Severe mortality impact of the 1957 influenza pandemic in Chile

INTRODUCTION: Epidemiological studies of the 1957 influenza pandemic are scarce, particularly from lower income settings.

METHODS: We analyzed the spatial-temporal mortality patterns of the 1957 influenza pandemic in Chile including detailed age-specific mortality data from a large city and investigated risk factors for severe mortality impact across regions.

RESULTS: Chile exhibited two waves of excess mortality in winter 1957 and 1959 with a cumulative excess mortality rate of 12 per 10,000, and a ~10-fold mortality difference across provinces. High excess-mortality rates were associated with high baseline mortality (R2=41.8%; P=0.02), but not with latitude (P>0.7). Excess mortality rates increased sharply with age. Transmissibility declined from R=1.4-2.1 to R=1.2-1.4 between the two pandemic waves.

CONCLUSIONS: The estimated A/H2N2 mortality burden in Chile is the highest on record for this pandemic - about 3-5 times as severe as that experienced in wealthier nations. The global impact of this pandemic may be substantially underestimated from previous studies based on high-income countries.