet doubled; CDC: 160, 13%, not yet doubled.
  10. 5/03 - WHO: 898, 36%, not yet doubled; CDC: 226, 41%, not yet doubled.

In summary, an exponential growth rate was clearly established, but my hunch is that it has already peaked.

In order to confirm that the exponential growth rate is still in place we would need to see the WHO number jump to 1,300 within a day or two and the CDC data would need to jump to 320 within a day or 450 within two days.

From want I have read, it sounds as if a large percentage of the "new" confirmed cases are simply lab confirmations of existing "probable" cases rather than the appearance of symptoms in additional persons.

-- Jack Krupansky

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