Respiratory tract infections can be divided into upper respiratory tract infections and lower respiratory tract infections according to their manifestations. They are common and frequently-occurring diseases in children under 5 years old. The clinical symptoms of upper respiratory tract infections mainly include sore throat, fever, cough, nasal congestion, runny nose, and fatigue. Due to the lack of obvious specificity of early symptoms and various factors such as poor nursing during the disease period, children with low immunity may develop bronchitis and pneumonia (lower respiratory tract infections), which may endanger the lives of patients. Therefore, early diagnosis of respiratory tract infections is of great significance.
There are many types of pathogens that cause respiratory tract infections, 90% of which are caused by viruses-common ones include novel coronavirus, influenza virus, respiratory syncytial virus, parainfluenza virus, adenovirus, etc., and there are differences in the prevalence of viruses in different seasons.
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SARS-CoV-2 has four main structural proteins: spike protein (S protein), nucleocapsid protein (N protein), membrane protein (M protein), and envelope protein (E protein). The S protein is a very important surface protein of coronaviruses, which is closely related to the virus's infectivity. The S protein contains S1, S2, and receptor binding domains (RBD). The N protein is abundant in coronaviruses and is a highly immunogenic protein that participates in genome replication and cell signaling pathway regulation. The S protein and N protein are key raw materials for COVID-19 immunoassay kits and have important value for the diagnosis and screening of COVID-19.
There are four types of influenza viruses: A, B, C, and D. Human influenza A and B viruses cause seasonal epidemics (called flu seasons). Influenza virus A is the only known influenza virus that causes influenza pandemics. Influenza viruses have an enveloped structure. Inside the virus, nucleoprotein (NP) binds to single-stranded genome fragments for RNA transcription, replication, and packaging. The virus envelope is a lipid bilayer membrane. Glycoproteins called HA (hemagglutinin) and NA (neuraminidase) are crucial for combating the virus's immune response. Influenza A viruses are classified into subtypes based on the properties of the HA and NA surface proteins. There are 18 different HA subtypes and 11 different NA subtypes. Subtypes are named by combining the numbers of H and N, such as A(H1N1). Due to antigenic drift of influenza viruses or transfer between HA and NA proteins leading to high mutation rates, vaccine strains are selected and updated every year based on global virus circulation monitoring throughout the year. Seasonal flu vaccines use 3-4 strains of influenza viruses, including H1N1 and H3N2 influenza viruses, with A-type influenza and one or two prevalent B-type influenza virus strains each season.
The nucleoprotein (NP) of influenza virus is a structural protein that plays a critical role in virus replication and host adaptation. Antibodies against NP protein are commonly used for immunological detection of influenza viruses, including ELISA, lateral flow assay (LFA), and direct fluorescent antibody testing.
Hemagglutinin (HA) contains antigenic sites recognized by the host immune system, cleavage sites cleaved by host proteases, receptor binding sites attached to sialic acid receptors on target cells, and fusion peptides that mediate membrane fusion. Hemagglutinin (HA) glycoprotein has become an important focus of influenza research due to its role in antigenic drift and shift.
Neuraminidase (NA) is an enzyme on the surface of type A and B influenza viruses. They cleave sialic acid residues from glycoproteins and are essential for influenza virus replication. In addition to mutations caused by antigenic drift, NA of type A influenza viruses can exist in different forms. It is the main target of anti-influenza antiviral drugs.
Respiratory syncytial virus (RSV) is the most common cause of lower respiratory tract infections in infants and children worldwide, which can lead to high morbidity and considerable mortality. In the elderly, RSV manifests as recurrent infections, leading to fever, pneumonia, and even death. According to CDC data, RSV alone causes 58,000 hospitalizations of children under 5 years old and 177,000 hospitalizations of people aged 65 or older in the United States each year.
RSV is a negative-strand RNA virus belonging to the family Pneumoviridae. A and B are the two main subtypes of RSV. Although strains of both subtypes often co-infect, only one subtype usually dominates in infection. RSV is an enveloped virus with a particle diameter of 100-300 nm. Its genome is about 15.2 kb long and contains 10 genes encoding 11 proteins. Two transmembrane proteins G protein and F protein are the two main protective antigens of the virus.
Envelope protein: G glycoprotein (mediates virus-cell contact, highly variable), Fusion (F) glycoprotein (mediates host receptor fusion, mediates envelope formation, HLA and T cell recognition targets, highly conserved), and Small hydrophobic (SH) protein
Structural protein: Large (L) protein, Nucleocapsid (N) (wraps viral genome), Phosphoprotein (P), Matrix (M), M2-1, and M2-2
Non-structural protein: NS1, NS2