Abstract
BackgroundThe historical Japanese influenza vaccination program targeted at schoolchildren provides a unique opportunity to evaluate the indirect benefits of vaccinating high-transmitter groups to mitigate disease burden among seniors. Here we characterize the indirect mortality benefits of vaccinating schoolchildren based on data from Japan and the US.MethodsWe compared age-specific influenza-related excess mortality rates in Japanese seniors aged ≥65 years during the schoolchildren vaccination program (1978–1994) and after the program was discontinued (1995–2006). Indirect vaccine benefits were adjusted for demographic changes, socioeconomics and dominant influenza subtype; US mortality data were used as a control.ResultsWe estimate that the schoolchildren vaccination program conferred a 36% adjusted mortality reduction among Japanese seniors (95%CI: 17–51%), corresponding to ∼1,000 senior deaths averted by vaccination annually (95%CI: 400–1,800). In contrast, influenza-related mortality did not change among US seniors, despite increasing vaccine coverage in this population.ConclusionsThe Japanese schoolchildren vaccination program was associated with substantial indirect mortality benefits in seniors.
Highlights
Despite current approaches to prevention and control, seasonal influenza remains a significant cause of morbidity and mortality worldwide
Over the entire period studied, crude excess mortality rates were higher in Japan than in the US (19 v. 16 per 100000, respectively), but this patterns reversed after adjustment for population demographics and socioeconomic trends (Table 1)
Adjustment for betweencountry differences in socio-economic conditions, using summer mortality rates experienced in the US in 2000 as a reference, resulted in a,50% reduction in Japanese winter-seasonal excess pneumonia and influenza (P&I) mortality rates
Summary
Despite current approaches to prevention and control, seasonal influenza remains a significant cause of morbidity and mortality worldwide. Analysis of US national vital statistics revealed that increasing vaccination coverage in seniors from 1980 to 2001 did not correlate with a decline in influenza-related mortality [5]; influenza-related hospitalization rates increased steadily in persons aged $50 years during this time period [6]. These findings suggest that the strategy of vaccinating only ‘‘high risk’’ populations may not be sufficient to decrease influenza transmission and severe burden at the population level, in part due to the weak immune response of seniors to vaccination [7].
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