ICV-associated hospitalization most commonly occurs in children under the age of 3 and is frequently accompanied by co-infection with another virus or a bacterium, which may increase the severity of disease. When considering all hospitalizations for respiratory illness among young children, ICV appears to account for only a small percentage of such cases. Large outbreaks of ICV infection can occur, so incidence varies significantly.Outbreaks of influenza caused by novel influenza viruses are common. Depending on the level of pre-existing immunity in the population, novel influenza viruses can spread rapidly and cause pandemics with millions of deaths. These pandemics, in contrast to seasonal influenza, are caused by antigenic shifts involving animal influenza viruses. To date, all known flu pandemics have been caused by IAVs, and they follow the same pattern of spreading from an origin point to the rest of the world over the course of multiple waves in a year. Pandemic strains tend to be associated with higher rates of pneumonia in otherwise healthy individuals. Generally after each influenza pandemic, the pandemic strain continues to circulate as the cause of seasonal influenza, replacing prior strains. From 1700 to 1889, influenza pandemics occurred about once every 50–60 years. Since then, pandemics have occurred about once every 10–50 years, so they may be getting more frequent over time. History
It is impossible to know when an influenza virus first infected humans or when the first influenza pandemic occurred.
In other words, people exposed to a certain strain in childhood still possess antibodies to that strain at a reasonable level later in life, which can provide some protection to related strains. There is, however, an "original antigenic sin", in which the first HA subtype a person is exposed to influences the antibody-based immune response to future infections and vaccines. Prevention
Vaccination
Annual vaccination is the primary and most effective way to prevent influenza and influenza-associated complications, especially for high-risk groups. Vaccines against the flu are trivalent or quadrivalent, providing protection against an H1N1 strain, an H3N2 strain, and one or two IBV strains corresponding to the two IBV lineages. Two types of vaccines are in use: inactivated vaccines that contain "killed" (i.e. inactivated) viruses and live attenuated influenza vaccines (LAIVs) that contain weakened viruses. There are three types of inactivated vaccines: whole virus, split virus, in which the virus is disrupted by a detergent, and subunit, which only contains the viral antigens HA and NA. Most flu vaccines are inactivated and administered via intramuscular injection. LAIVs are sprayed into the nasal cavity.Vaccination recommendations vary by country. Some recommend vaccination for all people above a certain age, such as 6 months, whereas other countries recommendation is limited for high at risk groups, such as pregnant women, young children (excluding newborns), the elderly, people with chronic medical conditions, health care workers, people who come into contact with high-risk people, and people who transmit the virus easily.