Quantifying long-range dissemination of infectious diseases is a key issue in their dynamics and control. Here, we use influenza-related mortality data to analyze the between-state progression of interpandemic influenza in the United States over the past 30 years. Outbreaks show hierarchical spatial spread evidenced by higher pairwise synchrony between more populous states. Seasons with higher influenza mortality are associated with higher disease transmission and more rapid spread than are mild ones. The regional spread of infection correlates more closely with rates of movement of people to and from their workplaces (workflows) than with geographical distance. Workflows are described in turn by a gravity model, with a rapid decay of commuting up to around 100 km and a long tail of rare longer range flow. A simple epidemiological model, based on the gravity formulation, captures the observed increase of influenza spatial synchrony with transmissibility; high transmission allows influenza to spread rapidly beyond local spatial constraints.

Influenza epidemics lead to increased mortality, principally among elderly persons and others at high risk, and in most developed countries, influenza-control efforts focus on the vaccination of this group. Japan, however, once based its policy for the control of influenza on the vaccination of schoolchildren. From 1962 to 1987, most Japanese schoolchildren were vaccinated against influenza. For more than a decade, vaccination was mandatory, but the laws were relaxed in 1987 and repealed in 1994; subsequently, vaccination rates dropped to low levels. When most schoolchildren were vaccinated, it is possible that herd immunity against influenza was achieved in Japan. If this was the case, both the incidence of influenza and mortality attributed to influenza should have been reduced among older persons.

This study sought to quantify influenza-related serious morbidity in pregnant women, as measured by hospitalizations for or death from selected acute cardiopulmonary conditions during predefined influenza seasons. The study population included women aged 15–44 years who were enrolled in the Tennessee Medicaid program for at least 180 days between 1974 and 1993. In a nested case-control study, 4, 369 women with a first study event during influenza season were compared with 21, 845 population controls. The odds ratios associated with study events increased from 1.44 (95% confidence interval (Cl) 0.97–2.15) for women at 14–20 weeks‘ gestation to 4.67 (95% Cl 3.42–6.39) for those at 37–42 weeks in comparison with postpartum women. A retrospective cohort analysis, which controlled for risk factors identified in the case-control study, identified 22, 824 study events during 1, 393, 166 women-years of follow-up. Women in their third trimester without other identified risk factors for influenza morbidity had an event rate of 21.7 per 10, 000 women-months during influenza season. Approximately half of this morbidity, 10.5 (95% Cl 6.7–14.3) events per 10, 000 women-months, was attributable to influenza. Influenza-attributable risks in comparable nonpregnant and postpartum women were 1.91 (95% Cl 1.51–2.31) and 1.16 (95% Cl -0.09 to 2.42) per 10, 000 women-months, respectively. The data suggest that, out of every 10, 000 women in their third trimester without other identified risk factors who experience an average influenza season of 2.5 months, 25 will be hospitalized with influenza-related morbidity. Am J Epidemiol 1998; 148: 1094-102.

Almost all deaths related to current influenza epidemics occur among the elderly. However, mortality was greatest among the young during the 1918–191

OBJECTIVES: The purpose of this study was to assess the impact of recent influenza epidemics on mortality in the United States and to develop an index for comparing the severity of individual epidemics. METHODS: A cyclical regression model was applied to weekly national vital statistics from 1972 through 1992 to estimate excesses in pneumonia and influenza mortality and all-cause mortality for each influenza season. Each season was categorized on the basis of increments of 2000 pneumonia and influenza excess deaths, and each of these severity categories was correlated with a range of all-cause excess mortality. RESULTS: Each of the 20 influenza seasons studied was associated with an average of 5600 pneumonia and influenza excess deaths (range, 0-11,800) and 21,300 all-cause excess deaths (range, 0-47,200). Most influenza A(H3N2) seasons fell into severity categories 4 to 6 (23,000-45,000 all-cause excess deaths), whereas most A(H1N1) and B seasons were ranked in categories 1 to 3 (0-23,000 such deaths). CONCLUSIONS: From 1972 through 1992, influenza epidemics accounted for a total of 426,000 deaths in the United States, many times more than those associated with recent pandemics. The influenza epidemic severity index was useful for categorizing severity and provided improved seasonal estimates of the total number of influenza-related deaths.