Respiratory syncytial virus surge in 2022 caused by lineages already present before the COVID-19 pandemic.

In 2022, Austria experienced a severe respiratory syncytial virus (RSV) epidemic with an earlier-than-usual start (Weeks 35/2021-45/2022) and increased numbers of pediatric patients in emergency departments. This surge came 2 years after a season with no cases detected as a result of coronavirus disease 2019 nonpharmaceutical interventions. We analyzed epidemiologic patterns and the phylodynamics of RSV based on approximately 30 800 respiratory specimens collected year-round over 10 years from ambulatory and hospitalized patients from 248 locations in Austria. Genomic surveillance and phylogenetic analysis of 186 RSV-A and 187 RSV-B partial glycoprotein sequences collected from 2018 to 2022 revealed that the 2022/2023 surge was driven by RSV-B in contrast to the surge in the 2021/2022 season that was driven by RSV-A. Whole-genome sequencing and phylodynamic analysis indicated that the RSV-B strain GB5.0.6a was the predominant genotype in the 2022/2023 season and emerged in late 2019. The results provide insight into RSV evolution and epidemiology that will be applicable to future monitoring efforts with the advent of novel vaccines and therapeutics.

Detailed Evolutionary Analyses of the F Gene in the Respiratory Syncytial Virus Subgroup A.

We performed evolution, phylodynamics, and reinfection-related antigenicity analyses of respiratory syncytial virus subgroup A (RSV-A) fusion (F) gene in globally collected strains (1465 strains) using authentic bioinformatics methods. The time-scaled evolutionary tree using the Bayesian Markov chain Monte Carlo method estimated that a common ancestor of the RSV-A, RSV-B, and bovine-RSV diverged at around 450 years ago, and RSV-A and RSV-B diverged around 250 years ago. Finally, the RSV-A F gene formed eight genotypes (GA1-GA7 and NA1) over the last 80 years. Phylodynamics of RSV-A F gene, including all genotype strains, increased twice in the 1990s and 2010s, while patterns of each RSV-A genotype were different. Phylogenetic distance analysis suggested that the genetic distances of the strains were relatively short (less than 0.05). No positive selection sites were estimated, while many negative selection sites were found. Moreover, the F protein 3D structure mapping and conformational epitope analysis implied that the conformational epitopes did not correspond to the neutralizing antibody binding sites of the F protein. These results suggested that the RSV-A F gene is relatively conserved, and mismatches between conformational epitopes and neutralizing antibody binding sites of the F protein are responsible for the virus reinfection.

Molecular characterization of respiratory syncytial viruses circulating in a paediatric cohort in Amman, Jordan.

Respiratory syncytial viruses (RSVs) are an important cause of mortality worldwide and a major cause of respiratory tract infections in children, driving development of vaccine candidates. However, there are large gaps in our knowledge of the local evolutionary and transmission dynamics of RSVs, particularly in understudied regions such as the Middle East. To address this gap, we sequenced the complete genomes of 58 RSVA and 27 RSVB samples collected in a paediatric cohort in Amman, Jordan, between 2010 and 2013. RSVA and RSVB co-circulated during each winter epidemic of RSV in Amman, and each epidemic comprised multiple independent viral introductions of RSVA and RSVB. However, RSVA and RSVB alternated in dominance across years, potential evidence of immunological interactions. Children infected with RSVA tended to be older than RSVB-infected children [30 months versus 22.4 months, respectively (P value = 0.02)], and tended to developed bronchopneumonia less frequently than those with RSVB, although the difference was not statistically significant (P value = 0.06). Differences in spatial patterns were investigated, and RSVA lineages were often identified in multiple regions in Amman, whereas RSVB introductions did not spread beyond a single region of the city, although these findings were based on small sample sizes. Multiple RSVA genotypes were identified in Amman, including GA2 viruses as well as three viruses from the ON1 sub-genotype that emerged in 2009 and are now the dominant genotype circulating worldwide. As vaccine development advances, further sequencing of RSV is needed to understand viral ecology and transmission, particularly in under-studied locations.

Respiratory syncytial virus causes more hospitalizations and deaths in equatorial Brazil than influenza (including during the 2009 pandemic).

Despite numerous studies evaluating influenza and Respiratory Syncytial Virus (RSV), there is still a lack of knowledge about them, especially in tropical countries. We compared the relative importance of respiratory viruses by examining their spatiotemporal patterns, age-specific hospitalization data and mortality data for 2007-2012 obtained from official sources. The data were aggregated into "respiratory infection seasonal zones" formed combining states that had similar seasonal patterns of pneumonia and influenza (P&I). Equatorial-North where P&I peaks in the middle of the year, Equatorial-South where P&I peaks in the first semester and coincides with the rainy-season, Subtropical where P&I peaks are clearly concentrated in the winter season, and Tropical Midwest and South-East where P&I peaks are a transition between the South-Equatorial and the Sub-tropical. Our analyses indicate that RSV has higher impact than influenza in equatorial region of Brazil, which was particularly evident during the circulation of the 2009pdm strain, and suggests that seasonal influenza may have a lower impact in Equatorial Brazil (and perhaps in other tropical regions of the world) than previously considered. Accordingly, we suggest that the broad assumption that influenza is the main cause of viral respiratory hospitalizations and death in equatorial regions be questioned with greater emphasis in future studies.

Analysis of phylogenetic diversity and in vitro adherence characteristics of respiratory syncytial virus and Streptococcus pneumoniae clinical isolates obtained during pediatric respiratory co-infections.

Respiratory syncytial virus (RSV) and Streptococcus pneumoniae are frequently co-associated during acute respiratory infections, particularly amongst infants and young children. In this study, we aimed to identify strains of RSV and serotypes/sequence types of S. pneumoniae associated with co-infections within a cohort of paediatric patients, and to assess RSV-mediated adhesion of pneumococcal isolates. The RSV glycoprotein sequence was determined for 58 RSV-positive samples and molecular serotyping and MLST was used to analyse 26 pneumococcal isolates. We also compared 23 pneumococcal isolates for their adherence to RSV-infected or mock-infected airway epithelia cells using immunofluorescence microscopy and automated particle counting. The tight association between RSV and S. pneumoniae was also visualized using scanning electron microscopy. This study did not identify any statistically significant trend in the strains of RSV and S. pneumoniae associated with co-infections. Furthermore, almost all isolates (22 of 23) showed significantly increased adherence to RSV-infected cells. The level of adherence did not appear to correlate with pneumococcal strain or sequence type, and isolates obtained from RSV-infected patients displayed a similar level of adherence as those from RSV-negative patients. The absence of particular S. pneumoniae or RSV strains associated with co-infection, together with the near ubiquitous presence of RSV-mediated adhesion throughout the pneumococcal clinical isolates, may indicate that the mechanisms governing the association with RSV are of sufficient importance to be maintained across much of the species.

[Etiological structure of influenza and other ARVI in St. Petersburg during epidemic seasons 2012-2016.]

The etiological structure of influenza and other acute respiratory viral infections including their rate of incidence in St. Petersburg and Leningrad region during 4 epidemic seasons has been studied. Seasonality of some respiratory viruses was shown and peaks of circulation of RSV, adenovirus, parainfluenza viruses, rhinovirus, bocavirus, metapneumovirus and coronavirus were marked. The interference of influenza A viruses and RSV, RSV and rhinoviruses was highlighted. A high incidence of adenovirus infection in organized communities and RSV infection in children was revealed.

Using a novel rapid viral test to improve triage of emergency department patients with acute respiratory illness during flu season.

BACKGROUND: Acute respiratory illnesses (ARI) are mostly viral in etiology and cause significant morbidity and mortality. Point of care PCR (POC-PCR) is a promising new technology for rapid virus identification but utility in the Emergency Department (ED) is not yet defined. OBJECTIVES: Primarily, to investigate the value of POC-PCR in rapidly identifying RSV and influenza in the setting of ED triage. Additionally, to assess whether rapid knowledge of accurate test results would improve patient management by preventing nosocomial transmission and optimizing the prescription of antimicrobials for ARIs. STUDY DESIGN: A prospective cohort study of consecutive ED patients with ARI symptoms during peak flu season was conducted. Patient nasopharyngeal swabs were collected and tested using a POC-PCR device; physicians and patients were blinded to results. Virus positive and negative groups were compared by ED patient room placement and antimicrobial therapy ordered. Specificity and sensitivity were calculated using laboratory-PCR as the gold standard. RESULTS: Of 119 participants, 52.9% were POC-PCR positive - Influenza A (42.9%), RSV (41.3%), influenza B (15.9%). Nearly 70% of virus positive patients were placed rooms shared with non-ARI patients. Antibiotics were prescribed for 27.3% of virus positive patients, and 77.8% of oseltamivir-eligible patients did not receive therapy. POC-PCR was 100% sensitive (95% CI, 80.5-100.0%) and 95.2% specific (95% CI, 76.2-99.9%). CONCLUSIONS: Rapid POC-PCR for influenza and RSV in ED triage has excellent sensitivity and specificity and the potential to improve social distancing practices through better triage and increase appropriate prescription of antimicrobials.

Respiratory syncytial virus infections in India: Epidemiology and need for vaccine.

Respiratory syncytial virus (RSV) has been identified as a leading cause of lower respiratory tract infections in young children and elderly. It is an enveloped negative-sense RNA virus belonging to Genus Orthopneumovirus. The clinical features of RSV infection range from mild upper-respiratory-tract illnesses or otitis media to severe lower-respiratory-tract illnesses. Current estimates show that about 33.1 million episodes of RSV-acute lower respiratory infection (ALRI) occurred in young children in 2015, of these majority that is, about 30 million RSV-ALRI episodes occurred in low-middle-income countries. In India, the rates of RSV detection in various hospital- and community-based studies mostly done in children vary from 5% to 54% and from 8% to 15%, respectively. Globally, RSV epidemics start in the South moving to the North. In India, RSV mainly peaks in winter in North India and some correlation with low temperature has been observed. Different genotypes of Group A (GA2, GA5, NA1 and ON1) and Group B (GB2, SAB4 and BA) have been described from India. The burden of RSV globally has kept it a high priority for vaccine development. After nearly 50 years of attempts, there is still no licensed vaccine and challenges to obtain a safe and effective vaccine is still facing the scientific community. The data in this review have been extracted from PubMed using the keywords RSV and Epidemiology and India. The data have been synthesised by the authors.

Trends of respiratory syncytial virus sub-types in children hospitalised at a tertiary care centre in Jaipur during 2012-2014.

Respiratory syncytial virus (RSV) causes high mortality and morbidity in infants. The study was planned to determine the trends of RSV sub-types in hospitalised children. Nasopharyngeal aspirate and throat swabs were collected from the hospitalised children up to 5 years of age. Viral nucleic acid was extracted using easyMAG automated extraction system, and real-time reverse transcription polymerase chain reaction was performed. Total positivity for RSV was found to be 25.40%, predominantly for RSV B (20.03%), followed by RSV A (2.90%) and RSV AB mixed infections (2.47%). Palivizumab prophylaxis can be planned to be given to infants from post-monsoon to end of winter.

The clinical and phylogenetic investigation for a nosocomial outbreak of respiratory syncytial virus infection in an adult hemato-oncology unit.

Although many reports have already shown RSV outbreaks among hemato-oncology patients, genomic studies detecting similar RSV strains prior to an outbreak in the hospital are rare. In 2014, the University of the Ryukyus hospital hemato-oncology unit experienced, and successfully managed, a respiratory syncytial virus (RSV) nosocomial outbreak. During the outbreak investigation, genotyping and phylogenetic analysis was used to identify a potential source for the outbreak. Nasopharyngeal swabs were tested for RSV using three tests: (1) rapid antigen test (RAT); (2) reverse transcriptase polymerase chain reaction (PCR); or (3) quantitative PCR (RT-qPCR); a positive PCR reaction was considered a confirmed case of RSV. Phylogenetic analysis of the G protein was performed for outbreak and reference samples from non-outbreak periods of the same year. In total, 12 confirmed cases were identified, including 8 hemato-oncology patients. Patient samples were collected weekly, until all confirmed RSV cases returned RSV negative test results. Median time of suspected viral shedding was 16 days (n = 5, range: 8-37 days). Sensitivity and specificity of the RAT compared with RT-qPCR were 30% and 91% (n = 42). Phylogenetic analysis revealed nine genetically identical strains; eight occurring during the outbreak time period and one strain was detected 1 month prior. A genetically similar RSV detected 1 month before is considered one potential source of this outbreak. As such, healthcare providers should always enforce standard precautions, especially in the hemato-oncology unit.

Respiratory syncytial virus subtype ON1/NA1/BA9 predominates in hospitalized children with lower respiratory tract infections.

Respiratory syncytial virus (RSV) infection is the leading cause of acute respiratory tract disease in children less than 5 years old. The aim of this study was to further elucidate the molecular properties and clinical characteristics of RSV infection. The study sample included 238 patients <5 years old who were hospitalized with clinical symptoms of upper or lower respiratory tract infection (URTI or LRTI) in the Pediatric Department at the First People's Hospital of Chenzhou, South China in 2014. We subjected nasopharyngeal aspirate (NPA) or nasal swab (NS) samples from the patients to indirect fluorescence assay screens. RSV G genes were amplified by reverse transcription-PCR (RT-PCR) and sequenced. Of the 238 patients screened, 64 (26.8%) were confirmed to have RSV infections. Of those 64 confirmed RSV infection cases, 39 (60.9%) had subtype BA9, 13 (20.3%) had the recently identified subtype ON1, 11 (17.2%) had subtype NA1, and 1 (1.6%) had subtype GB2. The predominant presentation was LRTI with coughing, sputum production, fever, and wheezing. RSV subtype NA1 and BA9 infections were found mostly in infants, whereas the age distribution of subtype ON1 infections was more uniform across the age bands. Phylogenetic analysis indicated that, compared with the prototype strain A2, all ON1 and most NA1 isolates had lost one potential N-glycosylation site at amino acid 251 and 249 due to T251K and N249Y substitution, respectively. These findings suggest that NA1, BA9, and ON1 are the dominant RSV subtypes causing respiratory tract infections in young children presenting to the hospital in South China. J. Med. Virol. 89:213-221, 2017. © 2016 Wiley Periodicals, Inc.

Participación de los virus respiratorios en la neumonía del adulto inmunocompetente adquirida en la comunidad / Importance of respiratory virus in immunocompetent adult patients hospitalized with community-acquired pneumonia

Background: Community-acquired pneumonia (CAP) is a relevant worldwide cause of morbidity and mortality in adult population, however its etiology is often not identified and therapy is empirical. Aim: To assess the etiology of CAP in immunocompetent adult hospitalized patients using conventional and molecular diagnostic methods. Material and Methods: We prospectively studied 240 adult patients who were hospitalized for CAP to identify the microbial etiology. Sputum and blood cultures were obtained as well as serology testing for Mycoplasma pneumoniae and Chlamydophila pneumoniae, urinary antigen testing for Legionella pneumophila and Streptococcus pneumoniae, and a nasopharyngeal swab for the detection of sixteen respiratory viruses by reverse transcriptase polymerase chain reaction (RT-PCR). Results: In 100 patients (41.7%) a single respiratory pathogen was identified. In 17 (7.1%) cases, a mixed bacterial and viral infection was detected and no pathogen was identified in 123 cases (51%). The most commonly identified pathogens identified were: influenza virus (15.4%), parainfluenza virus (10.8%), rhinovirus (5%), Streptococcus pneumoniae (5%), respiratory syncytial virus (2.9%) and Mycoplasma pneumoniae (2.5%). Infectious agent detection by RT-PCR provided greater sensitivity than conventional techniques. Viral respiratory infections were more prevalent in older patients with comorbidities and high risk patients, according to the Fine index at hospital admission. The clinical severity and outcome were independent of the etiological agents detected. Conclusions: The use of molecular diagnostic techniques expanded the detection of respiratory viruses in immunocompetent adults hospitalized with CAP.

Frequent Respiratory Viral Infections in Children with Febrile Neutropenia - A Prospective Follow-Up Study.

OBJECTIVE: Febrile neutropenia is common in children undergoing chemotherapy for the treatment of malignancies. In the majority of cases, the cause of the fever is unknown. Although respiratory viruses are commonly associated with this condition, the etiologic significance of this finding remains unclear and is therefore the subject of this study. STUDY DESIGN: Nasopharyngeal aspirates were collected during 87 episodes of febrile neutropenia in children age 0-18 years, being treated at a children's oncology unit between January 2013 and June 2014. Real-time polymerase chain reaction was used to determine the presence of 16 respiratory viruses. Follow-up samples were collected from children who tested positive for one or more respiratory viruses. Rhinoviruses were genotyped by VP4/VP2 sequencing. Fisher's exact test and Mann-Whitney U test were used for group comparisons. RESULTS: At least one respiratory virus was detected in samples from 39 of 87 episodes of febrile neutropenia (45%), with rhinoviruses the most frequently detected. Follow-up samples were collected after a median of 28 days (range, 9-74 days) in 32 of the 39 virus-positive episodes. The respiratory viral infection had resolved in 25 episodes (78%). The same virus was detected at follow-up in one coronavirus and six rhinovirus episodes. Genotyping revealed a different rhinovirus species in two of the six rhinovirus infections. CONCLUSION: The frequency of respiratory viral infections in this group of patients suggests an etiologic role in febrile neutropenia. However, these findings must be confirmed in larger patient cohorts.

[Importance of respiratory virus in immunocompetent adult patients hospitalized with community-acquired pneumonia]. / Participación de los virus respiratorios en la neumonía del adulto inmunocompetente adquirida en la comunidad.

BACKGROUND: Community-acquired pneumonia (CAP) is a relevant worldwide cause of morbidity and mortality in adult population, however its etiology is often not identified and therapy is empirical. AIM: To assess the etiology of CAP in immunocompetent adult hospitalized patients using conventional and molecular diagnostic methods. MATERIAL AND METHODS: We prospectively studied 240 adult patients who were hospitalized for CAP to identify the microbial etiology. Sputum and blood cultures were obtained as well as serology testing for Mycoplasma pneumoniae and Chlamydophila pneumoniae, urinary antigen testing for Legionella pneumophila and Streptococcus pneumoniae, and a nasopharyngeal swab for the detection of sixteen respiratory viruses by reverse transcriptase polymerase chain reaction (RT-PCR). RESULTS: In 100 patients (41.7%) a single respiratory pathogen was identified. In 17 (7.1%) cases, a mixed bacterial and viral infection was detected and no pathogen was identified in 123 cases (51%). The most commonly identified pathogens identified were: influenza virus (15.4%), parainfluenza virus (10.8%), rhinovirus (5%), Streptococcus pneumoniae (5%), respiratory syncytial virus (2.9%) and Mycoplasma pneumoniae (2.5%). Infectious agent detection by RT-PCR provided greater sensitivity than conventional techniques. Viral respiratory infections were more prevalent in older patients with comorbidities and high risk patients, according to the Fine index at hospital admission. The clinical severity and outcome were independent of the etiological agents detected. CONCLUSIONS: The use of molecular diagnostic techniques expanded the detection of respiratory viruses in immunocompetent adults hospitalized with CAP.

RSV infections: State of the art.

Respiratory syncytial virus (RSV) is one of the most common lower respiratory disease in infants and young children worldwide. Despite its long history, a safe and effective cure for RSV remains elusive. Nonetheless, further understanding of RSV pathogenesis and risk factors have led to advances in prophylaxis and management. The leading risk factor for RSV is premature birth, primarily because fewer protective antibodies are transmitted from the mother to the infant. For full-term born infants, susceptibility to RSV increases as maternal IgG titers decline to a nadir at 2 to 3 months of age. Because of the unique pathophysiologic mechanisms involved in this infection, it is likely that future treatment strategies will focus on modulating the host immune response to the virus, rather than virucidal or virustatic molecules.

Detection of respiratory syncytial virus and rhinovirus in healthy infants.

BACKGROUND: Despite the research importance of rhinovirus detection in asymptomatic healthy infants, the literature remains sparse. OBJECTIVE: To investigate the prevalence of respiratory syncytial virus (RSV) and rhinovirus (and its species). METHODS: We conducted a cross-sectional study of 110 healthy, non-hospitalized infants without acute illness at an academic medical center from November 2013 through May 2014. We tested nasal swab specimens by using polymerase chain reaction and genetic sequencing. RESULTS: Overall, the median age was 3.8 months (IQR 2.0-5.1 months), 56 % were male, and 90% were born >37 weeks. RSV was detected in nasal swabs from infants (1.8%). By contrast, rhinovirus was detected in nasal swabs from 16 infants (14.5%). Molecular typing assay revealed rhinovirus species: six rhinovirus-A (5.5%), one rhinovirus-B (0.9%), eight rhinovirus-C (7.3%), and one untypeable (0.9%). CONCLUSIONS: In this cross-sectional study of healthy, community-based infants, RSV was rare (<2%) in nasal swabs, while rhinovirus was detected in 14.5% with a predominance of rhinovirus-A and -C. These finding are important for understanding the clinical significance of rhinovirus detection among infants hospitalized for bronchiolitis.

Detection of 12 respiratory viruses by duplex real time PCR assays in respiratory samples.

Different viruses can be responsible for similar clinical manifestations of respiratory infections. Thus, the etiological diagnosis of respiratory viral diseases requires the detection of a large number of viruses. In this study, 6 duplex real-time PCR assays, using EvaGreen intercalating dye, were developed to detect 12 major viruses responsible for respiratory diseases: influenza A and B viruses, enteroviruses (including enterovirus spp, and rhinovirus spp), respiratory syncytial virus, human metapneumovirus, coronaviruses group I (of which CoV 229E and CoV NL63 are part) and II (including CoV OC43 and CoV HKU1), parainfluenza viruses type 1, 2, 3 and 4, human adenoviruses and human bocaviruses. The 2 target viruses of each duplex reaction were distinguishable by the melting temperatures of their amplicons. The 6 duplex real time PCR assays were applied for diagnostic purpose on 202 respiratory samples from 157 patients. One hundred fifty-seven samples were throat swabs and 45 were bronchoalveolar lavages. The results of the duplex PCR assays were confirmed by comparison with a commercial, validated, assay; in addition, the positive results were confirmed by sequencing. The analytical sensitivity of the duplex PCR assays varied from 10(3) copies/ml to 10(4) copies/ml. For parainfluenza virus 2 only it was 10(5) copies/ml. Seventy clinical samples (35%) from 55 patients (30 children and 25 adults) were positive for 1 or more viruses. In adult patients, influenza A virus was the most frequently detected respiratory virus followed by rhinoviruses. In contrast, respiratory syncytial virus was the most common virus in children, followed by enteroviruses, influenza A virus and coronavirus NL63. The small number of samples/patients does not allow us to draw any epidemiological conclusion. Altogether, the results of this study indicate that the 6 duplex PCR assays described in this study are sensitive, specific and cost-effective. Thus, this assay could be particularly useful to identify the main respiratory viruses directly from clinical samples, after nucleic acid extraction, and, also, to screen a large number of patients for epidemiological studies.

Detecting respiratory viral RNA using expanded genetic alphabets and self-avoiding DNA.

Nucleic acid (NA)-targeted tests detect and quantify viral DNA and RNA (collectively xNA) to support epidemiological surveillance and, in individual patients, to guide therapy. They commonly use polymerase chain reaction (PCR) and reverse transcription PCR. Although these all have rapid turnaround, they are expensive to run. Multiplexing would allow their cost to be spread over multiple targets, but often only with lower sensitivity and accuracy, noise, false positives, and false negatives; these arise by interactions between the multiple nucleic acid primers and probes in a multiplexed kit. Here we offer a multiplexed assay for a panel of respiratory viruses that mitigates these problems by combining several nucleic acid analogs from the emerging field of synthetic biology: (i) self-avoiding molecular recognition systems (SAMRSs), which facilitate multiplexing, and (ii) artificially expanded genetic information systems (AEGISs), which enable low-noise PCR. These are supplemented by "transliteration" technology, which converts standard nucleotides in a target to AEGIS nucleotides in a product, improving hybridization. The combination supports a multiplexed Luminex-based respiratory panel that potentially differentiates influenza viruses A and B, respiratory syncytial virus, severe acute respiratory syndrome coronavirus (SARS), and Middle East respiratory syndrome (MERS) coronavirus, detecting as few as 10 MERS virions in a 20-µl sample.

The impact of viral dynamics on the clinical severity of infants with respiratory syncytial virus bronchiolitis.

The impact of dynamic respiratory syncytial virus (RSV) load on the clinical severity of hospitalized infants with bronchiolitis has not been clarified. Nasopharyngeal aspirates were obtained from 60 infants who were diagnosed with bronchiolitis within 96 hr of wheezing onset upon admission and on days 3, 5, and 7 in the hospital, and 17 respiratory viruses were detected. The RSV load was quantified by real-time qPCR for RSV subtypes A and B at different time points. Scoring criteria were used to evaluate the degree of severity. A total of 40 infants were determined to be RSV-positive, nine were identified as RSV subtype A (RSVA), and 31 were RSV subtype B (RSVB). The peak RSV load was observed upon admission, and the RSV load decreased significantly over time; in addition, this decrease began to have significant differences on day 5. There was a positive correlation between the RSV load and the clinical score (r(2) = 0.121 and P < 0.001). According to the clinical scores, the infants in the severe group tended to have higher RSV loads than those in the moderate and mild groups. Multivariate logistic regression models revealed that the viral load on day 3 was independently associated with the degree of severity. This study elucidated that a higher mean RSV load was associated with a more severe disease and a longer duration of hospitalization and symptoms. This study also clarified RSV replication in infants and provides a theoretical basis for specifying an anti-RSV therapy strategy.