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.
The envelope is reinforced structurally by matrix proteins on the interior that enclose the RNPs, and the envelope contains HA and NA (or HEF) proteins extending outward from the exterior surface of the envelope. HA and HEF proteins have a distinct "head" and "stalk" structure. M2 proteins form proton ion channels through the viral envelope that are required for viral entry and exit. IBVs contain a surface protein named NB that is anchored in the envelope, but its function is unknown. Life cycle
The viral life cycle begins by binding to a target cell. Binding is mediated by the viral HA proteins on the surface of the evelope, which bind to cells that contain sialic acid receptors on the surface of the cell membrane. For N1 subtypes with the "G147R" mutation and N2 subtypes, the NA protein can initiate entry. Prior to binding, NA proteins promote access to target cells by degrading mucous, which helps to remove extracellular decoy receptors that would impede access to target cells. After binding, the virus is internalized into the cell by an endosome that contains the virion inside it. The endosome is acidified by cellular vATPase to have lower pH, which triggers a conformational change in HA that allows fusion of the viral envelope with the endosomal membrane. At the same time, hydrogen ions diffuse into the virion through M2 ion channels, disrupting internal protein-protein interactions to release RNPs into the host cells cytosol.