Post-COVID-19 Epidemiology of Bronchiolitis: We Are (Not) Returning to the Past

To the Editors: In 2024, the Pediatric Infectious Disease Journal presented the findings of an Italian study analyzing how the coronavirus disease 2019 (COVID-19) pandemic influenced viral bronchiolitis’ characteristics.1 The authors recommended that hospitals maintain a high level of surveillance of epidemiological changes to ensure a prompt response and proper allocation of resources. Considering the profound impact of bronchiolitis epidemiology modifications in our setting,2,3 these observations prompted us to monitor bronchiolitis admissions during the 2023–2024 epidemic season to continue describing the epidemiological trends of bronchiolitis that we have already reported since before the start of the pandemic.2,3 For this purpose, we included all children 0–2 years with an International Classification of Diseases, 9th revision diagnosis of bronchiolitis admitted to Istituto Gaslini, Italy, from 1 October 2023 to 30 April 2024 and compared with the previous 6 seasons. In our region, bronchiolitis typically followed a seasonal pattern in the prepandemic era, starting in December, peaking in January and February, and decreasing during spring.2 These trends have been drastically disrupted by the COVID-19 pandemic and related preventive interventions resulting in the disappearance of bronchiolitis during the year 2020–2021. In the 2021–2022 season, after the relaxation of pandemic-related measures, a huge and significantly early peak was detected, followed by a gradual shift of the curve during the 2022–2023 season with an always early and increased although less anticipated peak. In the last season, the hospitalizations distribution seemed to be realigning to prepandemic years starting in late November, peaking in December and January, and ending during spring, thus suggesting a possible “return to the past” (Fig. 1A).FIGURE 1.: Trend of bronchiolitis admissions during the epidemic seasons (1 October–30 April) of the 2017–2024 period at Gaslini Institute, Genoa, Italy (A) with the corresponding absolute numbers, the annual rate of bronchiolitis admissions per 1000 admissions and the prevalence rate of RSV per 100 bronchiolitis tested (B). RSV indicates respiratory syncytial virus.However, total admissions remained largely over 300, three times more than in prepandemic years. The admission rate for bronchiolitis increased from 7.7 to 20 × 1000 (Fig. 1B), in the absence of rising regional birth rates,4 with the incidence ratio being 2.7 times (95% confidence interval: 2.1–3.3, P < 0.001) higher in the 2023–2024 season than in the 2017–2018 season. Although the temporal distribution of admissions seems to resemble the trend we were used to, the impact of bronchiolitis appears to be persistently enhanced. The reason for this phenomenon is not easily understandable. A partial explanation could be found in the increased prevalence of respiratory syncytial virus, which in our study went persistently from being the cause of 50–60% of bronchiolitis in the prepandemic period to 70–80% in the postpandemic era. This could reflect the reduced population exposure to respiratory viruses during the pandemic, resulting in higher vulnerability among infants and susceptibility in the postpandemic seasons. A more sustained circulation of respiratory syncytial virus could also support the supposed increased severity of bronchiolitis in postpandemic years.5 In conclusion, although a transition to prepandemic epidemiology is beginning to be observed, in our setting the bronchiolitis burden remained overwhelming during the 2023–2024 season, in the absence of the universal prevention measures already available in other countries. Close surveillance and monitoring of the impact of bronchiolitis remains crucial to fully understand the long-term effects of the COVID-19 pandemic and to assess the hopefully significant impact of universal prophylaxis.