This study described and analyzed the epidemiology of respiratory pathogens in children under 18 years of age who were hospitalized for ARIs in Quzhou City, China, more than 4 years after the implementation of COVID-19 pandemic restrictions. Of all children hospitalized with ARIs, nearly two-thirds were under three years old, with the highest proportion in the 1–3 year age group (40.98%), which was inconsistent with Julia Tabatabai et al. [18], who had the highest proportion in the < 6 month age group (39.0%).
Regarding the profile of respiratory pathogen detected in the present study, the most common pathogen was Mp, followed by HRV, RSV, HPIV and HMPV, with a low rate of influenza virus detection. At least one viral pathogen was detected in 62.12% of the hospitalized children. It was worth noting that in our study, the most frequently detected respiratory pathogen was Mp, rather than HRV or RSV, as found in some other studies [9, 18–20]. Cheng Gong et al. [21] found an increase in the incidence of respiratory diseases among children in Beijing during the fall and winter seasons in 2023, with the highest detection rate of Mp, which was consistent with our results. The delayed re-emergence of Mp after the containment of the COVID-19 pandemic may be related to the attenuation of cell-mediated herd immunity [14]. In addition, the atypical characteristics of Mp compared to other pathogens, including a slow generation time (6h), a relatively low transmission rate, and an incubation period of up to 1–3 weeks, may be contributing factors to the long intervals required for Mp to re-establish infection in the population [14].
Several studies have noted that the introduction of NPIs during the COVID-19 pandemic resulted in a significant reduction in the detection of influenza viruses and RSV at a time when they should have been at their peak of transmission [22–24], whereas HRV was consistently detected throughout the study period [25]. In our study, the detection of influenza viruses remained low, while the detection of RSV (19.67%) increased but was lower than HRV (25.14%). This may be due to the fact that HRV is an non-enveloped virus. Compared with enveloped viruses such as influenza virus and RSV, HRV is highly resistant to ethanol-based disinfectants, and a single application of ethanol-based disinfectant does not reduce the amount left on the skin, increasing the risk of surface contamination [26, 27]. In addition, wearing masks during NPIs prevented the transmission of human coronaviruses and influenza viruses, but did not reduce the transmission of rhinovirus-containing droplets and aerosols, increasing the risk of HRV infection [28]. It has been suggested that HRV itself may interfere with Severe acute respiratory syndrome Coronavirus 2 (SARS-CoV-2) infection, whereas HRV is not affected by the presence of SARS-CoV-2 [29]. The above reasons partly explain the continued detection of HRV after the relaxation of NPIs during the pandemic, whereas the detection rate of RSV increased as a result of the relaxation of NPIs. The decrease in influenza virus activity after relaxation from NPIs may be due to viral interference, usually caused by another respiratory virus that can compete with influenza viruses in the respiratory tract [30]. Wu et al. [31] found a negative interaction between HRV and influenza. In addition, widespread local influenza vaccination in Quzhou may have contributed to the low influenza virus detection rate in our study.
In monoinfection and coinfections, different pathogens were detected at different rates. Among them, HRV, Mp, HPIV and HMPV were more inclined to be detected in coinfections. In this study, 30.02% (127/423) of the Mp-positive patients had one or more other respiratory pathogens detected in their respiratory specimens at the same time, which was closer to the results of Wang Xue et al. [32]. The highest percentage of Mp + virus co-detection was for Mp + HRV (35.71%), while the highest percentage of virus + virus co-detection was for HRV + RSV (13.03%), the latter expected because HRV and RSV are the most common causes of upper and lower respiratory tract infections in children, respectively, and are the two most common respiratory viruses in viral surveillance studies [33–36].
Age and month of infection are important factors influencing the distribution of pathogens. Our study found that in children younger than 18 years, the detection rate of Mp tended to increase with increasing age, while the detection rate of RSV was the opposite, which was consistent with the results of studies in Beijing, Shanghai, and Huzhou [9, 32, 37]. The coinfections rate was higher in children aged 1–3 years than in other age groups. This finding was consistent with that reported by Song et al. in a previous study [38]. It is well known that pathogen distribution can be affected by seasonality, and climate as an important factor can alter the duration of the pathogen in the environment [39]. In our study, we found that there were differences in the distribution of pathogens in different months. Mp had the highest detection rate in October and showed an opposite trend to RSV in June-October. This may be related to the different epidemiological seasons of Mp and RSV, with Mp being detected in all seasons, with a slightly higher rate in spring and autumn, and RSV being detected at a higher rate in winter and spring [40].
Our study has several limitations. First, our study lacked clinical symptoms of respiratory infections and laboratory data to investigate the relationship between pathogenic coinfections and severity of respiratory symptoms. Second, we did not test for typing of Mp and common respiratory viruses such as HRV/RSV/HPIV, nor did we test for respiratory bacteria or fungi. This would limit detection of the full spectrum of present respiratory viruses and underestimate viral and/or bacterial coinfections in patients with ARIs. Finally, the scope of our single-center study included only six districts and counties in Quzhou City, and the enrollment period was less than one year. Therefore, our results may not be generalizable to other areas of the country. In addition, the pathogenicity of many viruses, such as HRV and RSV, is affected by seasonal variations; therefore, the scope of our study was an important limitation in studying seasonal variations of respiratory pathogens, and future studies should cover multiple seasons.