Objective To understand the epidemiological characteristics of an outbreak of respiratory tract infections caused by Mycoplasma pneumoniae （MP） in a primary school in Huzhou City， Zhejiang Province， and to provide scientific reference for future prevention and control of school outbreaks of upper respiratory tract infections caused by M. pneumoniae.

Methods According to the requirements for investigating respiratory infectious disease outbreaks， basic information， clinical history and epidemiological information were collected from the cases. Throat swab samples of cases for COVID-19 nucleic acid testing， influenza A and B virus antigen testing， and serum samples for detecting total antibody of mycoplasma were also collected.

Results A total of 47 cases related to M. pneumoniae infection were identified. The first case occurred on May 4， 2022， and the cases were concentrated between May 22 and May 31， 2022. The last case occurred on June 21， marking a duration of 48 days for the outbreak. All cases were second-grade students， with an incidence rate of 35.07% （47/134） in the affected classes， with Class 206 at 53.330% （24/45）， Class 207 at 38.64% （17/44）， and Class 211 at 13.33% （6/45）. There was a statistically significant difference in the incidence rates among the classes （χ²=16.57，P<0.05）. The incidence rate was 32.39% （23/71） for boys and 38.10% （24/63） for girls， with no statistically significant difference by gender （χ²=0.48，P>0.05）. The main clinical manifestations included cough， fever， and sore throat， and some cases exhibited signs of pneumonia. Pharyngeal swabs and serum samples were collected from 27 patients. The results were negative for pharynx swab samples， and positive for total mycoplasma antibody in 12 serum samples， with a positive rate of 44.44%.

Conclusion Symptoms caused by M. pneumoniae are diverse in clinical manifestations， with atypical signs and a slow progression. Early cases tend to exhibit mild symptoms. The onset of the disease is not detected in time when students are attending classes. The school morning health checks fail to promptly detect it and trigger home isolation control measures， which all contribute to the outbreak of the epidemic. Schools should conduct relevant prevention and control campaigns， improve morning and afternoon health checks， and prevent students from attending classes while sick.