Discussion
In January of 2020, Zhejiang province activated its Level 1 emergency reaction to the COVID-19 pandemic. Several NPIs were adopted for the purpose of suppressing or mitigating the virus spread. Respiratory tract virus infection in children has always been the focus of attention at home and abroad. Tremendous efforts were made to minimize common childhood respiratory virus prevalence apart from COVID-19 containment [2,12,13]. Herein, virus-positive sample numbers increased by 7.2% compared to before COVID-19. The detection percentages were relatively unaffected; this increase in numbers may be the result of a combination of factors, including a presumed decline in population immunity and a loosening of COVID-19 limitations.
More specifically, we found that the test positivity rate is extremely high over the period of the COVID-19 outbreak; the strongest contribution is provided by RSV, whose prevalence has experienced a significant increase, consistent with results described by reports from Australia[13,14], and New Zealand, likely corresponding to relaxing NPIs. These phenomena may be associated with the concept of “immunity debt”[15,16], emphasizing the necessity of recognizing NPIs as a double-edged sword; this highlights the criticality of active and ongoing epidemiological surveillance and timely adjustment of immunization approaches. The epidemiological trend of RSV infection in children had undergone a significant alternation before and after COVID-19 in East China. A significant reduction in the ADV, Flu B, and PIV-2 and -3 during COVID-19 was identified, which was in the same line with observations from other reports[17,18].
Herein, children between the ages of 1-12 months are the most susceptible to respiratory viruses, particularly RSV, both prior to and throughout COVID-19; surprisingly, compared to before COVID-19, respiratory infection prevalence was almost higher after COVID-19 in this age group, where it decreased significantly in other age groups. When a virus infects a host, the immune system is activated to fight the pathogenic microorganism[19]. In children under one year old, lacking complete immune memory and decreased innate and adaptive immunity due to the immature immune system may be liable to their respiratory-virus vulnerability, with NPIs having little influence[20]. Nevertheless, neonates receiving massive passive antibodies from mothers and older children whose immune systems are more thoroughly developed may benefit to a great extent from NPIs implementation in their protection against respiratory viral infections.
This study showed that some respiratory viruses had obvious seasonality, and the detection rates of RSV and Flu A were high in winter.  During COVID-19, PIV-3 was detected mainly in autumn and winter, while before COVID-19, PIV-3 was detected mainly in summer, followed by spring. Since Flu is mainly transmitted by droplets and contact, the prevention and control measures taken in the early stage of the novel coronavirus outbreak were just in spring and summer, basically the same as those taken for flu prevention and control. It can effectively block the transmission of Flu. Nevertheless, at the beginning of the fall semester in September 2020, in the light of the Chinese domestic COVID-19 outbreak in remission, people generally went back to normal work and life in low-risk areas, like those prior to COVID-19; this may clarify the spike in PIV-3 infection in summer of 2020, consistent with relevant research results[21].
This study has some drawbacks as it is only a single-center study; besides conducting a retrospective analysis and collecting the data of children admitted to our hospital, hence the other factors which may relate to respiratory virus infection were not analyzed.
In conclusion, RSV infection is the most frequent respiratory tract infection, with significant differences in distribution between different ages and different seasons. COVID-19 prevention and control measures can effectively contain ADV, Flu A and B, and PIV1, 2, and 3 transmissions. Herein, the respiratory virus was detected and analyzed in hospitalized children with acute respiratory tract infection to provide evidence for the clinical diagnosis and treatment of respiratory tract virus infection during the epidemic period.