Influenza outbreaks in the United States tend to be concentrated and intense in small cities and more evenly spread throughout the season in large cities, results of a recent study show.
Swings in humidity further intensified the influenza spikes in small cities, but didn’t seem to have as much of an effect in large cities, the results suggest.
These findings help explain differences in influenza transmission patterns between cities that have similar climates and virus epidemiology, according to researcher Benjamin D. Dalziel, PhD, of the departments of integrative biology and mathematics at Oregon State University in Corvallis.
“City size and structure can play a role in determining how other factors such as climate affect and influence transmission,” Dr. Dalziel said in a press conference.
“Our results show how metropolises play a disproportionately important role in this process, as epidemic foci, and as potential sentinel hubs, where epidemiological observatories could integrate local strain dynamics to predict larger-scale patterns. As the growth and form of cities affect their function as climate-driven incubators of infectious disease, it may be possible to design smarter cities that better control epidemics in the face of accelerating global change,” the researchers wrote in their study.
Dr. Dalziel and his coauthors analyzed the weekly incidence of influenza-like illness across 603 U.S. ZIP codes using data obtained from medical claims from 2002 to 2008. They used epidemic intensity as a summary statistic to compare cities. By this variable, low epidemic intensity indicated a diffuse spread evenly across weeks of the flu season, whereas high epidemic intensity indicated intensively focused outbreaks on particular weeks.
In small cities, epidemics were more intensely focused on shorter periods at the peak of flu season, they found. In large cities, incidence was more diffuse, according to results published in.
Patterns of where people live and work in a city may account for the more diffuse and prolonged outbreaks seen in large cities, the authors wrote. Large cities have organized population movement patterns and crowding. In more highly established work locations, for example, the population density is pronounced during the day.
“We found the structure makes a difference for how the flu spreads at different times of year,” Dr. Dalziel said of the study, which used U.S. Census data to evaluate spatial population distributions. “In large cities with more highly organized patterns, conditions play a relatively smaller role in flu transmission.”
Humidity’s lower impact on outbreaks in large cities might also be explained by population effects: “If an infected person is sitting beside you, it matters less what the specific humidity is,” Dr. Dalziel said, adding that the proximity helps the virus find hosts even when climatic conditions are not at their most favorable.
The study findings may have implications for health care resources in small cities, which could be strained by intense outbreaks, said coinvestigator Cecile Viboud, PhD, of the Division of International Epidemiology and Population Studies, Fogarty International Center, National Institutes of Health, Bethesda, Md.
Intense outbreaks could overload the health care system, making it challenging to respond, especially around the peak of the epidemic. Pressure on the health care system may be less intense in cities such as Miami or New York, where flu epidemics are more diffuse and spread out during the year, she said.
Variations in vaccination coverage were not associated with variations in epidemic intensity at the state level. However, the data period that was analyzed ended in 2008, a time when flu vaccination rates were much lower than they are today, according to Dr. Viboud.
“It would be important to revisit the effect of city structure and humidity on flu transmission in a high vaccination regime in more recent years, especially if there is a lot of interest in developing broadly cross-protective flu vaccines, which might become available in the market in the future,” she said.
The researchers declared no competing interests related to their research, which was supported by a grant from the Bill & Melinda Gates Foundation, the RAPIDD program of the Science and Technology Directorate Department of Homeland Security, and the Fogarty International Center, National Institutes of Health.
SOURCE: Dalziel BD et al. .