PROVIDENCE, R.I. [Brown University] — Providing year-round access to RSV immunization would minimize the risk of large seasonal outbreaks across the nation, including in both urban and rural areas.

That’s according to a new study, published in Science Advances, which examined differences in viral spread in areas with different population density. The study showed that in urban areas, higher rates of interpersonal contact led to a higher proportion of hospitalizations in infants under age 1 and a more prolonged, lower-intensity RSV outbreak. Rural areas, on the other hand, saw shorter, spikier outbreaks.

The researchers used those insights to create mathematical models that ultimately showed providing year-round access to RSV immunization would minimize the risk of large seasonal outbreaks across the country, regardless of population density.

Understanding outbreaks of infectious disease requires exploring different variables, said study author Rachel Baker, an assistant professor of epidemiology and environment and society at Brown. 

“There's what happens within a country, and then what happens within a state, and what happens within a city, and it's important to examine all of those situations and the involved factors to really understand what's happening in terms of the outbreaks that we observe so that we can develop effective protections,” Baker said. 

Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract infections in infants, accounting for approximately 80,000 hospitalizations in the U.S. annually for children under age 5. In recent years, a new RSV vaccine for pregnant women and an antibody infusion for infants have been offered seasonally. To inform national immunization strategies, the researchers examined transmission patterns of RSV among children in rural and urban communities.

The study builds on previous work by Baker, who has a joint academic appointment with Brown’s School of Public Health and the Institute at Brown for Environment and Society. One prior study, focused on geography and climate, showed that RSV outbreak patterns range from a large biennial epidemic peak every two years in northern U.S. states to annual or year-round in southern parts of the country and the tropics. Another study predicted that easing pandemic-era disease transmission measures like masking would lead to RSV outbreak spikes — which is what happened in 2023 and 2024.

For the new analysis, the research team analyzed hospital records to learn how population density affected how the virus spread among young children. 

“We saw a higher proportion of infants under the age of 1 going to the hospital in urban areas compared to rural areas,” Baker said. “The urban environment seems to be amplifying the risk because kids there are more likely to get their first infection at an age when their respiratory system is less developed.”

The observed transmission patterns of RSV (year-round persistence in cities, with more concentrated intense outbreaks in the countryside) are consistent with those of the influenza virus, Baker said. Yet unlike flu, RSV is mainly transmitted among very young children.

“The hypothesis for why flu is more persistent year-round in urban environments is because people consistently have more contact with one another — in crowds, on public transportation, in communal spaces,” Baker said. “But very young children aren’t out socializing and mixing in the same way as adults.”

However, city families are more likely to use daycare, Baker said, which brings many young children in contact with one another and provides ample opportunities for viral transmission. 

 Immunization timing and RSV outbreaks

With that explanation for urban and rural differences, researchers led by study author Presley Kimball, a Ph.D. candidate in applied mathematics at Brown, modeled how RSV outbreak dynamics could change under two types of immunization schedules: seasonal and year-round.

They found that while any level of additional vaccine coverage lowers RSV hospitalizations, a seasonal implementation may actually increase the risk of a summertime RSV outbreak.

“A phased vaccine can lead to a net reduction in hospitalizations, which is a good thing, but we predict that we could also see infection rates creep up when those vaccine protections are not in place,” Baker said.

The timing of RSV varies quite a bit across the United States, Baker said, and therefore it doesn't make sense to think of the virus as having a fixed season with an optimal vaccination window.

“To fully minimize the risk of a large seasonal outbreak, our results suggest that access to RSV immunizations be provided year-round,” Baker said. “It seems from our analysis that it would be more risk-averse to offer the RSV vaccine at any time of the year to women who are within the recommended gestational window, and to young children in the appropriate age group, in any location.”

The study received funding from the National Institute of General Medical Sciences (R35GM156856) and the National Science Foundation (DMS-2038039).