1. What is HRSV?

Human respiratory syncytial virus (HRSV) is a member of the Pneumoviridae genus and contains a single-stranded non-segmented negative-sense RNA genome approximately 15,200 not in length. Its genome contains 10 open reading frames (ORFs) which encode 11 proteins. From 3' to 5', these genes include two non-structural proteins (NS1 and NS2), two nucleocapsid proteins (N and P), one inner envelope membrane protein (M1), three surface proteins that coat the virion—small hydrophobic (SH), attachment (G), and fusion (F)—M2 which contains overlapping ORFs, resulting in the production of M2.1 and M2.2, and large (L) protein. G and F proteins are the major antigenic proteins. HRSV is pleomorphic, i.e., spherical, asymmetrical, and filamentous, and categorized into subgroups A and B based on the sequence of the G protein. HRSV subtypes co-circulate and often cause reinfection with the same strain.

Although HRSV is a normal childhood disease, it is a current concern in many countries because it appears to be behaving abnormally. Cases usually start to rise in the fall, peaking in the middle of the winter, before decreasing in late spring. However, since COVID-19 occurred, this predictable trend seems to have been challenged.

2.What symptoms can HRSV cause?

Overall, HRSV is a common cause of lower airway infections (which affect both the lungs and airways) in all age groups. The early HRSV infection shows the clinical manifestations such as nasal congestion, runny nose and cough. Most of the symptoms of children with HRSV infection will disappear within 1-2 weeks, and a small part of them will develop a lower respiratory tract infection. Clinical symptoms involve coughing, wheezing. Severe cases can become respiratory failure, which can affect the external organs of the respiratory system, and even result in death.

3.Which people are susceptible to HRSV?

People of all ages may be infected with VRSH, but groups at high risk of VRSH infection are mostly concentrated in children, the elderly and people with low immune function.

4.How does HRSV spread?

Both symptomatic HRSV infected persons and asymptomatic infected persons can transmit the virus:

(i) Contact transmission: It is the most common mode of transmission, primarily by contact of the nasopharyngeal mucosa or ocular mucosa with virus-containing secretions or pollutants.

(ii) HRSV can be transmitted through respiratory droplets or fomites which infect the upper respiratory tract (URT) via nasopharyngeal or conjunctival mucosa. From the URT, HRSV spreads to the lower respiratory tract, primarily infecting polarized ciliated human airway epithelial cells (hAECs), leading to lower respiratory tract infection (LRTI), bronchiolitis, and/or pneumonia. Along with respiratory epithelial cells, HRSV has also been reported to infect CX3CR1+ neonatal regulatory B lymphocytes, primary neurons, alveolar macrophages, dendritic cells, neutrophils, mast cells, and T cells.

5. Global infection of HRSV

Since 2016, WHO has carried out HRSV monitoring in 14 member countries based on the Global Influenza Surveillance and Response System (GISRS), including the United Kingdom, Russia, Brazil, Canada, South Africa, India. The USA, Japan and other countries also followed up on the HRSV.

It is reported that the positive rate of HRSV detection in Canada was significantly higher than that in the same period of previous years in the autumn and winter of 2022. According to the data released by the US CDC, the positive rate of HRSV nucleic acid detection in the United States increased significantly in the autumn and winter of 2022, with the peak ahead of previous years. 

Despite the known disease burden, there is no FDA-approved HRSV vaccine available. As of July 2019, there were 121 clinical trials evaluating HRSV vaccines. In brief, vaccine platforms including virus particle based, nucleic acid, live attenuated, subunit and vector based are being pursued for infants, children, and the elderly as well as maternal vaccines to protect newborn infants.

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