Giuseppe
Emeritus
NEJM -- The Signature Features of Influenza Pandemics -- Implications for Policy (Extract)
The Signature Features of Influenza Pandemics ? Implications for Policy
Mark A. Miller, M.D., Cecile Viboud, Ph.D., Marta Balinska, Ph.D., and Lone Simonsen, Ph.D.
Vast amounts of time and resources are being invested in planning for the next influenza pandemic, and one may indeed have already begun. Data from past pandemics can provide useful insights for current and future planning. Having conducted archeo-epidemiologic research, we can clarify certain "signature features" of three previous influenza pandemics ? A/H1N1 from 1918 through 1919, A/H2N2 from 1957 through 1963, and A/H3N2 from 1968 through 1970 ? that should inform both national plans for pandemic preparedness and required international collaborations.
Past pandemics were characterized by a shift in the virus subtype, shifts of the highest death rates to younger populations, successive pandemic waves, higher transmissibility than that of seasonal influenza, and differences in impact in different geographic regions. Although influenza pandemics are classically defined by the first of these features, the other four characteristics are frequently not considered in response plans.
Yet the second feature, the shift in mortality toward younger age groups, was the most striking characteristic of the 20th-century pandemics.1,2 Exposure to influenza A/H1 subtypes before 1873 may have offered some protection to adults over 45 years of age during the pandemic of 1918 and 1919. A similar mechanism of antigen recycling might explain the partial protection against influenza-related death that was observed among people over 77 years of age during the 1968?1970 pandemic ? a possibility supported by the prepandemic presence of antibodies to H3, which were isolated in people born before 1892.1 Another possible mechanism is immune potentiation, leading to an increased likelihood of lethal outcomes after influenza infection in specific age groups. Still other hypotheses include the possibility of bacterial superinfection due to asymmetric carriage rates, given that higher rates were found among young people in 1918 and 1919.1,2 Although the elderly frequently have the highest death rates during seasonal epidemics, their relative sparing during pandemics has not been generally appreciated. Advance knowledge of which subpopulations are most likely to be at increased risk for death can shape the optimization of control strategies.
The third feature, a pattern of multiple waves, characterized all three 20th-century pandemics, each of which caused increased mortality for 2 to 5 years (see chart).1 The lethal wave in the autumn of 1918 was preceded by a first wave in the summer that led to substantial morbidity but relatively low mortality in both the United States and Europe. Recent studies suggest that these early mild outbreaks partially immunized the population, decreasing the mortality impact of the main pandemic wave in the fall of 1918.2 In the United States, the 1957 influenza A/H2 pandemic had three waves in the United States, with notable excess mortality in the nonsuccessive winter seasons of 1959 and 1962 ? the latter being 5 years after the initial emergence of the pandemic strain.1 From 1968 through 1970, Eurasia had a mild first influenza season, with the full effects on morbidity and mortality occurring in the second season of pandemic-virus circulation. The reasons for multiple waves of varying impact are not precisely understood, but they probably include adaptation of the virus to its new host, demographic or geographic variation, seasonality, and the overall immunity of the population.1,2 The occurrence of multiple waves potentially provides time for health authorities to implement control strategies for successive waves.
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<cite cite="http://content.nejm.org/cgi/content/full/NEJMp0903906?query=TOC">NEJM -- The Signature Features of Influenza Pandemics -- Implications for Policy</cite>Mark A. Miller, M.D., Cecile Viboud, Ph.D., Marta Balinska, Ph.D., and Lone Simonsen, Ph.D.
Vast amounts of time and resources are being invested in planning for the next influenza pandemic, and one may indeed have already begun. Data from past pandemics can provide useful insights for current and future planning. Having conducted archeo-epidemiologic research, we can clarify certain "signature features" of three previous influenza pandemics ? A/H1N1 from 1918 through 1919, A/H2N2 from 1957 through 1963, and A/H3N2 from 1968 through 1970 ? that should inform both national plans for pandemic preparedness and required international collaborations.
Past pandemics were characterized by a shift in the virus subtype, shifts of the highest death rates to younger populations, successive pandemic waves, higher transmissibility than that of seasonal influenza, and differences in impact in different geographic regions. Although influenza pandemics are classically defined by the first of these features, the other four characteristics are frequently not considered in response plans.
Yet the second feature, the shift in mortality toward younger age groups, was the most striking characteristic of the 20th-century pandemics.1,2 Exposure to influenza A/H1 subtypes before 1873 may have offered some protection to adults over 45 years of age during the pandemic of 1918 and 1919. A similar mechanism of antigen recycling might explain the partial protection against influenza-related death that was observed among people over 77 years of age during the 1968?1970 pandemic ? a possibility supported by the prepandemic presence of antibodies to H3, which were isolated in people born before 1892.1 Another possible mechanism is immune potentiation, leading to an increased likelihood of lethal outcomes after influenza infection in specific age groups. Still other hypotheses include the possibility of bacterial superinfection due to asymmetric carriage rates, given that higher rates were found among young people in 1918 and 1919.1,2 Although the elderly frequently have the highest death rates during seasonal epidemics, their relative sparing during pandemics has not been generally appreciated. Advance knowledge of which subpopulations are most likely to be at increased risk for death can shape the optimization of control strategies.
The third feature, a pattern of multiple waves, characterized all three 20th-century pandemics, each of which caused increased mortality for 2 to 5 years (see chart).1 The lethal wave in the autumn of 1918 was preceded by a first wave in the summer that led to substantial morbidity but relatively low mortality in both the United States and Europe. Recent studies suggest that these early mild outbreaks partially immunized the population, decreasing the mortality impact of the main pandemic wave in the fall of 1918.2 In the United States, the 1957 influenza A/H2 pandemic had three waves in the United States, with notable excess mortality in the nonsuccessive winter seasons of 1959 and 1962 ? the latter being 5 years after the initial emergence of the pandemic strain.1 From 1968 through 1970, Eurasia had a mild first influenza season, with the full effects on morbidity and mortality occurring in the second season of pandemic-virus circulation. The reasons for multiple waves of varying impact are not precisely understood, but they probably include adaptation of the virus to its new host, demographic or geographic variation, seasonality, and the overall immunity of the population.1,2 The occurrence of multiple waves potentially provides time for health authorities to implement control strategies for successive waves.
(...)
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