This study investigated the epidemiological trends of MP infection before, during and after COVID-19 pandemic. In addition, the analysis of 14 years clinical data showed that the number of children with acute respiratory tract infections (ARTIs) who underwent MP DNA-PCR assay sharply rase in 2019 and 2023. However, it plummeted sharply and remained at a low level during COVID-19 pandemic (from 2020 to 2022), which indicated that non-pharmaceutical interventions (NPIs) have to some extent reduced the spread of acute respiratory diseases, which consisted with previous studies [5, 11]. Interestingly, the inpatients with ARTIs were more commonly infants from 2010 to 2022. However, following the discontinuation of strict NPIs, the trend shifted towards school-aged children (6–16years). The reason may be that young children had poor compliance with NPIs, while older children were more socially active after the discontinuation of NPIs [12].

In this study, the proportion of ARTIs caused by MP varies by age, with school-age (>6 years) children being the most common age group affected during the 14 years (before, during and after COVID-19 pandemic), which was consist with previous studies [1, 8]. In addition, girls were more susceptible to MP than boys in this study, which was also consistent with the previous research by Cheng et al. and Zhang et al. [13, 14]. The reason was believed to be related to the different lifestyles of them that boys spend more time engaging in outdoor activities than girls [13]. Overall, the NPIs have not changed the demographic characteristics of MP infection in children.

Previous studies indicated that MP infection complied with an epidemic cycle of approximately 3–7 years, with each outbreak spanning 1–2 years and can occur throughout the year [4]. In this study, there were epidemic of MP in 2012 and 2013. According to the trend of MP, the recent MP epidemic was expected to prolong from 2019 to 2021 [15, 16]. In fact, there was indeed a pandemic of MP in Suzhou, China in 2019. However, compared to 2019, the positive cases and detection rates of MP in 2020, 2021 and 2022 have marked declined. It suggested that strict NPIs have the potential to remarkable reduce the prevalence of MP in semi-closed or closed populations [17, 18]. Since cancellation of NPIs in December 2022 in Suzhou, China, the detection rate of MP was at a low level from January to February 2023, but it continued to rise until June 2023 and persisted until the end of the investigation. The slow generation time (6 h) and transmission (1–3 weeks incubation period) of MP result in a longer time interval than the virus to re-establishment in the population after stopping NPIs [19]. So, the cancellation of NPIs did not immediately lead to the spread of MP infection in the population. However, due to the low MP infection rate for 3 years, the population lacked immune protection (also known as immune debt) [9], and was susceptible to infection, leading to a gradual increase in the number of MP infections in 2023.

According to the monthly MP-positivity rates, a peak was in August of each year before the COVID-19 epidemic, except 2011. It indicated that the prevalence of MP showed seasonal variation. Previous studies have shown that the peak of MP detection in eastern China (such as Zhejiang) was in summer and autumn [9], while in northern China was in winter [20]. A research report from Japan stated that there was a positive correlation between rising temperatures and the occurrence of MP infection, which also explains why the number of infections increases during warmer months [21, 22]. However, the diversity of MP epidemic seasons appearing in the north and south of China may be related to the living environment, public health measures, and activity patterns [4]. It is worth noting that the detection rate of MP infection in 2011 in this study remained low throughout the year without a peak. Considering the local epidemiological characteristics, it may be due to the high incidence of hand, foot, and mouth disease [23] and the competition between pathogens.

When implementing NPIs and lifting the policy within one year, the prevalence of MP infection showed no obvious seasonal, which was consistent with previous research [24, 25]. It indicates that NPIs can not only reduce the MP infection in children, but also interfere with its seasonal characteristics.