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⇱ Severe mortality impact of the 1957 influenza pandemic in Chile - PubMed

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Abstract

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=.02), but not with latitude (P>.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 three to five 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.

Keywords: 1957 influenza pandemic; Chile; baseline mortality rates; excess mortality rates; latitude; reproduction number; transmissibility.

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Figures

👁 Figure 1
Figure 1
Temporal evolution of all‐cause mortality rates per 10 000 people during 1953‐1959 across 25 provinces of Chile, sorted in geographic order from north to south of Chile. For visualization purposes, the time series are log‐transformed
👁 Figure 2
Figure 2
Monthly time series of all‐cause and respiratory mortality per 10 000 people in Chile, 1953‐1959 (black curve). Shaded areas highlight three winter periods (Jun‐Dec) during 1957‐1959. The Serfling seasonal regression model baseline (blue curve) and corresponding upper limit of the 95% confidence interval of the baseline (red curve) are also shown
👁 Figure 3
Figure 3
Age‐stratified weekly respiratory mortality rates per 10 000 in the city of Concepcion, Chile, 1953‐1959 (black curve). Shaded areas highlight three winter periods (Jun‐Dec) during 1957‐1959. The Serfling seasonal regression model baseline (blue curve) and corresponding upper limit of the 95% confidence interval of the baseline (red curve) are also shown. Excess deaths are above the upper limit of the baseline mortality curve calibrated using mortality levels prior to the 1957 influenza pandemic
👁 Figure 4
Figure 4
Weekly all‐age all‐cause and respiratory death counts in Santiago during weeks 29‐35 in 1957 and corresponding mortality baseline in 1956 during the same period (top). The basic reproduction number was estimated from the exponential growth fit to the initial phase of the excess mortality curve, which was computed by subtracting the 1956 baseline mortality curve to the 1957 pandemic period (bottom)
👁 Figure 5
Figure 5
Weekly all‐cause and respiratory death counts in Concepcion during weeks 28‐38 in 1957 and the corresponding mortality baseline based on data from 1953 to 1957 during the same period (top). The basic reproduction number was estimated from the exponential growth fit to the initial phase of the excess mortality curve, which was computed by subtracting the baseline mortality curve to the 1957 pandemic period (bottom)

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