When claims are made that the new H1N1 virus is “mild”, we are often told that between 20,000 and 50,000 people die every year of seasonal influenza. What we are rarely told is that 90% of these deaths occur in people over 65 years old (Foppa and Hossain, 2008). Further, people over 65 represent a relatively small percent of the total US population:
- 0-14 years: 20.2%
- 15-64 years: 67%
- 65 years and over: 12.8%
Since most of the remaining deaths occur in babies and infants, this means that the case fatality rate for people in the 15-64 age range from seasonal flu is vanishingly small.
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. 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.
For countries that lack Tamiflu or vaccines, a 1918 level pandemic is entirely possible.