Javier Dávila-Torres1, Gerardo Chowell2, Víctor H Borja-Aburto1, Cécile Viboud3, Concepción Grajalez-Muñiz4, Mark A Miller3. 1. Direction of Medical Benefits, Mexican Institute of Social Security, Mexico City, Mexico. 2. Division of Epidemiology and Population Studies, Fogarty International Center, National Institutes of Health, Bethesda, Maryland, USA; Mathematical, Computational and Modeling Sciences Center, School of Human Evolution and Social Change, Arizona State University, Tempe, Arizona, USA. Electronic address: gchowell@asu.edu. 3. Division of Epidemiology and Population Studies, Fogarty International Center, National Institutes of Health, Bethesda, Maryland, USA. 4. Coordination of Epidemiological Surveillance and Contingency Support, Mexican Institute of Social Security, Mexico, City, Mexico.
Abstract
BACKGROUND AND AIMS: A recrudescent wave of pandemic influenza A/H1N1 affected Mexico during the winter of 2013-2014 following a mild 2012-2013 A/H3N2 influenza season. METHODS: We compared the demographic and geographic characteristics of hospitalizations and inpatient deaths for severe acute respiratory infection (SARI) and laboratory-confirmed influenza during the 2013-2014 influenza season compared to previous influenza seasons, based on a large prospective surveillance system maintained by the Mexican Social Security health care system. RESULTS: A total of 14,236 SARI hospitalizations and 1,163 inpatient deaths (8.2%) were reported between October 1, 2013 and March 31, 2014. Rates of laboratory-confirmed A/H1N1 hospitalizations and deaths were significantly higher among individuals aged 30-59 years and lower among younger age groups for the 2013-2014 A/H1N1 season compared to the previous A/H1N1 season in 2011-2012 (χ(2) test, p <0.001). The reproduction number for the winter 2013-2014 influenza season in central Mexico was estimated at 1.3-1.4, in line with that reported for the 2011-2012 A/H1N1 season but lower than during the initial waves of pandemic A/H1N1 activity in 2009. CONCLUSIONS: We documented a substantial increase in the number of A/H1N1-related hospitalizations and deaths during the period from October 2013-March 2014 in Mexico and a proportionate shift of severe disease to middle-aged adults, relative to the preceding A/H1N1 2011-2012 season. In the absence of clear antigenic drift in globally circulating A/H1N1 viruses in the post-2009 pandemic period, the gradual change in the age distribution of A/H1N1 infections observed in Mexico suggests a slow build-up of immunity among younger populations, reminiscent of the age profile of past pandemics.
BACKGROUND AND AIMS: A recrudescent wave of pandemic influenza A/H1N1 affected Mexico during the winter of 2013-2014 following a mild 2012-2013 A/H3N2 influenza season. METHODS: We compared the demographic and geographic characteristics of hospitalizations and inpatient deaths for severe acute respiratory infection (SARI) and laboratory-confirmed influenza during the 2013-2014 influenza season compared to previous influenza seasons, based on a large prospective surveillance system maintained by the Mexican Social Security health care system. RESULTS: A total of 14,236 SARI hospitalizations and 1,163 inpatient deaths (8.2%) were reported between October 1, 2013 and March 31, 2014. Rates of laboratory-confirmed A/H1N1 hospitalizations and deaths were significantly higher among individuals aged 30-59 years and lower among younger age groups for the 2013-2014 A/H1N1 season compared to the previous A/H1N1 season in 2011-2012 (χ(2) test, p <0.001). The reproduction number for the winter 2013-2014 influenza season in central Mexico was estimated at 1.3-1.4, in line with that reported for the 2011-2012 A/H1N1 season but lower than during the initial waves of pandemic A/H1N1 activity in 2009. CONCLUSIONS: We documented a substantial increase in the number of A/H1N1-related hospitalizations and deaths during the period from October 2013-March 2014 in Mexico and a proportionate shift of severe disease to middle-aged adults, relative to the preceding A/H1N1 2011-2012 season. In the absence of clear antigenic drift in globally circulating A/H1N1 viruses in the post-2009 pandemic period, the gradual change in the age distribution of A/H1N1 infections observed in Mexico suggests a slow build-up of immunity among younger populations, reminiscent of the age profile of past pandemics.
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