How the flu spreads within cities

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The spread of respiratory diseases, such as the flu, is affected by temperature and school-aged children, according to Swiss researchers, who looked at how the flu spread within a city. The researchers looked at the genetic sequences of the flu virus and compared this to patient data, humidity, temperature, and whether schools were open or closed, to figure out how the spread changed throughout the season. They found the same flu was introduced to the city repeatedly, and the spread increased as temperature did, but humidity did not affect spread. They also found the elderly were infected within their own transmission networks, and school-aged children drove local spread more than preschool-aged children did. The researchers say this kind of data can help in understanding how respiratory viruses spread and could lead to more efficient public health interventions, including for the current COVID-19 pandemic while we await better data on how SARS-CoV-2 is spread.

Journal/conference: PLOS Pathogens

Link to research (DOI): 10.1371/journal.ppat.1008984

Organisation/s: ETH Zürich, Switzerland

Funder: This project was funded by the Swiss National Science Foundation (SNF; grant number CR32I3_166258), the Freiwillige Akademische Gesellschaft Basel and the Swiss Red Cross. AE received additional grants for this project: Freiwillige Akademische Gesellschaft Basel (with TS), Blutspende Zentrum SRK, and partially salary grant from SNSF Ambizione (PZ00P3_154709/1). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Media release

From: PLOS

How the flu virus spreads within cities

New insights into the local transmission of seasonal influenza may be valuable for planning interventions to combat the spread of respiratory diseases within cities, according to a study published November 19, 2020 in the open-access journal PLOS Pathogens by Nicola Müller and Tanja Stadler of ETH Zürich, and colleagues.

As shown with the current severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic, respiratory diseases can quickly spread around the world. While it can be important to understand how diseases spread globally, local spread is most often the main driver of novel infections of respiratory diseases such as SARS-CoV-2 or influenza. In the absence of deep knowledge about the important drivers of the local spread of SARS-CoV-2, governments around the world have resorted to closing down societies to reduce the burden of coronavirus disease (COVID-19). Similar to SARS-CoV-2, relatively little is known about the local spread of seasonal influenza, even though it annually infects a large portion of the global population.

In this new study, researchers addressed this question by collecting influenza samples and demographic information from 663 patients at two hospitals and various private practices in Basel, Switzerland, during the 2016/2017 influenza season. Genetic sequences of the influenza virus revealed that hundreds of introductions into the city drove the influenza season. Some evidence suggested that transmission dynamics in Basel may be associated with temperature. Moreover, the elderly were to a large extent infected within their own transmission network. In the remaining transmission network, additional analyses suggested that school-aged children likely played a more central role in driving the local spread of influenza than did preschool-aged children. According to the authors, the findings may lead to more streamlined, efficient public health interventions to reduce the great societal burden caused by the seasonal flu.

The authors conclude, “By allowing us to see how individual cases are connected, genetic sequences of influenza viruses isolated in Basel, Switzerland showed us that influenza viruses were repeatedly introduced into Basel from around the world within a single season”

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