Tweet
Citation: Tamerius J, Viboud C, Shaman J, Chowell G (2015) Impact of School Cycles and Environmental Forcing on the Timing of Pandemic Influenza Activity in Mexican States, May-December 2009. PLoS Comput Biol 11(8): e1004337. doi:10.1371/journal.pcbi.1004337
Abstract
While a relationship between environmental forcing and influenza transmission has been established in inter-pandemic seasons, the drivers of pandemic influenza remain debated. In particular, school effects may predominate in pandemic seasons marked by an atypical concentration of cases among children. For the 2009 A/H1N1 pandemic, Mexico is a particularly interesting case study due to its broad geographic extent encompassing temperate and tropical regions, well-documented regional variation in the occurrence of pandemic outbreaks, and coincidence of several school breaks during the pandemic period. Here we fit a series of transmission models to daily laboratory-confirmed influenza data in 32 Mexican states using MCMC approaches, considering a meta-population framework or the absence of spatial coupling between states. We use these models to explore the effect of environmental, school?related and travel factors on the generation of spatially-heterogeneous pandemic waves. We find that the spatial structure of the pandemic is best understood by the interplay between regional differences in specific humidity (explaining the occurrence of pandemic activity towards the end of the school term in late May-June 2009 in more humid southeastern states), school vacations (preventing influenza transmission during July-August in all states), and regional differences in residual susceptibility (resulting in large outbreaks in early fall 2009 in central and northern Mexico that had yet to experience fully-developed outbreaks). Our results are in line with the concept that very high levels of specific humidity, as present during summer in southeastern Mexico, favor influenza transmission, and that school cycles are a strong determinant of pandemic wave timing.
Author Summary
An influenza pandemic virus emerged in North America in 2009. Although the virus spread worldwide within months of its emergence, the timing of peak pandemic activity varied by nearly a year across global populations. In fact, the most intense period of pandemic activity occurred earlier in temperate countries of the southern hemisphere (e.g., South Africa, Argentina and Australia) than in countries that reported the first infections (Mexico and the US). This indicates that the timing of peak pandemic activity did not directly align with the arrival of the virus in a location, suggesting the influence of social and environmental factors on pandemic influenza transmission. In this study we examined two large pandemic outbreaks (?waves?) that occurred in summer and fall of 2009 in Mexico. The summer wave occurred in the tropical southeastern states of Mexico, whereas the larger fall wave occurred in the central and northern states. We find that the distinct pandemic waves were likely caused by complex interactions between regional differences in specific humidity, population level susceptibility, and school term cycles. Improving our understanding of the factors modulating regional transmission patterns will improve prediction of the spatial and temporal progression of future pandemics.
full article
Abstract
While a relationship between environmental forcing and influenza transmission has been established in inter-pandemic seasons, the drivers of pandemic influenza remain debated. In particular, school effects may predominate in pandemic seasons marked by an atypical concentration of cases among children. For the 2009 A/H1N1 pandemic, Mexico is a particularly interesting case study due to its broad geographic extent encompassing temperate and tropical regions, well-documented regional variation in the occurrence of pandemic outbreaks, and coincidence of several school breaks during the pandemic period. Here we fit a series of transmission models to daily laboratory-confirmed influenza data in 32 Mexican states using MCMC approaches, considering a meta-population framework or the absence of spatial coupling between states. We use these models to explore the effect of environmental, school?related and travel factors on the generation of spatially-heterogeneous pandemic waves. We find that the spatial structure of the pandemic is best understood by the interplay between regional differences in specific humidity (explaining the occurrence of pandemic activity towards the end of the school term in late May-June 2009 in more humid southeastern states), school vacations (preventing influenza transmission during July-August in all states), and regional differences in residual susceptibility (resulting in large outbreaks in early fall 2009 in central and northern Mexico that had yet to experience fully-developed outbreaks). Our results are in line with the concept that very high levels of specific humidity, as present during summer in southeastern Mexico, favor influenza transmission, and that school cycles are a strong determinant of pandemic wave timing.
Author Summary
An influenza pandemic virus emerged in North America in 2009. Although the virus spread worldwide within months of its emergence, the timing of peak pandemic activity varied by nearly a year across global populations. In fact, the most intense period of pandemic activity occurred earlier in temperate countries of the southern hemisphere (e.g., South Africa, Argentina and Australia) than in countries that reported the first infections (Mexico and the US). This indicates that the timing of peak pandemic activity did not directly align with the arrival of the virus in a location, suggesting the influence of social and environmental factors on pandemic influenza transmission. In this study we examined two large pandemic outbreaks (?waves?) that occurred in summer and fall of 2009 in Mexico. The summer wave occurred in the tropical southeastern states of Mexico, whereas the larger fall wave occurred in the central and northern states. We find that the distinct pandemic waves were likely caused by complex interactions between regional differences in specific humidity, population level susceptibility, and school term cycles. Improving our understanding of the factors modulating regional transmission patterns will improve prediction of the spatial and temporal progression of future pandemics.
full article