Although epidemiological studies on M. pneumoniae infection have indicated that epidemics usually occur every 3–7 years [12–14], infection incidence in Europe and Asia significantly increased in 2011–2012, 2015, and 2017 [2, 9, 14, 15]. Our study retrospectively analyzed the impact of the public health response to COVID-19 on the occurrence of M. pneumoniae infection among children in western China, based on analysis of data from 2017 to 2020. In particular, we found that a small-scale epidemic outbreak of 3 months occurred in 2017, while a large-scale outbreak of 10 months occurred in 2019, confirming the uniform global epidemic pattern of M. pneumoniae infection. It has also been reported that a long epidemic affecting a large area can lead to a secondary peak in the same epidemic [16]. The average number of M. pneumoniae infections per month was approximately two times higher during each epidemic than between the epidemics.
Although substantial numbers of children were diagnosed with M. pneumoniae infection throughout the study period, the epidemic peaked in the fourth quarter of each year between 2017 and 2019, which was consistent with the results obtained previously in South Korea [2, 13], USA [3], Israel, and 11 countries of Europe [16]. However, the tendencies in these studies differ from the data reported in epidemiological studies in Italy [17], South Africa [18], and other regions of China [8, 19, 20]. The peaks of M. pneumoniae infection between 2017 and 2019 in our study coincided with the school semesters, and the number of infections fell significantly after schools were closed to limit the COVID-19 pandemic. These results indicate that closed settings with closer contacts promoted the M. pneumoniae transmission, consistent with studies reporting that M. pneumoniae infections are transmitted mainly through droplets spread during close contact [21], and that closed or semi-closed communities, such as military bases, hospitals, religious communities, schools, and institutions are areas associated with the highest rates of transmission, which can more easily lead to epidemics [22–24].
It has also been reported that climate conditions, such as humidity and temperature, can significantly affect the survival and spread of airborne M. pneumoniae [19, 25–27]. However, these studies have come to conflicting conclusions, suggesting that climatic factors are not the primary determinants of M. pneumoniae transmission patterns.
Furthermore, no clear differences were observed in sex distribution of pediatric cases of M. pneumoniae infection, but the number of positive cases varied significantly depending on age. Some studies have shown that M. pneumoniae infections are more common in children over 5 years of age [13, 22], although they also occur in infants [14, 17, 28, 29]. However, other studies have variably suggested higher rates of infection among preschool children or among school-age children [2, 8, 19, 20]. In the present study, the highest number of infections was detected among children 3–6 years old, which included children sent to a childcare center in Chengdu, where inter-child contact was closer than in primary and secondary schools [23], thus favoring the transmission of M. pneumoniae.
In the present study, the rate of hospitalization due to M. pneumoniae infection was within the rates reported in recent studies (18–67%) [13, 30, 31]. The significant increase in 2019 suggests that infections were more severe during an M. pneumoniae epidemic. Nevertheless, the incidence of infections decreased significantly in 2020 due to the restrictive measures and strong isolation policy applied from February 2020 by the Chinese government after the COVID-19 outbreak. In fact, the number of M. pneumoniae infections in the second quarter of 2020 was 63.3%, 60.3%, and 77.5% smaller, respectively, than the numbers in the second quarter of 2017, 2018, and 2019. This suggests that a comprehensive public health response can effectively control M. pneumoniae transmission.
Our study had some limitations, including the fact that M. pneumoniae infection was diagnosed based only on a single passive agglutination method [16] and a single acute-phase serum test for the presence of anti-M. pneumoniae IgM. In children with pneumonia, testing paired samples of acute- and convalescent-phase sera for M. pneumoniae antibody may be more sensitive than testing a single sample [32], and a titer of 1:640 may be a more suitable diagnostic criterion [13]. However, paired serum samples are difficult to obtain from inpatients and outpatients with self-limiting M. pneumoniae infection, and the optimal antibody level is difficult to define, because it can depend on the patient’s immune state [33]. Therefore, our retrospective study relied mainly on clinical manifestations of respiratory infection combined with a single antibody titer of ≥1:160, as recommended in a Chinese study [34].
In conclusion, we demonstrate that two epidemic outbreaks of M. pneumoniae infection occurred during 2017–2020 in western China. Preschool children were more susceptible to infection, and the predominant factor influencing M. pneumoniae transmission appeared to be close contact, especially in childcare centers. The significant differences in the temporal distribution and the decrease in the number of positive cases in the first three quarters of 2020 indicated that the public health response to the COVID-19 pandemic may have effectively controlled the transmission of M. pneumoniae infection.