Introduction
Bronchiolitis, an acute infection of the lower respiratory tract involving mainly the bronchioles, is one of the most substantial disease burdens for infants and young children worldwide (1). It is a common clinical syndrome occurs in children under 2 years of age, most often in the first year of life. A typical disease course begins with rhinitis and cough, which may progress to tachypnea, wheezing, rales, use of accessory muscles, grunting, or nasal flaring (2). Viral infection is the main cause of bronchiolitis; the most common causative viruses are respiratory syncytial virus (RSV), followed by human rhinovirus (HRV), parainfluenza virus (PIV, especially PIV III), adenovirus (ADV), influenza virus, human metapneumovirus (hMPV), and human bocavirus (HBoV) (3). Additionally, Mycoplasma pneumoniae (MP), Chlamydia pneumoniae (CP) and Bordetella pertussis (B. pertussis) can also cause bronchiolitis (4–7).
At the end of 2019, a novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was identified in hospitalized patients in Wuhan, China, and the illness caused by this virus was later named coronavirus disease 2019 (COVID-19) (8). In late January 2020, Suzhou municipal government implemented the first-level public health emergency response and ordered a stringent lockdown. Measures, such as prohibiting people from gathering, closing public places, postponing the resumption of work and school, wearing masks, and maintaining physical distancing, were taken to prevent the rapid spread of SARS-CoV-2. These measures impacted not only COVID-19, but also the dynamics of various other infectious diseases, including bronchiolitis (9, 10). However, it is unclear whether, in the second year of the pandemic when the prevention measures were relaxed, the pathogens associated with bronchiolitis continued to be markedly reduced and whether the clinical manifestations changed.
Because Suzhou municipal government enacted lockdown measures at the end of January 2020, here, we took January 31, 2020 as the time node and compared the clinical characteristics and etiology of children hospitalized with bronchiolitis over 3 consecutive years, i.e., 2019 (pre-pandemic), 2020 (first year of the pandemic), and 2021 (second year of the pandemic), from February 1 to January 31.
Research subjects and methods
Subjects
We conducted a retrospective analysis of the data from children who were admitted to the Department of Respiratory Disease at the Children's Hospital of Soochow University with a final diagnosis of bronchiolitis during the period from February 1 to January 31 in 2019, 2020, and 2021. It is important to note that only patients with negative SARS-CoV-2 nucleic acid tests could be admitted to the hospital. The inclusion criteria for this study were: age of between 1 month and 1 year old; occurrence of first episode of wheezing; and clinical evidence of bronchiolitis (tachypnea, wheeze, prolonged expiratory phase, and crackles on auscultation). The exclusion criteria were: recurrent wheezing; bronchopulmonary dysplasia; neuromuscular disease; congenital disease; congenital immunodeficiency; or evidence suggesting that the wheezing is caused by non-infectious factors, such as a bronchial foreign body. This study was approved by the Medical Ethics Committee of the Children's Hospital of Soochow University.
Specimen collection
Nasopharyngeal secretions, collected from all children within 24 h of hospital admission, were analyzed. For aspiration, a suction catheter was introduced through the nose and advanced into the lower portion of the pharynx, up to a distance of approximately 7 cm–9 cm, and approximately 2 ml of nasopharyngeal secretions were obtained and sent for analysis within 30 min. The retrieved specimen was centrifuged (500 g, 10 min) and suspended in 2 ml saline before being divided into two aliquots for pathogen identification using a direct immunofluorescence assay (DFA) and polymerase chain reaction (PCR).
Microbe detection
Seven common respiratory viruses, i.e., RSV, influenza virus A, influenza virus B, PIV I, PIV II, PIV III, and ADV were all detected using DFA. Chemicon (United States) provided the assay kits, and all staining methods were carried out according to the manufacturer's instructions. After that, immunofluorescence experiments were carried out (Leica 020-518.500, Germany). Trizol reagent (Invitrogen, United States) was used to extract RNA from the specimens, which was followed by reverse transcription to create cDNA. The cyclic temperature settings were 94 °C for 30 s, 55 °C for 30 s, 68 °C for 30 s, and a final extension at 68 °C for 7 min after 45 amplification cycles. A fluorescent real-time PCR (BIO-RAD iCycler) was used to detect hMPV and HRV. To identify HBoV, DNA extraction and real-time PCR were used. The 16 s rRNA gene of MP extracted from nasopharyngeal samples was identified using a quantitative diagnostic kit (Daan Gene Co., Ltd. Of Sun Yat-Sen University) for MP DNA.
Data collection
The medical records of the patients were reviewed, and data regarding the following parameters were recorded: (1) demographic and clinical characteristics, including age, gender, symptom duration prior to admission, and hospital stay length; (2) results of blood tests for inflammatory profiles, including white blood cell (WBC) count, neutrophil percentage, and serum C-reactive protein (CRP) level; and (3) results of etiological examination of nasopharyngeal secretions. Tachypnea was defined as follows: >60 breaths/min in children aged <2 months, >50 breaths/min in children aged 2–11 months.
Statistical analysis
Statistical analyses were performed using the Statistical Package for the Social Science (version 26.0, IBM Corporation, Armonk, NY, United States). Descriptive continuous outcome variables are presented here as medians (IQR). If the measurement data met the requirements of normality and homogeneity of variance, two independent sample T tests were performed; if not, a Kruskal-Wallis H test was performed. A chi-squared (χ2) test or Fisher's exact test was used to analyze categorical data. Post-hoc multiple comparisons were performed to determine the origins of significant differences, and the results were adjusted by using the Bonferroni method. P-values of <0.05 were taken to indicate statistical significance. Graphs were produced with GraphPad Prism (Version 8.4.3, GraphPad Software Inc).
Discussion
COVID-19, a coronavirus disease caused by SARS-CoV-2, broke out in Wuhan, China in December 2019 (8). SARS-CoV-2 is a highly contagious respiratory virus that spreads from person to person through contact with respiratory droplets and aerosols from infected individuals (11). In late January 2020, the Chinese government issued a national public health intervention policy, which included closing schools and other public places, prohibiting people from gathering, and encouraging mask wearing, hand washing, and disinfecting, to help prevent the spread of the epidemic. These measures also effectively suppressed the transmission of pathogens that cause bronchiolitis because the transmission pattern of bronchiolitis-causing pathogens, in particular RSV, is similar to that of SARS-CoV-2 (12). However, with the resumption of work and school, the epidemic prevention and control measures have been gradually relaxed.
In this study, we conducted a retrospective investigation of children hospitalized with bronchiolitis over 3 consecutive years (2019, 2020, and 2021) from February 1 to January 31, to compare the clinical features and etiological composition of this population before and during the COVID-19 pandemic. We found that the pathogen detection rate was the lowest in 2020, likely because 2020 was the first year of the COVID-19 pandemic and the associated interventions also reduced the spread of other respiratory pathogens. However, the pathogen detection rate in 2021 was the highest among the 3 study years, possibly because the COVID-19 control measures were eased in the second year of the COVID-19 pandemic.
In this 3-year study, we found that RSV was the predominant pathogen, a finding which is in agreement with many previous studies. Recently, a systematic review and meta-analysis of the prevalence of common respiratory viruses in children under 2 years of age with bronchiolitis in the pre-COVID-19 pandemic era found that RSV was the dominant etiologic agent of bronchiolitis (13). Correspondingly, the RSV detection rate changed in parallel with the total detection rate over our study years. In the first year of the COVID-19 pandemic, the RSV detection rate was the lowest, which is consistent with findings from previous work (14, 15). In the most severe months of the COVID-19 pandemic in 2020, there was a high uptake of enhanced hygiene and physical distancing measures. Handwashing can damage the lipid envelope that surrounds RSV, thereby impairing its ability to infect host cells (16). Another potential explanation for this difference is that there may have been a change in the ecology of respiratory viruses during the beginning of the COVID-19 pandemic. It is thought that rhinoviruses can stimulate the antiviral defenses of the mucous membrane of the respiratory tract, interfering with and blocking infection by other viruses (17). However, in the second year of the COVID-19 pandemic, the RSV detection rate increased significantly. Seasonal distribution studies conducted on participants in different age groups also found a significant increase in RSV-positive cases during the winter of 2021, and this increase began as early as October. In 2020, researchers from Princeton University and the National Institutes of Health predicted that a decline in common respiratory pathogens, such as RSV and influenza, could increase the population's susceptibility to these diseases, leading to future pandemics when they flare up again. They also predicted that RSV infections would be delayed after the end of the non-pharmaceutical interventions (NPIs) phase of the COVID-19 pandemic response, with a peak of RSV cases expected in many places during the winter of 2021–2022 (18). Our results are in line with their expectations, but this pattern is not unique to China; other regions, including Australia and Tokyo, had already observed an RSV resurgence in September 2020 and July 2021, respectively (19, 20). The timing of RSV prevalence is inconsistent in different regions, which may be related to regional and climate differences. Similarly, we also found that the number of bronchiolitis cases and the positive RSV detection rate dropped sharply beginning in February 2020 when the lockdown measures were taken and then increased again when the intervening measures were relaxed, especially in the winter of 2021. The resurgence of RSV may be attributed to “immune debt”, a term established to characterize the lack of protective immunity resulting from prolonged periods of low exposure to a certain pathogen, making a larger percentage of the population susceptible to the disease (21). Due to the transitory immunity established by RSV through virus exposure, maternal antibodies deplete quickly, and without seasonal exposure, immunity declines and susceptibility to subsequent and potentially more severe infections rises (22).
The positive rate of MP detection in 2021 was lower than that in 2019 but similar to that in 2020. This result is similar to those from a study conducted in Chengdu, China. In that study, researchers analyzed the prevalence of MP infection in children with respiratory symptoms from January 2017 to December 2020, and they found that public health measures adopted during the COVID-19 pandemic response could effectively control the spread of MP (23). MP, which is the smallest independent pathogenic microorganism between bacteria and viruses, often affects school-aged children. It continues to circulate and spread periodically, exhibiting an epidemic peak every 3 to 7 years.MP is a common causative pathogen of children's community-acquired pneumonia and, like RSV, is an important causative pathogen of bronchiolitis. However, in our study, the prevalence of MP was not found to rebound like that of RSV. It is unclear whether this occurrence is because of the epidemiological characteristics of MP itself or a low susceptibility to MP in children under 1 year of age.
Our study found that the detectable rate of HRV in 2021 was higher than that in 2019 but similar to that in 2020. Rhinoviruses are non-enveloped organisms that remain on environmental surfaces longer than many other pathogens (24), and they are not sensitive to ethanol-based disinfectants (25). So it is possible that these factors contributed to the rise in the positive rate of rhinovirus, despite the widespread use of disinfectants during the pandemic. However, the precise mechanism is still unknown.
We found that, except for differences in the percentage of patients with stuffy nose rhinorrhea, and gastrointestinal symptoms, there were no significant differences in the prevalence of bronchiolitis clinical features over the 3-year study period. The increase in the percentage of patients with bronchiolitis who reported stuffy nose rhinorrhea during the COVID-19 pandemic from that in 2019 may be due to the increased detection rate of rhinovirus, given that the most common symptoms of HRV infection are runny nose, nasal congestion, sore throat, and cough. During the COVID-19 pandemic, the percentage of patients with bronchiolitis who reported gastrointestinal symptoms (vomiting/diarrhea) decreased, possibly owing to the small sample size and the COVID-19 intervention measures.
Our study has some limitations. First, we included only patients with bronchiolitis who had undergone nasopharyngeal secretion examinations, and we did not enroll any patients with only a bacterial infection. Second, our samples were all collected from Suzhou City; thus, they do not fully reflect the epidemiological changes owing to geographical limitations. Finally, all of the children enrolled in our study were inpatients because it was not possible to obtain the epidemiology of respiratory pathogens from outpatients.