Climate change and influenza: the likelihood of early and severe influenza seasons following warmer than average winters

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The 2012-13 influenza season had an unusually early and severe start in the US, succeeding the record mild 2011-12 influenza season, which occurred during the fourth warmest winter on record. Our analysis of climate and past US influenza epidemic seasons between 1997-98 to present indicates that warm winters tend to be followed by severe epidemics with early onset, and that these patterns are seen for both influenza A and B. We posit that fewer people are infected with influenza during warm winters, thereby leaving an unnaturally large fraction of susceptible individuals in the population going into the next season, which can lead to early and severe epidemics.

In the event of continued global warming, warm winters such as that of 2011-12 are expected to occur more frequently. Our results thus suggest that expedited manufacture and distribution of influenza vaccines after mild winters has the potential to mitigate the severity of future influenza epidemics.

Quantifying the transmissibility of human influenza and its seasonal variation in temperate regions

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Seasonal influenza has considerable impact around the world, both economically and in mortality among risk groups. The long term patterns of disease are hard to capture with simple models, while the interplay of epidemiological processes with antigenic evolution makes detailed modelling difficult and computationally intensive. We identify a number of characteristic features of flu incidence time series in temperate regions, including ranges of annual attack rates and outbreak durations. We construct pseudo-likelihoods to capture these characteristic features and examine the ability of a collection of simple models to reproduce them under seasonal variation in transmission. Results indicate that an age-structured model with non-random mixing and co-circulating strains are both required to match time series data. The extent of matching behaviour also serves to define informative ranges for parameters governing essential dynamics. Our work gives estimates of the seasonal peak basic reproduction, R0, in the range 1.7-2.1, with the degree of seasonal variation having limited impact of these estimates. We find that it is only really possible to estimate a lower bound on the degree of seasonal variation in influenza transmissibility, namely that transmissibility in the low transmission season may be only 5-10% less than the peak value. These results give some insight into the extent to which transmissibility of the H1N1pdm pandemic virus may increase in Northern Hemisphere temperate countries in winter 2009. We find that the timescale for waning of immunity to current circulating seasonal influenza strain is between 4 and 8 years, consistent with studies of the antigenic variation of influenza, and that inter-subtype cross-immunity is restricted to low levels.

Tracking the evolution and geographic spread of Influenza A

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The 2009 swine-origin strain of Influenza A H1N1 has spread to nearly all parts of the world, with 175 countries reporting confirmed cases thus far. Consistent with seasonal flu outbreaks, the current pandemic strain has shown rapid dispersal, with multiple examples of introduction into different geographic regions. Here we use an automated pipeline to collect data for analysis in the geospatial package GenGIS, which allows the geographic and temporal tracking of new sequence types and polymorphisms. Using this approach, we examine a pair of amino acid changes in the neuraminidase protein that are implicated in antibody recognition, and exhibit global dispersal with little or no geographic structure.

Next generation syndromic surveillance: molecular epidemiology, electronic health records and the pandemic Influenza A (H1N1) virus

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In the early phase of the 2009 A (H1N1) pandemic a marked increase in severity and a shift in the age distribution toward younger persons was found, with higher severity reported in patients with pre-existing medical conditions and pregnant women. Consistent with previous pandemics, the age and clinical history of the patients play a critical role in the morbidity and mortality associated with the pandemic virus. This is the first influenza pandemic in the information era, where enormous amounts of information will be available from the pathogen and the patient. Recent advances in molecular techniques have provided an enormous amount of information about pathogens in near real time and at relatively low cost. Electronic Health Records (EHRs) provide another enormously rich set of information about patients, which include patient preconditions, previous exposures, immunization history, presenting complaints, duration and severity of illness, treatment history, and geographic location. An infectious disease is a complex interplay between host and pathogen. The morbidity and mortality of a virus depend on the virus, the patient, and the environment. To evaluate and understand the severity of the pandemic virus and to identify the populations at risk of mild or severe, life-threatening illness, it is compulsory to integrate viral and patient information in a fast and accurate way. Both advances in biomedical informatics with the creation of EHRs and molecular techniques provide the framework to achieve these aims.

Transmission of influenza virus in temperate zones is predominantly by aerosol, in the tropics by contact

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Using the guinea pig model, we have previously shown that the aerosol transmission of a seasonal human influenza virus is blocked by humid (80% relative humidity) or warm (30°C) ambient conditions. In contrast, we found that transmission by a contact route proceeded at high efficiency despite increased temperature or humidity. Based on these findings, and the observed seasonal behavior of influenza viruses in various regions of the world, we hypothesize herein that the predominant mode of influenza virus transmission differs in temperate and tropical climates. Specifically, we predict that aerosol transmission predominates during the winter season in temperate regions, while contact is the major mode of spread in the tropics. With this idea in mind, possible explanations for the current summer-time spread of swine-origin influenza viruses are discussed.