When Did the H1N1 Flu Bug Go Away?

August 12, 2009
When Did the H1N1 Flu Bug Go Away?

By Peter Laird, MD

It's inexplicable to hear many of the H1N1 flu commentators talk about swine flu returning in the fall and their continuing to downplay the impact that this virus will have. That sort of commentary makes no sense. H1N1 has never gone away and it has continued to spread rapidly during what is normally flu's off season. Here in North America's summer months, where flu's spread should have abated by past experience we continue to have a drum beat of illness and death.

Despite this ongoing crises many experts continue to sound the all clear, announcing the end of the current H1N1 pandemic before we have seen the full brunt of its fall/winter rampage:
Harsh second H1N1 wave not inevitable: U.S. experts
We do think -- everybody thinks -- the virus will come back in the (northern hemisphere) fall," Morens said.

"Beyond that, it's hard to say what will happen -- whether it will be more severe, less severe or just the same. If you look at past pandemics, any one of those things happened at a particular time," he said.

This calming proclamation came as we saw a 24% increased mortality in one week alone here in the United States. During the summer! The United States continues be a global hot spot for H1N1 flu deaths and hospitalizations even as the flu season in the southern hemisphere is picking up strength.

The news that H1N1 flu is spreading rapidly during the usual flu free summer months here in America is a bad omen of the unabated and unrelenting assault we can expect to see in the coming months. The initial attack rate in the spring disproportionately affected children who have the lowest rate of death giving a misleadingly low total death rate.
Age Structure and Case Fatality Rate
The new H1N1 virus is very different. The vast majority of the deaths have been in people under the age of 65. Perhaps the most detailed calculation of case fatality rates by age was provided in a leaked CDC Director’s brief (from July 16, 2009) that was apparently uploaded to Cryptome (PDF link). According to this document, here are the case fatality rates for different ages (n = 262):

* 0 -4 years: 0.17%
* 5-24 years: 0.22%
* 25-49 years: 1.5%
* 50-64 years: 3.33%
* 65 years and over: 5.24%

In the Spring and early Summer, most of the cases in the US were in children because the virus was spreading rapidly in schools and summer camps. Note, the case fatality rate for this age range is 0.22%. However, in the Fall, we can expect that although the virus will likely primarily affect school children in the beginning, it will eventually spread to the older age groups.

Based on the leaked information on case fatality rates from the CDC and the known population structure of the US, it is reasonable to project that the unmitigated case fatality rate for the US in the Fall/Winter will be much higher than in the Spring, even with no change in the killing power of the virus.

With rapidly increasing deaths across America long before the start of school that many predict will cause the H1N1 virus to return, speaking of the return of this virus belies the simple fact it has never left us.

http://www.billpeckham.com/from_the_sharp_end_of_the/2009/08/when-did-the-h1n1-flu-bug-go-away.html