Effect of COVID-19 on childhood Mycoplasma pneumoniae infection in Chengdu, China

Background Following the outbreak of the COVID-19 pandemic, a change in the incidence and transmission of respiratory pathogens was observed. Here, we retrospectively analyzed the impact of COVID-19 on the epidemiologic characteristics of Mycoplasma pneumoniae infection among children in Chengdu, one of the largest cities of western China. Method M. pneumoniae infection was diagnosed in 33,345 pediatric patients with respiratory symptoms at the Chengdu Women’s & Children’s Central Hospital between January 2017 and December 2020, based on a serum antibody titer of ≥1:160 measured by the passive agglutination assay. Differences in infection rates were examined by sex, age, and temporal distribution. Results Two epidemic outbreaks occurred between October-December 2017 and April-December 2019, and two infection peaks were detected in the second and fourth quarters of 2017, 2018, and 2019. Due to the public health response to COVID-19, the number of positive M. pneumoniae cases significantly decreased in the second quarter of 2020. The number of M. pneumoniae infection among children aged 3–6 years was higher than that in other age groups. Conclusions Preschool children are more susceptible to M. pneumoniae infection and close contact appears to be the predominant factor favoring pathogen transmission. The public health response to COVID-19 can effectively control the transmission of M. pneumoniae.

respiratory pathogens, including M. pneumoniae. In this study, we conducted a retrospective epidemiologic analysis of data from January 2017 to December 2020 in order to evaluate the impact of the public health response to COVID-19 on the epidemiological characteristics and transmission of M. pneumoniae among children in western China.

Study subjects
Data were retrospectively analyzed for children between 1 month and 18 years of age who came to Chengdu Women's & Children's Central Hospital from January 2017 to September 2020 due to respiratory symptoms. The patients' demographic features, clinical information, and laboratory data were retrospectively collected from the hospital records. The pediatric patients were divided into four groups depending on their age in years: 0-2, 3-6, 7-12, and 13-18.

Detection of M. pneumoniae
Serum antibodies against M. pneumoniae in serum were detected using a passive agglutination kit (Fujirebio, Japan) based on the manufacturer's instructions. A single titer of ≥1:160 was considered an indicator of M. pneumoniae infection.

Statistical analysis
All data were analyzed using the SPSS software package (version 20.0, IBM, USA). Categorical data were reported as ratios or n (%).

Results
Demographic characteristics of pediatric patients with M. pneumoniae infection A total of 34,977 pediatric patients were enrolled in the study, including 17,005 males and 17,972 females. The male/female ratios were 0.92:1 for 2017, 0.94:1 for 2018, 0.96:1 for 2019, and 0.97 for 2020 (Fig. 1). In each year, the number of M. pneumoniae infection was higher for the age group of 3-6 years than for other age groups, especially in 2019 (Fig. 2).

Temporal distribution of pediatric patients with M. pneumoniae infection
Our data provide the first evidence that two M. pneumoniae epidemic outbreaks occurred in western China between 2017 and 2020; the first between October 2017 and December 2017, and the second between April 2019 and January 2020. Analysis of the monthly distribution in the indicated period revealed that the number of M. pneumoniae positive cases was the highest in January 2020 and decreased sharply after February 2020 (Fig. 3). In addition, two epidemic peaks were identified in the second and fourth quarters of 2017, 2018 and 2019 (Fig. 4). Interestingly, these peaks decreased significantly after the COVID-19 pandemic outbreak, especially during the second quarter of 2020 (Fig. 4). 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 [2,17], 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 [18], South Africa [19], and other regions of China [8,20,21]. 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 [22], 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 [10,[23][24][25].
It has also been reported that climate conditions, such as humidity and temperature, can significantly affect the survival and spread of airborne M. pneumoniae [26,27]. 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 [12], although they also occur in infants [11,18,28]. However, other studies have variably suggested higher rates of infection among preschool children or among school-age children [2,8,14,29,30]. In the present study, the highest number of infections was detected among preschool children especially 3-6 years old, who spent most of the day playing with other children in their same age group in community or daycare settings, where inter-child contact was closer than  In the present study, the rate of hospitalization due to M. pneumoniae infection was within the rates reported in recent studies (18-67 %) [13,17,25]. 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 in the condition that the number of M. pneumoniae infections in the first quarter was higher than 2017 and 2018. This suggests that a comprehensive public health policy which response to the COVID-19 can effectively reduce M. pneumoniae infections in children [24,25].
Our study had some limitations, including the fact that M. pneumoniae infection was diagnosed based only on a single acute-phase serum antibody titer ≥ 1:160 which was tested by passive agglutination. A 4-fold increase in antibody titer and a single titer ≥ 1:640 were most specific for the diagnose of current or recent M. pneumoniae infection [12], while RNA or DNA tested by polymerization chain reaction (PCR) was most sensitive [31]. Given antibodies against M. pneumoniae could be affected by co-infection and patients' immune state [31,32], and results of PCR could be affected by antibiotics, techniques, and asymptomatic carriage [33,34], a combination of multiplex-PCR and serology helps to reduce each other's 'false positive' and 'false negative' rates and was considered to have the highest specificity and sensitivity [31]. However, paired serum samples are difficult to obtain in pediatric, and study demonstrated that titer of 1:160 had a high sensitivity and the highest Youden index and Kappa value using PCR as the standard indicating that it was conducive to screening for M. pneumoniae infection [35], coupled with that multiplex-PCR is expensive, therefore, our retrospective study relied mainly on clinical manifestations of respiratory infection combined with a single antibody titer ≥ 1:160. Another, we have to make longer observation and get more systematic surveillance data for better understanding of the epidemiology of M. pneumoniae in COVID-19 pandemic.

Conclusions
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.