Re: Heat wave of 1915-1916
We obtained annual P&I mortality rates by age group for<SUP> </SUP>1915-1917 to look for the typical "U-shaped" mortality curve<SUP> </SUP>(increased rates at the extremes of age) and for 1918 to look<SUP> </SUP>for the "W-shaped" curve characteristic of the pandemic (highest<SUP> </SUP>annual mortality rates among young children, young adults, and<SUP> </SUP>the elderly). To provide an alternative measure of the age-specific<SUP> </SUP>impact on interpandemic and pandemic influenza-related deaths,<SUP> </SUP>excess mortality rates were plotted by age and compared for<SUP> </SUP>the severe 1915/
1916 influenza season, the March and April 1918<SUP> </SUP>epidemic months, and the 1918/1919 pandemic season. As done<SUP> </SUP>recently to characterize mortality age patterns for the 1968<SUP> </SUP>pandemic (
31), we calculated the age-specific ratio of pandemic<SUP> </SUP>to interpandemic excess rates to estimate age-specific relative<SUP> </SUP>risk by using 45 years of age as the cutoff.<SUP> </SUP>
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Abstract
Materials and Methods
Results
Discussion
References
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Excess Mortality. Severe epidemic mortality occurred in 5 of<SUP> </SUP>the 10 influenza seasons during 1911-1921. The 1911/1912 influenza<SUP> </SUP>season was reported by the health department as distinctly free<SUP> </SUP>of epidemic influenza, and the 1915/
1916 influenza epidemic<SUP> </SUP>was noted at the time as the worst to date that century (
4).<SUP> </SUP>During the 1915-1920 epidemic seasons, the temporal pattern<SUP> </SUP>and peaks in all-cause and P&I deaths coincided (
Fig. 1 A and B).<SUP> </SUP>Epidemic period estimates for all ages in the 1915/
1916<SUP> </SUP>and
1916/1917 seasons were 2,100 and 2,800 excess all-cause<SUP> </SUP>deaths, respectively (
Table 1). Excess all-cause deaths in the<SUP> </SUP>1918/1919 pandemic and 1919/1920 recrudescence were 29,200 and<SUP> </SUP>8,200, respectively (
Table 1). An estimated 4,600 excess all-cause<SUP> </SUP>deaths occurred during the 1917/1918 influenza season (
Table 1);<SUP> </SUP>epidemic mortality in this season occurred in two peaks,<SUP> </SUP>in January and March 1918 (
Fig. 1 A and B, arrowheads).<SUP> </SUP>
<SUP></SUP>
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Fig. 1. All-cause and P&I monthly mortality rates for all ages (
A and
B) and all-cause mortality rates by age group (
C-
H) are calculated per 10,000 population. Observed rates are days-per-month adjusted. Expected model baselines (solid lines) are derived from each series of nonepidemic months. Epidemic thresholds (dashed lines) are the upper 95% confidence limit above each baseline. The major epidemic influenza season months are indicated (shaded). Two 1917/1918 influenza season peaks (arrowheads) show excess mortality primarily confined to the
65-years age group in January (
C), and to the groups <45 years old in March 1918 (
E-
H). Other severe mortality events are evident: summer diarrheal disease epidemics were confined to young children (
H), the
1916 polio epidemic to all children (
G and
H), and the summer 1917
heat wave and diarrheal disease epidemic among the youngest and oldest age groups (
C and
H).
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Table 1. Influenza season excess deaths
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Age-Specific Analyses. The age pattern of mortality shifted<SUP> </SUP>profoundly during the 1917/1918 season. In the 1915/
1916 and<SUP> </SUP>
1916/1917 influenza seasons and in January 1918, excess mortality<SUP> </SUP>incidence was greatest in those
65 years old (
Fig. 1 C). During<SUP> </SUP>February-April 1918 and the 1918/1919 season, those
65 years<SUP> </SUP>old experienced little or no excess mortality (
Fig. 1C), whereas<SUP> </SUP>those aged 15-24 and 25-44 years experienced sharply elevated<SUP> </SUP>death rates (
Fig. 1 E and F). Mortality data for children <5<SUP> </SUP>and 5-14 years old did not exhibit winter peaks before the 1917/1918<SUP> </SUP>season (
Fig. 1 G and H). Before 1918, children experienced mortality<SUP> </SUP>peaks from summer diarrheal disease epidemics, a severe polio<SUP> </SUP>epidemic in the summer of
1916 (
Fig. 1 G and H), and a coincident<SUP> </SUP>diarrheal epidemic and
heat wave in the summer of 1917 (
Fig.<SUP> </SUP>1H).<SUP> </SUP>
Because of the striking contrast in mortality impact by age<SUP> </SUP>during the 1918/1919 pandemic season, we calculated the ratio<SUP> </SUP>of <45 to
45 years epidemic period excess death rates (
Table 1).<SUP> </SUP>The ratio of age-specific epidemic deaths shifted abruptly<SUP> </SUP>in February 1918 and reached values >20-fold higher in March<SUP> </SUP>and April 1918 and in the 1918/1919 season than in the 1915/
1916<SUP> </SUP>and
1916/1917 epidemic seasons (
Table 1). The age-specific ratio<SUP> </SUP>in the 1919/1920 season was closer to prepandemic levels (
Table 1);<SUP> </SUP>however, the total burden of excess deaths among people<SUP> </SUP><45 years old remained extremely high, compared with the<SUP> </SUP>interpandemic period (
Fig. 1 E-H).<SUP> </SUP>
The W-Shaped Curve. Average annual P&I death rates by age<SUP> </SUP>for 1915-1917 showed the classic U-shaped mortality pattern<SUP> </SUP>characteristic of influenza epidemic years (
Fig. 2A). For calendar<SUP> </SUP>year 1918, the age-specific mortality pattern showed the classic<SUP> </SUP>W-shaped distribution of the pandemic (
Fig. 2 A). In contrast,<SUP> </SUP>when we plotted seasonal age-specific excess mortality rates<SUP> </SUP>(specifically representing influenza-related mortality), a strikingly<SUP> </SUP>different pattern emerged. For the 1915/
1916 interpandemic season,<SUP> </SUP>the U pattern is flattened on one end because children <5<SUP> </SUP>years had little excess epidemic-period impact, whereas the<SUP> </SUP>U pattern was elevated on the other, because the greatest increase<SUP> </SUP>in epidemic period mortality was among the elderly (
Fig. 2B).<SUP> </SUP>For the 1918/1919 pandemic season, mortality rates in young<SUP> </SUP>children and young adults were sharply higher than for the 1915/
1916<SUP> </SUP>epidemic, whereas mortality rates were lowest among the oldest<SUP> </SUP>age group, presenting an age pattern resembling an attenuated<SUP> </SUP>W shape (
Fig. 2B).<SUP> </SUP>
<SUP></SUP>
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Fig. 2. Annual and epidemic period mortality rates by age group. (
A) Average age-specific calendar year P&I death rates are plotted in a U-shaped age distribution for 1915-1917 (
), and in the characteristic W-shaped distribution for the pandemic year 1918 (
). (
B) Influenza-season-attributable excess deaths are plotted for the 1915/
1916 epidemic influenza season (
), the epidemic months March and April 1918 (
), and the pandemic season from September 1918 through April 1919 (
). (
C) The excess rates are plotted on a log<SUB>10</SUB> scale. (
D) Relative risk of death is plotted by age group on a log<SUB>10</SUB> scale for the March and April 1918 epidemic period (
) and the pandemic from September 1918 through April 1919 (
) relative to the severe 1915/
1916 epidemic season.</TD></TR></TBODY></TABLE></TD></TR></TBODY></TABLE></CENTER>
http://www.pnas.org/cgi/content/full/pnas;102/31/11059