Death rate ratios from pneumonia and influenza (P&I) during the 1968–1969 pandemic and percent without antibody titers against H1 and H3 influenza viruses.

Serological data on H1 and H3 viruses were adapted from Masurel [8,9,25] and Doodle et al. [10]. Years of birth were deduced from data originally presented by age by Masurel (HI > 9–19) [8,9,25]. We used life tables from the Netherland Bureau of Statistics, available in the Human Life-Table Database [26], in order to estimate average ages (and years of birth) for donor sera grouped in age bins larger than five years. The death rate ratio by age (or by birth cohort) in 1968 was estimated by dividing the P&I death rate calculated for the December 1968 to January 1969 pandemic flu season [27] with the average P&I death rates of the same seasons from 1959–1960 to 1967–1968. These ratios represent the increase of mortality due to the 1968 pandemic (a ratio of 1.5 means a mortality increment of 50% in comparison with the previous ten-year’s average). Monthly P&I death counts were taken from the National Center for Health Statistics, available on the National Bureau of Economic Research website [28], while the populations exposed to risk were interpolated from the Human Mortality Database [29]. In order to account for secular mortality improvements, we detrended the time series from 1959 to 1968 with quadratic regressions of mortality rates based on the 20 epidemic seasons from 1959 to 1978, excluding the 1968–1969 pandemic season.