Influenza viruses have a significant impact on respiratory health, resulting in hundreds of thousands of deaths worldwide during seasonal epidemics. In particular, influenza A virus (IAV) has the potential to cause widespread infection, affecting up to 50% of the population during a pandemic and leading to a substantial number of fatalities. The acute onset of upper respiratory tract symptoms, including fever and headache, is a common feature of influenza infections.
One of the primary concerns during IAV infection is the development of secondary bacterial infections, which contribute to mortality rates. However, complications can also manifest in other distant organ systems, such as the cardiovascular and nervous systems. Various risk factors, including age, socioeconomic status, pregnancy, race, and pre-existing medical conditions, increase the likelihood of influenza-related fatalities. Notably, the risk of death associated with influenza rises significantly after the age of 65, largely due to the elevated risk of complications.
Structural Features and Subtypes of Influenza Viruses
Influenza viruses possess an enveloped structure and consist of single-stranded RNA surrounded by nucleoproteins (NP). The main surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA), determine the virus subtype. HA facilitates the attachment of the virus to target cells, while NA plays a crucial role in the release of newly formed virus particles from infected cells.
The HA protein serves as the primary target for antibody-mediated protection as it enables viral entry into host cells. Through a process known as antigenic drift and shift, HA and NA glycoproteins undergo genetic variations, leading to seasonal epidemics and pandemics, respectively. There are 18 different HA subtypes (H1 to H18) and 11 different NA subtypes (N1 to N11). Influenza viruses are categorized into subtypes A, B, and C based on their core proteins, and they differ in their global distribution and pathogenicity. Influenza B and C subtypes primarily affect humans, while influenza A subtypes can infect a wide range of host species, serving as a natural reservoir for infections.
