Pandemic severity index: Does the 2009 H1N1 influenza pandemic measure up with past pandemics?

The 2009 H1N1 influenza pandemic is the first pandemic in over 40 years. While its severity has been low, its severity's relation to past pandemics remains uncertain. This study will investigate the severity of the recent 2009 influenza pandemic in the U.S. and compare it with that of the past three influenza pandemics (e.g. 1918-19, 1957-58, 1968-69). We will use U.S. surveillance data on laboratory-confirmed and reported pneumonia and influenza (P&I) cases and deaths during the first wave of each of the last four pandemics (i.e. August – November 1918, October – December 1957, December 1968 – March 1969, and April – July 2009) to estimate five indicators of influenza pandemic severity: case-fatality rate (CFR), attack rate, all-cause mortality rate, excess mortality rate, and P&I mortality rate. Preliminary results indicate that the estimated attack rate (71 cases/1000 persons) of the first wave of the 2009 pandemic in the U.S. was far less than the attack rates of the first waves of the 1918 pandemic (attack rate=280/1000; Odds Ratio [OR] = 0.20, 95% Confidence Interval [CI]: 0.15-0.26), the 1957 pandemic (attack rate=250/1000; OR=0.23, 95% CI: 0.17-0.31), and the 1968 pandemic (attack rate=200/1000; OR=0.31, 95% CI: 0.23-0.41). Future analyses will calculate and compare ratios for the other four severity variables, control for demographic variables (e.g. age, underlying medical conditions), and perform sensitivity analyses for a range of values for severity variables. We intend to distinguish if the H1N1 influenza epidemic was a pandemic of severity as well as of geography.