Paediatric RSV infection risk rises, drops depending on weather, air quality

Meteorologic factors appear to influence the risk of respiratory syncytial virus (RSV) infection in children, such that the risk is exacerbated when ambient temperature is low and absolute humidity is high, but the risk is decreased in the presence of increased air pollutant concentrations, according to a study in Singapore.

“We observed a long-term increase in the daily number of RSV infections from 2009 to 2019 (p<0.001) [and] found evidence of cyclical variations in the pattern of RSV infections at the 4-, 6-, and 12-monthly intervals, with the highest peaks falling between June and August,” the investigators reported.

A complex, nonlinear cubic relationship between maximum temperature and RSV infection risk emerged. Specifically, the risk increase was significant on days that fell into the 10th percentile of daily maximum temperature (29.7 °C [85.46 °F]; relative risk [RR], 1.168, 95 percent confidence interval [CI], 1.068–1.277) and into the 90th percentile of absolute humidity (RR, 1.170, 95 percent CI, 1.102–1.242). [Sci Rep 2023;13:1001]

Moreover, RSV infection risk was low on days where the levels of particulate matter (PM) with aerodynamic diameter of 2.5 µm (PM_2.5) or 10 µm (PM₁₀), carbon monoxide (CO), and sulfur dioxide (SO₂) were increased.

There was a 16.3-percent cumulative decrease in the risk of RSV infections at ambient PM_2.5 concentration of 26.2 μg/m³ (RR, 0.837, 95 percent CI, 0.794–0.884), 14.8-percent cumulative decrease at ambient PM₁₀  concentration of 40.4 μg/m³ (RR, 0.852, 95 percent CI, 0.807–0.900), and a 12.2-percent cumulative decrease at the 90th percentile of SO₂ concentration (RR, 0.878, 95 percent CI, 0.807–0.956). The risk of RSV infections was also down when CO concentrations were high (RR_{90th percentile}, 0.863, 95 percent CI, 0.801–0.929).

Mechanisms underlying the associations

The association of RSV infections with daily maximum temperature and absolute humidity found in the present study was consistent with previous studies conducted in China, Brazil, and Hong Kong. [Environ Sci Pollut Res 2016;23:20178-20185; Biosci Rep 2020;40:1-10; Thorax 2021;76:360-369].

“Lower temperatures enhance virus viability in the environment, thus leading to an increased risk of infection. The lower heat makes the RSV lipid envelop more stable in the secretions through which it is transmitted… [and] also enhance virus susceptibility by triggering changes in human physiology,” the investigators explained.

On the other hand, higher humidity may facilitate RSV infections by increasing virus survivability, they added. “RSV is suspected to be transmitted via the aerosols, and earlier studies showed higher RSV stability in large particles aerosols at higher humidity levels.” [Environ Res 2017;158:188-193; Sci Rep 2020;10:1-7; Epidemiol Infect 2007;135:1077-1090]

With respect to the effects of particulate matter, the investigators pointed out that exposure to higher levels of pollutant in the air can aid infections with RSV by impairing lung function. However, the National Environment Agency in Singapore recommends vulnerable people, including children, to minimize strenuous outdoor activities when PM_2.5 concentrations exceed 55 μg/m³ or when the Pollutant Standards Index value exceeds 100. [Ital J Pediatr 2013;39:1-6; Pediatrics 2005;116:e235-e240; https://www.haze.gov.sg/]

It is possible that parents and their children might have chosen to increase their dwelling times indoors in their homes when PM levels were elevated, which explains the inverse relationship between RSV infection risk and air pollution levels observed in the study, according to the investigators.

“RSV is an important cause of respiratory illness… The health burden of lower respiratory infections is particularly high in South-East Asia where the incidence rate is estimated at 17.8 per 1,000 children per year,” they said. [Lancet 2013;381:1380-1390]

Despite the need for more studies to understand the mechanisms behind the air quality-RSV relationship, the meteorologic factors identified in the present study, “together with the predictable seasonality of RSV infections, can inform the timing of mitigation measures aimed at reducing transmission,” the investigators said.

For the study, laboratory-confirmed reports of RSV infections in children no more than 5 years old from the largest public hospital specializing in paediatric healthcare in Singapore were obtained. The independent cumulative effects of air quality and meteorologic factors on RSV infection risk were then evaluated using the Distributed Lag Non-Linear Model (DLNM) framework in negative binomial models, adjusted for long-term trend, seasonality, and changes in the diagnostic systems. A total of 15,715 laboratory-confirmed RSV reports from 2009 to 2019 were included.