Impact of School Cycles and Environmental Forcing on the Timing of Pandemic Influenza Activity in Mexican States, May-December 2009

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.

Identifying factors that may improve mechanistic forecasting models for influenza

“…Our results are broadly consistent with the work of [26] in which both school 240 closures and specific humidity were both considered. They estimated the impact of both 241 of these processes during the 2009 influenza pandemic in Mexican states, finding that 242 the the spatial structure of the pandemic could be explained by a combination of 243 factors: high specific humidity on some states driving activity, and school vacations 244 during the summer preventing further transmission.…”

“…Heuristically at least, this is supported by the radically 25 different evolutions of ILI profiles in tropical versus temperate zones [16]. Theoretically, 26 if at least some of the virus transmission is airborne [17], support for such a relationship 27 comes from the idea that the effective "lifetime" of the virus in a droplet is sensitive to 28 the local conditions within which it is embedded. Experimentally, it has been shown 29 that the transmission rate amongst guinea pig hosts increased as the relative humidity 30 decreased [18].…”

Model-based reconstruction of an epidemic using multiple datasets: understanding influenza A/H1N1 pandemic dynamics in Israel

“…Interface 13: 20160099 also be related to behavioural differences that render young children more likely to be infected when contacting an infectious individual. We obtained an estimate of R ¼ 1.4 at the beginning of the pandemic, which is in the range of other estimates from different parts of the world (1.2 R 2.2) [10,12,16,19,20,22,24,44]. In the beginning of July, owing to the combined effect of SVs and increasing AH levels, our estimate for R is just above 1 (figure 5b), which is in line with previous estimates made based on incidence of confirmed cases at the initial phase of the pandemic in Israel [3].…”

“…Evaluating the effect of SVs on the spread of influenza is difficult, as typically the influenza season does not overlap with the school summer vacation. The early initiation of the pandemic has enabled one to examine and confirm the curtailing impact of SVs on influenza transmission [10,12,[15][16][17][18][19][20]. In Israel, the early initiation resulted in the pandemic overlapping not only with the summer vacation during July and August, but also with the Jewish New Year holidays in October (figure 1a).…”

Does influenza drive absolute humidity?

“…Multiple lines of evidence suggest that absolute humidity and school terms affect the timing of influenza epidemics, although the contributions of these and other factors are still unresolved (1)(2)(3)(4)(5). In PNAS, Deyle et al (6) applied convergent crossmapping (CCM) to measure the impact of environmental variables on influenza activity.…”