Shopes research reinvigorated research on human influenza, and many advances in virology, serology, immunology, experimental animal models, vaccinology, and immunotherapy have since arisen from influenza research. Just two years after influenza viruses were discovered, in 1933, IAV was identified as the agent responsible for human influenza. Subtypes of IAV were discovered throughout the 1930s, and IBV was discovered in 1940.During the Second World War, the US government worked on developing inactivated vaccines for influenza, resulting in the first influenza vaccine being licensed in 1945 in the United States. ICV was discovered two years later in 1947. In 1955, avian influenza was confirmed to be caused by IAV. Four influenza pandemics have occurred since WWII, each less severe than the 1918 pandemic. The first of these was the Asian flu from 1957 to 1958, caused by an H2N2 strain and beginning in Chinas Yunnan province. The number of deaths probably exceeded one million, mostly among the very young and very old. Notably, the 1957 pandemic was the first flu pandemic to occur in the presence of a global surveillance system and laboratories able to study the novel influenza virus. After the pandemic, H2N2 was the IAV subtype responsible for seasonal influenza. The first antiviral drug against influenza, amantadine, was approved for use in 1966, with additional antiviral drugs being used since the 1990s.In 1968, H3N2 was introduced into humans as a result of a reassortment between an avian H3N2 strain and an H2N2 strain that was circulating in humans.
Additionally, febrile seizures and Reye syndrome can occur, most commonly in children. Influenza-associated encephalopathy can occur directly from central nervous system infection from the presence of the virus in blood and presents as suddent onset of fever with convulsions, followed by rapid progression to coma. An atypical form of encephalitis called encephalitis lethargica, characterized by headache, drowsiness, and coma, may rarely occur sometime after infection. In survivors of influenza-associated encephalopathy, neurological defects may occur. Primarily in children, in severe cases the immune system may rarely dramatically overproduce white blood cells that release cytokines, causing severe inflammation.People who are at least 65 years of age, due to a weakened immune system from aging or a chronic illness, are a high-risk group for developing complications, as are children less than one year of age and children who have not been previously exposed to influenza viruses multiple times. Pregnant women are at an elevated risk, which increases by trimester and lasts up to two weeks after childbirth. Obesity, in particular a body mass index greater than 35–40, is associated with greater amounts of viral replication, increased severity of secondary bacterial infection, and reduced vaccination efficacy. People who have underlying health conditions are also considered at-risk, including those who have congenital or chronic heart problems or lung (e.g. asthma), kidney, liver, blood, neurological, or metabolic (e.g. diabetes) disorders, as are people who are immunocompromised from chemotherapy, asplenia, prolonged steroid treatment, splenic dysfunction, or HIV infection. Current or past tobacco use also places a person at risk.