Your city structure may determine the length and intensity of your local flu season, according research published in October’s Science Magazine by OSU Population Biologist Ben Dalziel, assistant professor of Integrative Biology and Mathematics. This research explores the connection between city structure, climate, and flu epidemics.
As a population biologist, Dalziel’s research often explores topics that are “too big for the lab,” so he analyzes pre-existing data sets. He and his collaborators compared six years of data collected from influenza related hospital visits in 603 population centers throughout the USA, as designated by the post office’s three-digit zip codes. This data was compiled and anonymized so researchers didn’t have access to any personal details about patients. Although most people don’t seek medical attention for the flu, Dalziel said the data set offers a consistent way to compare flu patterns to among cities.
Climate factors like humidity were already known to hinder or assist the spread of influenza. The virus thrives in cool, dry conditions, allowing it to survive longer outside the host’s body, but Dalziel’s research discovered another layer to the infection pattern.
After adjusting for climate conditions, some cities have a longer, but less intense flu season year after year, while other cities have shorter, but more intense flu seasons. An intense flu season refers to one where the majority of people catch the flu within a relatively short period of time as observed in the smaller cities. On the other hand, the larger cities with higher population densities had longer, but less intense flu seasons.
The connection between city structure and flu epidemic intensity was previously unexplored. Dalziel first became interested in researching this while working on measles-related post-doctoral research with one of his “absolute heroes” Bryan T. Grenfell of Princeton University. After forming his own group at Oregon State, he collaborated again with his mentor and other researchers at the University of Cambridge, Pennsylvania State University, the Fogarty International Center, and the National Institutes of Health.
After much debate and analysis, the team identified commonalities among the cities with a longer flu season. These cities are larger metropolises where residents frequently travel in between sprawling residential areas and centralized areas where they work and socialize. The travel typically happens in specific organized times as observed during rush hour in many cities. In these cities, the flu spreads from person to person earlier in the year even when the climate conditions are not yet optimal for the virus.
He compared the larger, densely populated cities to a small elevator.
“If you are close to someone in a small elevator and someone with the flu sneezes, the humidity doesn’t matter since the virus goes directly from one person to another,” said Dalziel.
He then compared the smaller cities to a larger room-sized elevator where the virus would need more optimal climate conditions to survive outside the body long enough to reach the next person.
More research is needed to determine the impact of the concentrated flu season in comparison to more spread-out flu seasons. The bio-diversity in larger cities may strengthen the resident’s immune responses to certain diseases, however, these ideas are currently speculation requiring more research. Dalziel noted the possibility that early in the season the weather conditions are suboptimal for the flu virus to thrive, early exposure may improve immune response among the city residents.
Dalziel compared the longer flu season observed in dense population centers to forest fire season on the west coast. “It is kind of like the flu is fire and humans are fuel for the flu virus.” He explained, “If you had smoldering fires early that use up the ‘fuel’ like a controlled burn before the prime fire reason resulting in fewer less intense fires.”
Conversely, a smaller city’s season is more intense because the majority of the people catch the flu within a shorter time frame.
He noted similar principles may apply to other infectious disease epidemics. However, more research is required to further investigate the relationship among population patterns, climate, and disease.
Dalziel said he is especially interested in conversations with city planners, exploring ways to structure cities to be more resilient against epidemics like the flu. He explained that some city planners already use modeling to predict how city structure or design may impact their carbon footprint.
The largest cities may in the future serve as “sentinel cities” that can be used to follow flu evolution and more accurately predict which strains may later occur in other cities. In time, information gained from tracking sentinel cities may contribute to better preparation including improved vaccines or increased hospital staffing during flu seasons.
The next steps involve expanding the research to a more global level. For example, Dalziel wants to look into whether similar patterns apply to tropical climates where the flu seems to thrive in warm, humid conditions in paradox to the cool, dry conditions that seem optimal for flu epidemics. Dalziel is curious whether this is connected to urbanization and population patterns, or if something else causes this difference.
“This is potentially significant because urbanization and climate interacting are what determine flu transmission, but are also huge factors in global change.” Said Dalziel, “Going forward, we are going to try to expand the analysis to understand how present and future urbanization patterns across the globe are influencing the circulation of flu.”
By Samantha Sied