Thursday, August 19, 2010

The Pertussis Outbreak in California Revised

About a week ago I presented some data about the pertussis (whooping cough) outbreak in California and the US as a whole and suggested that the overall picture for the country was mixed.  Media outlets had been calling this the worst outbreak in 50 years but the number of cases for the country as a whole and for the majority of states were down from last year.

Of course as soon I wrote that, the CDC published new data and made a liar out of me.  That new data more than doubled the number of cases in California and pushed the country as a whole past prior years and on par with the last outbreak year.  I said in the update to my prior post that I was confused by the huge spike in the total number of cases because it didn't make much sense.

Well, it turns out I was right to be dubious.  The CDC has published data for the current week and revised their total numbers for the year significantly downward.  At the end of July (2 weeks ago), the total number of cases in California stood at 809 - a number that has been stagnant for 3 weeks.  Last week, that number jumped to 1,964 - a 150% change.  Well, in the current week, that number was slashed to 1,210 - a drop of 38%.

The number of cases for the country showed a similar pattern - the total number of cases went from 7,781 to 9,412 and then back down to 9,194.  I am including the updated charts below in case anyone is curious

I can't imagine what is behind these massive revisions in the numbers, you wouldn't think that it would be possible to be mistaken about 754 cases.  Maybe the cases are being counted too aggressively or there are a few labs that were doing some very bad testing.  Regardless of the reason, these changes don't inspire confidence.

If you look at how other epidemics, such as H1N1, have been handled recently, I think we are seeing a shift towards declaring epidemics first and counting cases later.


  1. You are seeing firsthand how shaky the pertussis statistics can be. This is an illness which is easily misdiagnosed. When doctors aren't thinking whooping cough, they can overlook cases with great ease. Now that whooping cough is being "promoted" as the illness of the year, doctors are obviously diagnosing everything that coughs as an example of the disease. But once lab test results come in many diagnoses turn out to be something else as the CDC numbers attest.

  2. Minority,

    I was hoping that by the time the CDC posted the numbers that they cases had already been verified by lab testing. I have to wonder if they are basing these numbers of the the results of PCR tests rather than the more accurate culture tests. Or maybe they are taking the doctor's word for it, who knows.

    Whatever the reason, you are right, these numbers aren't as good as I had thought they were.