Regional, altitudinal, and age-specific heterogeneity of bronchiolitis hospitalization seasonality in Ecuador, 2007-2024, and implications for RSV immunoprophylaxis timing.

BACKGROUND

Acute bronchiolitis, mainly attributed to respiratory syncytial virus (RSV), is the leading cause of infant hospitalization for lower respiratory tract disease worldwide. Nirsevimab and maternal RSVpreF (Abrysvo) confer about 6 months of protection, so timing relative to the season affects their impact. Ecuador spans the equator from sea level to valleys above 4,000 m and is considering introducing these products, yet lacks subnational evidence on the seasonality of bronchiolitis, altitudinal patterning, or post-pandemic stability.

METHODS

We conducted a national ecological time-series analysis of hospital discharges from the Ecuadorian Statistics Institute, 2007-2024 (19,788,097 discharges; 204 of 224 cantons had complete data and formed the analytic panel). The primary outcome was acute bronchiolitis (ICD-10 code J21) in children younger than 12 months (n = 23,190 nationally; 22,724 in the panel), with secondary outcomes across three RSV-relevant age bands, ICD-10 code J21 under 5 years, and two RSV-specific ICD-10 codes (J12.1 and J20.5; 2018-2024). K35 (appendicitis) was a negative control. Analyses combined circular seasonality metrics with block-bootstrap confidence intervals (CIs), year-by-year disaggregation, and complementary sensitivity analyses [Moving Epidemics Method (MEM), Bai-Perron breaks, and lagged climate correlations].

RESULTS

The weighted circular peak month of infant bronchiolitis hospitalizations shifted after the pandemic in Costa (+2.16 months), Sierra (+1.22), and Amazonía (+1.91), attenuating with altitude from +2.19 months below 500 m to +0.88 at 1,500-3,000 m. Costa was stable across 2022-2024; Sierra and, more so, Amazonía varied widely year to year (Amazonía peak ranged March-July, n = 37-42/year), so the pooled estimate should be read with caution. The composite RSV-coded outcome tracked ICD-10 code J21's timing closely in Costa (0.15-month difference) and Sierra (0.40), supporting ICD-10 code J21 as a reasonable RSV proxy in both, and the three regional peaks now span 2 months.

CONCLUSION

Bronchiolitis seasonality in Ecuador changed after the pandemic, most clearly and stably in Costa; Sierra showed a transient 2022 perturbation with partial reversion by 2023-2024, and Amazonía's estimates remained unstable and underpowered. As region-tailored estimates are less stable in Sierra and Amazonía than in Costa, a single national window (nirsevimab February-April; maternal RSVpreF for pregnancies expected to deliver January-May) is a more defensible starting point, pending confirmation from further seasons and, ideally, laboratory-confirmed surveillance.