Epidemiology of Mycoplasma pneumoniae in children with acute respiratory infections in Hangzhou during January 2021 to June 2023

Introduction

Respiratory tract infections are the most common infectious diseases in humans, with children being particularly susceptible. The common pathogens include bacteria, viruses, chlamydia and mycoplasma. MP infection is among the most common infections in children [1], [2], [3]. MP is the smallest cell wall deficient prokaryote. The microbes are wall-less and malleable organisms which are able to grow and proliferate under cell-free conditions [4]. MP is contagious and can be spread through aerosols produced by coughing and sneezing. The main symptoms of MP infection are typical symptoms such as fever, wheezing, cough and expectoration [5]. These symptoms cannot be distinguished clearly from those of other respiratory virus infections, and their clinical diagnosis is limited [6]. The incidence of MPP has been increasing annually [7], with some progressing to severe or refractory pneumonia. Therefore, monitoring the epidemiological characteristics of MP is imperative.

In this study, we conducted a retrospective analysis of the positivity rate of MP in our hospital from January 2021 to June 2023 using data from a Children’s Hospital in Hangzhou. We explored the epidemiology in children with respiratory tract infection and assessed the prevalence of MP. Early intervention to be developed based on the findings from the present study can help reduce the incidence of MP in children.

Materials and methods

Results

Overall detection of MP

From the overall trend of medical treatment, the number of medical visits in 2023 increased significantly, and the total number and positivity rate were also significantly higher than the same period, i.e., January–June, in 2021 and 2022 (p<0.001, shown in Table 2). In 2021, the positivity rates of MP fluctuated less throughout the year, with a minor peak in May (23.1 %) (p<0.001). In 2022, MP positivity rates were highest in July, August, and September, at 38.0 , 42.2, and 31.4 %, respectively (p<0.05). The highest rate was reached in August. Positivity rates decreased slightly in October and November but started to increase significantly in December. In January and February 2023, the positivity rates continued to rise, reaching 2.4 times that of the same period in 2022 and 2021 (p<0.001). Moreover, in June 2023, it reached the highest value for the first half of 2023 (Figure 1).

Table 2:

Monthly distribution of MP from 2021 (January-June) to 2023 (January–June).

Month	2021 (n=2,444)	2022 (n=3,468)	2023 (n=5,272)	χ2	p-Value
1	99/631 (15.7 %)	105/676 (15.5 %)	315/859 (36.7 %)	126.21	<0.001
2	52/295 (17.6 %)	59/480 (12.3 %)	108/316 (34.2 %)	58.396	<0.001
3	47/244 (19.3 %)	102/688 (14.8 %)	204/756 (27.0 %)	32.665	<0.001
4	80/391 (20.5 %)	128/640 (20.0 %)	206/815 (25.3 %)	6.839	0.033
5	109/472 (23.1 %)	123/521 (23.6 %)	395/1,205 (32.8 %)	23.708	0<0.001
6	78/411 (19.0 %)	135/463 (29.2 %)	523/1,321 (39.6 %)	64.796	<0.001

1. Data are shown as the positive number/total number (%). p-Values Pearson χ2 test.

Figure 1:

The monthly distribution of MP from January 2021 to June 2023.

Monthly distribution

The monthly distribution of MP positivity rates for the first half of 2021–2023 is as follows: the MP positivity rate for 2021 reaches a small peak of 23.1 % in May, and the highest for 2022 is in June at 29.2 %. In January 2023, it started to increase significantly, reaching 36.7 % and increased to 34.2 % in February, 39.6 % in June (p<0.001). Although the highest positivity rate was in June 2023, the more significant increase in positivity rates occurred in January and February compared to the same period in the previous two years (Figure 2, Table 2).

Figure 2:

Monthly and age distributions of MP from 2021 to 2023 (January–June).

Discussion

In our study, we found that there was no significant epidemic peak in MP positive rates in 2021. This could be attributed to the strict non-pharmaceutical interventions implemented in China, which not only reduced the spread of COVID-19 but also changed the epidemic pattern of common respiratory viruses [11].

However, on December 7, 2022, the State Council of China issued a set of “New 10 Measures” for COVID-19 prevention and control, which included the cessation of nucleic acid testing and the discontinuation of checking for nucleic acid certificates in public places. Since then, people have no longer been required to wear masks in public places. Meanwhile, the positivity rate of MP increased in December 2022. From January 2023, the MP positivity rate increased significantly to 34.2 %, which was 133.8 and 136.8 % higher than the rates in the same period of 2021 and 2022, respectively (p<0.001). The increases were most pronounced and consistent with the changes in COVID-19 prevention and control measures issued by the government.

It found that MP infection occurs in a regional outbreak globally every 3–7 years, with each outbreak lasting for 1–2 years [12], [13], [14], [15]. According to data in Beijing, we observed that MP epidemic in Beijing peaked in 2016, followed by that in 2019 between 2015 and 2022, the last outbreak was in 2019 [16].An official report from Public Health England showed that the most recent epidemic peak was in winter 2019 to 2022 in England. Those studies may provide an explanation for our finding that MP positivity rates significantly increased in December 2022 compared to the same period in 2021. The interval between the last epidemic in 2019 and the current one was about 4 years, which is consistent in principle with the cyclical prevalence of MP.

More studies have shown no gender difference in MP infection [17], but our study found a higher detection rate in girls, which is consistent with some previous studies [18, 19], which suggested that it was possibly due to regional and environmental factors, hormonal differences, or contact behaviors in daily life.

As we know, MP can cause cough and bronchitis, and may progress to pneumonia and extrapulmonary manifestations in severe cases [20]. In our study, pneumonia was the most common type of disease in children with MP infection. It was 74.0 , 87.7, and 86.4 % in 2021, 2022, and 2023, respectively. This result was consistent with a previous study [21].

It was apparent that MP was a common cause of pneumonia. It has been reported that during the COVID epidemics, up to 20–40 % of pneumonia cases were caused by MP [3, 22, 23], which is similar to other studies [24, 25] and ours, showing that MPP accounted for 36.5 % of all pneumonia in the first half of 2023, significantly increased compared with the same period of 2021 (18.7 %) and 2022 (30.0 %) (p<0.001).These results indicated that the incidence of MPP in children with respiratory tract infection was high. Because the manifestations of MP infection in children varied in severity, and the imaging features were not specific, often ignored or misdiagnosed [26, 27], how to detect the pathogen quickly and effectively should be studied in the future.

The present study also found that the epidemics could be advanced or delayed by 1–2 months in different years. There was a small peak from April to May in 2021, while in 2022, the onset of the epidemic was delayed to July to September. In reality the positivity rate of MP in 2023 (January–June) was highest in June. It has been noted that MP has occurred in different climates conditions and different times of a year [28]. Japanese studies have suggested that temperature increases were positively correlated with infection rates of MP [29, 30], which was consistent with our research. Hangzhou experienced relatively high temperatures in the months of July to September, coincided with the summer vacation when children often spend time in air-conditioned shopping malls or amusement parks. Enclosed or semi-enclosed spaces may facilitate the spread of MP.

Children may be infected with MP at any age. We found that pathogen detection rates varied among children of different ages in the first half of 2021–2023 (p<0.001) except for infants aged 0–28 days. MP infection was most common in children aged 6–17 years. This is basically consistent with previous studies [7, 31, 32]. Children aged 6 years older are school-age children who spend more time in classrooms or collective settings than children under 6 years old. This also corroborated previous reports that MP spread more easily in close contact and closed environments [33]. In contrast, the infection rate in the infant group (<1 year) was the lowest, which was probably related to their simple living environment in this age group, as most are breastfed. One study has shown that breast milk contains a variety of immunomodulatory and antibacterial substances, which can reduce the risk of acute respiratory infection risk in children [34]. However, it was worth noting that in 2023, compared to the same period in 2022 and 2021, the positivity rate of MP in children under one year old increased by 92.5 %, the largest increase among age groups. We speculated that after the government-issued changes in COVID-19 prevention and control in China, the risk of adult respiratory tract infections in families increased and subsequently the infection rate among infants and young children also increased. Another possible explanation was that less protecting IgG antibodies via the placenta by reduced MP-infections of the mother allowed the increase in infections of the infants.

Conclusions

Our retrospective analysis showed that MP was a major causative agent of respiratory tract infection. The main disease type was pneumonia. It was found that the prevalence of MP infection was significantly higher in girls than in boys, and higher positivity rates were mainly observed in children aged 6 years older. The positivity rate of MP increased significantly after the adjustment of COVID-19 prevention and control in China, and the most significant increase was seen in the infant group (<1 year). Thus, we called for focus on the new epidemiological characteristics of MP infection after easing the restrictive measures. Effective prevention and control measures should be implemented to reduce the prevalence of MP infection among children aged 6 years older and the infant group (<1 year).