This study was conducted in the Children's Hospital affiliated with Shandong University (Jinan Children's Hospital) in Jinan, Shandong Province, China, from 1 January 2016 to 31 December 2022. Shandong Province has the largest population in northern China, and it is one of only two provinces in the country with a population of over 100 million (9). Jinan Children's Hospital is the children's medical centre in Shandong Province, serving children from various parts of the province. There are 1,400 established beds and 40 clinical and medical technology departments, with an annual outpatient volume of more than 1.1 million and 39,000 discharged patients (10). It is a large general hospital integrating medical treatment, teaching, scientific research, prevention, rehabilitation and health care.

Data on the number of diseases, sex and age among the children with common respiratory infectious diseases in Jinan Children's Hospital from 1 January 2016 to 31 December 2022, including pertussis, measles, scarlet fever, pulmonary tuberculosis, mumps and influenza, were obtained from the public health department in the hospital. The department is an internal department of public medical institutions responsible for public health work. It is responsible for collecting information on different infectious diseases and reporting it to the superior health administrative department. Infectious disease data are based on ICD-10 codes. Because of the lack of test reagents, data on influenza in JCH in 2016 are missing.

The number of people affected by common respiratory infectious diseases in China was collected according to CDC data in China. This database collects all data reported on infectious diseases since the Chinese infectious disease network direct reporting system was implemented in 2004, mainly including the number of patients, age group, sex, and number of deaths, among others. The network reporting system covers 100% of the Centers for Disease Control, 98% of medical and health institutions at and above the county level and 88% of township health centres nationwide (11).

Children were diagnosed with pertussis, measles, scarlet fever, pulmonary tuberculosis, mumps or influenza based on recommendations for the diagnosis and treatment of Chinese paediatric infectious diseases. (1) The criteria for defining a pertussis case were as follows: cough lasting for more than 14 days associated with at least one of the other symptoms: paroxysmal cough, inspiratory whooping and posttussive vomiting. A suspected case of pertussis was considered a confirmed case if the child presented a persistent cough and a positive nasopharyngeal swab culture or a positive throat swab PCR or if the antibody against pertussis toxin (anti-PT) IgG was positive (12). (2) The clinical diagnostic criteria for measles were as follows: ① contact history with measles patients or travelling to high incidence areas of measles within 7–21 days; ② Koplik's spots seen in buccal mucosa at the early stage of the disease. A laboratory-confirmed case was considered when one of the following conditions was met: positive specific IgM antibodies in serum or detection of viral RNA in throat swabs or urine (13). (3) A scarlet fever case was characterized by a red rash after 12–48 h and was usually accompanied by a sore throat and flushed cheeks. Group A streptococcus (GAS) was cultured using throat swab materials (14). (4) To define a clinical case of paediatric pulmonary tuberculosis, the following three conditions must be met: ① signs and symptoms related to pulmonary tuberculosis, such as a chronic cough or wheezing, long-term fever, weight loss or failure to thrive; ② chest imaging showing the characteristics of active pulmonary tuberculosis; ③ positive tuberculin skin test (TST) results and/or recombinant Mycobacterium tuberculosis fusion protein (EC) results and/or interferon-gamma release assay (IGRA) results. The laboratory criteria of tuberculosis were as follows: positive stains of sputum/sputum induction/bronchoalveolar lavage fluid/gastric aspirates, positive culture of Mycobacterium tuberculosis (MTB), positive nucleic acid amplification test (NAAT), or positive pathological examination results (15). (5) The clinical diagnosis of mumps can be based on painful swollen salivary glands, particularly the parotid glands in front of the ears on both sides of the face. The laboratory criteria were isolation of virus from a swab inside the cheek, a positive PCR of the swab, or specific IgM mumps antibodies positive in serum (16). (6) The criteria for defining a clinical case of influenza were abrupt onset of fever, chills or sweats, muscle pain, cough and malaise. Influenza can be diagnosed by laboratory testing when meeting one of the following three criteria: positive rapid molecular assays, a positive viral culture or positive reference laboratory polymerase chain reaction testing (17).

Pertussis, measles, pulmonary tuberculosis, mumps and influenza are all vaccine-preventable diseases; scarlet fever is not. It should be noted that based on the national immunization programme in China, the bacillus Calmette-Guerin (BCG) vaccine against tuberculosis is given to infants at birth. Children receive the diphtheria, tetanus, and acellular pertussis (DTaP) vaccine at the ages of 3–5 months, and a booster dose is given to children at an age of 18 months. Infants receive the measles-rubella (MR) vaccine at the age of 8 months and the measles-mumps-rubella (MMR) vaccine at the age of 18–24 months. In addition, children in Beijing city, Tianjin city, Shanghai city and Shandong Province are given a 2-dose mumps-containing vaccine (MuCV) by the age of 6 years as part of Expanded Program on Immunization (EPI) (18). All vaccines mentioned above are mandatory and free. China recommends that children aged more than 6 months receive the influenza vaccine annually, though the vaccine is optional and paid out of pocket (19). Vaccination may be delayed due to requirements of home quarantine.

WPS Excel 2019 (Beijing Jinshan Office Software Co., Ltd., China) was used to organize the data and draw diagrams such as line charts and bar graphs. The χ² test and Fisher's exact test were applied to statistically compare the composition ratios of children with different respiratory infectious diseases in JCH before and during COVID-19 using SPSS version 25.0 (SPSS Inc., Chicago, IL, USA). A cut-off of P < 0.05 was considered statistically significant.

Common respiratory infectious diseases among children were pertussis, measles, scarlet fever, pulmonary tuberculosis, mumps and influenza. A total of 12,225 cases from 1 January 2016, to 31 December 2022, were included in this study at Jinan Children's Hospital. The numbers of children with pertussis, measles, scarlet fever, pulmonary tuberculosis, mumps and influenza before and during COVID-19 were as follows: 2,200 cases and 1,488 cases, 650 cases and 30 cases, 4,001 cases and 687 cases, 86 cases and 28 cases, 340 cases and 109 cases, and 1,051 cases and 1,555 cases, respectively. Scarlet fever, pertussis and influenza were the most common respiratory infectious diseases in this children's hospital. However, based on the data of the China CDC, influenza, tuberculosis and mumps were most common in China (Table 1).

The age distribution of pertussis in JCH showed that more children aged >6 years had pertussis in 2020–2022 than in 2016–2019, and the difference was statistically significant (χ² = 312.234, P < 0.001). More children aged >8 months had measles during COVID-19 than before the pandemic (χ2 = 15.395, P < 0.001). The onset of scarlet fever was most frequent at the age of 3–10 years old before and during COVID-19. Pulmonary tuberculosis was distributed among children of all ages between 2016 and 2022. There were many more children aged ≥2 years with mumps from 2016 to 2022 than children aged <2 years. Children aged 1–6 years old were more susceptible to influenza before and during COVID-19 (Table 2).

Thus, the high incidence rates by age of scarlet fever, pulmonary tuberculosis, mumps and influenza were similar before and during COVID-19 (2016–2019 vs. 2020–2022), except that the proportion of children older than 6 years of age with pertussis increased markedly and the proportion of children aged >8 months with measles accounted for a larger proportion from 2020 to 2022.

There was overall male predominance regarding the respiratory infectious diseases of pertussis, measles, scarlet fever, mumps and influenza from 2016 to 2022. However, the number of tuberculosis cases was too small to come to a conclusion (Table 3).

There was an a increase in the number of cases from September to November 2017 in JCH, while CH had a downward trend at the same time according to the data of China CDC. In 2018 and 2019, there was a high incidence of pertussis in both JCH and CH, and there were two peaks of pertussis incidence in JCH in 2018 and 2019 (March to May and September to November), yet there was only one peak in August in CH. During the COVID-19 pandemic (2020 and 2022), the morbidity of pertussis decreased sharply in both JCH and CH compared with that before the epidemic. However, the number of pertussis cases had increased since the beginning of 2022 (Figure 1).

The first half of 2016, especially spring, was the peak period of measles in JCH and CH. Then, in JCH, the monthly morbidity of measles was similar and remained at a quite low level from 2017 to 2022. However, according to the CH data, there were still outbreaks in the spring from 2017 to 2019, while the trend plateaued during the COVID-19 pandemic from 2020 to 2022 (Figure 2).

The peak seasons of scarlet fever were spring and winter from 2016 to 2019 in both JCH and CH, with scarlet fever occurring mainly from April to June and from November to January of the next year. However, the trend was not as obvious in 2020 when the number of cases was lowest. After 2020, the confirmed number of cases rebounded in 2021 and 2022 (Figure 3).

The seasonal distribution of tuberculosis was irregular in JCH and CH. The monthly number of children with TB remained in the single digits in JCH from 2016 to 2022. However, in CH, the number of confirmed cases remained quite large every month, ranging from 160,000 to 40,000, and showed a significant downward trend beginning in 2016 according to China CDC data (Figure 4).

April-July and November to the following January were the two peaks of mumps in JCH and CH. Before the epidemic, there were many more children confirmed in 2016 than in the other years in JCH, yet the number of patients in CH increased yearly from 2016 to 2019. The number of mumps cases fell during the COVID-19 epidemic compared with that in 2016–2019 in JCH and CH. However, at the beginning of the epidemic in 2020, the number of mumps cases fell to the lowest point in March in JCH and in April in CH and then increased gradually over the next few months (Figure 5).

Winter and spring were the most common seasons for influenza, and the number of influenza cases increased in January each year in JCH (2017–2019) and CH (2016–2019) during the prepandemic period. However, the number of cases declined sharply at the beginning of 2020 and increased markedly from December 2021 to 2022 (Figure 6).

The numbers of children in JCH with pertussis, measles, scarlet fever, mumps and influenza decreased substantially during COVID-19 from 2020 to 2022 compared with the numbers in 2016–2019, while the numbers of patients with all six respiratory infectious diseases, including pulmonary tuberculosis, declined in CH during the pandemic. In 2022, the peak season of influenza started from the early summer.