In this study, a history of BA, wheeze, BA exacerbation, combined history and exacerbation of BA, and systemic steroid use were associated with RSV co-infection with other respiratory pathogens. In contrast, among patients with hMPV, no differences were observed between single infection and co-infection.

RSV co-infection was more prevalent in age under 6 years groups, and co-infection with coronavirus, parainfluenza virus, adenovirus, and rhinovirus/enterovirus was associated with increased risk of BA exacerbation compared to that with RSV single infection. Furthermore, age under 6 years, testing period, and RSV co-infection were identified as independent risk factors for BA exacerbation, as assessed by multiple logistic regression analysis.

We found that RSV co-infection was associated with a higher prevalence of BA history, wheeze, BA exacerbation, combined history of BA and exacerbation, and systemic steroid use, all of which are related to BA, compared to that via single infection. RSV is a well-recognized trigger for BA exacerbation [1, 4] because of its involvement in lower respiratory tract infections, such as bronchiolitis, bronchitis, and pneumonia [3, 8, 25, 26]. Previous studies have also shown that RSV co-infection with rhinovirus or parainfluenza virus is associated with an increased risk of lower respiratory tract infections, although these studies involved smaller sample sizes and fewer respiratory pathogens than that in ours [27]. In addition, RSV targets bronchial epithelial cells, leading to disruption of the epithelial barrier, dysregulated immune responses, excessive mucin production, and impaired ciliary function [25, 28], thereby contributing to wheeze, BA exacerbation, and systemic steroid use, as observed in our study (Table 1). Furthermore, coronavirus, parainfluenza virus, adenovirus, and rhinovirus/enterovirus were commonly detected in patients with BA exacerbations [29,30,31]. Notably, RSV co-infection with rhinovirus was associated with a markedly higher prevalence of BA, with an odds ratio of 5.47 [32]. These findings suggest a possible synergistic interaction between RSV and these respiratory viruses, potentially contributing to BA exacerbation and supporting the associations observed in our study (Fig. 1). Consistent with our findings, previous study reported that RSV co-infection with other respiratory viruses is not linked to severe outcomes such as ICU admission, use of mechanical ventilation, or death [19]. We also examined age-related patterns of RSV co-infection (Table 3). Earlier studies have shown that RSV single infection are more common in children than in adults [33]. Our previous work also demonstrated that SARS-CoV-2 and other respiratory pathogens was most frequent in the youngest age group (0–4 years) [12]. Additionally, other reports have documented that respiratory viral co-infection are especially prevalent in children under 5 years of age [34]. Our findings support this age-based trend in RSV co-infection. This vulnerability may be explained by increased exposure to respiratory pathogens in communal environments such as nurseries and kindergartens, as well as biologically immature immune systems and limited adherence to preventive behaviors such as consistent hand hygiene and mask use [12].

A multiple logistic regression analysis identified age, testing period, and RSV co-infection as independent risk factors for BA exacerbation (Fig. 2).

An RSV vaccine has been shown to prevent RSV-related lower respiratory tract disease in adults over 60 years of age for at least three seasons [35, 36]. Additionally, an RSV vaccine administered to pregnant women between 24 and 36 weeks of gestation has demonstrated protective efficacy in their new-borns and infants, preventing severe RSV-related lower respiratory tract disease during the first six months of life [37]. Furthermore, prophylactic administration of monoclonal antibody targeting RSV has proven effective in reducing RSV-related symptoms and hospitalizations in high-risk new-borns, infants, and children under 24 months of age [38]. These findings suggest that RSV-related lower respiratory tract disease can be prevented through vaccination and antibody-based strategies, including in pediatric populations. In contrast, respiratory viruses that were also associated with BA exacerbation in the context of RSV co-infection, namely coronavirus, parainfluenza virus, adenovirus, and rhinovirus/enterovirus, currently lack effective vaccines, antivirals, or antibody-based prophylaxis. As a result, symptomatic management remains the standard of care. Moreover, childhood BA has been shown to persist into adulthood and may progress to severe adult BA [39, 41].

While current prophylactic approaches, such as maternal immunization and monoclonal antibody administration, offer indirect or time-limited protection, the development and deployment of pediatric RSV vaccine could provide direct, long-term immunity. This approach may significantly reduce RSV co-infection rates, decrease the risk of BA exacerbation in vulnerable pediatric populations, and may help prevent the progression to severe or persistent BA later in life.

In addition, RSV was notably prevalent in Japan in mid-2021—an earlier phase of this study period—to a degree not observed in the preceding seven years. This prevalence may have acted as an independent risk factor for BA exacerbation.Although this study identified several novel associations between RSV co-infection and other respiratory pathogens supported by robust statistical analyses, a large sample size, and an extended study period, some limitations should be acknowledged. First, the retrospective design limits the ability to infer causality. Prospective trials are necessary to validate these findings. Second, although pathogen detection was performed using comprehensive multiplex PCR testing, this analysis did not incorporate temporal or seasonal variations in the circulation of respiratory pathogens, which may have introduced bias. Third, since this investigation relied on clinical epidemiological data, mechanistic validation using in vivo and in vitro models is warranted to determine whether viral co-infection directly contribute to adverse clinical outcomes.

In conclusion, this study demonstrated that RSV co-infection with other respiratory pathogens was associated with a history of BA, BA exacerbation, wheeze, systemic steroid use, and age under 6 years, compared to that with RSV single infection. Notably, co-infection with coronavirus, parainfluenza virus, adenovirus, and rhinovirus/enterovirus in the context of RSV infection was identified as a contributing factor to BA exacerbation. In contrast, no such associations were observed among patients with hMPV co-infection. Multiple logistic regression analysis further confirmed that age under 6 years and RSV co-infection were independent risk factors for BA exacerbation. The implementation of RSV vaccination strategies in children may help reduce the burden of RSV-related respiratory illness.