7.5.4: Influenza Viruses
Of the 4 classifications of flu viruses (A, B, C, and D), only A and B tend to cause illness in humans, with a variety of these viruses causing the typical “seasonal flu” (World Health Organization: WHO, 2023). The subtypes of flu virus that circulate in a given community vary each year, and thus the flu vaccine efficacy also varies depending on the “popular” strains in that community and season. Even so, flu vaccines have been shown to be between 40-60% effective at reducing doctor visits, and also reduce severe illness, hospitalization, and admittance into the ICU for various age groups - especially children and older adults ( Vaccine Effectiveness: How Well Do Flu Vaccines Work? , 2024).
Type A influenza viruses also include zoonotic strains that are known to have pandemic potential, including “swine flu” (H1N1) and “bird flu” or “avian flu” (H5N1). The swine flu is particularly deadly, causing a mortality rate close to 50%, whereas the avian flu was much less deadly but easily spread between humans. There is significant concern that a new influenza virus could emerge that employs both properties of virulence and severe disease, which could cause a deadly pandemic around the world ( Emerging Infectious Diseases , Baylor College of Medicine, n.d.).