viral non-structural proteins including ns1, pb1-f2, and pa-x regulate host cellular processes to disable antiviral responses. pb1-f2 aso interacts with pb1 to keep polymerases in the nucleus longer. m1 and nep proteins localize to the nucleus during the later stages of infection, bind to viral rnps and mediate their export to the cytoplasm where they migrate to the cell membrane with the aid of recycled endosomes and are bundled into the segments of the genome. progeny viruses leave the cell by budding from the cell membrane, which is initiated by the accumulation of m1 proteins at the cytoplasmic side of the membrane. the viral genome is incorporated inside a viral envelope derived from portions of the cell membrane that have ha, na, and m2 proteins. at the end of budding, ha proteins remain attached to cellular sialic acid until they are cleaved by the sialidase activity of na proteins. the virion is then released from the cell. the sialidase activity of na also cleaves any sialic acid residues from the viral surface, which helps prevent newly assembled viruses from aggregating near the cell surface and improving infectivity. similar to other aspects of influenza replication, optimal na activity is temperature- and ph-dependent. ultimately, presence of large quantities of viral rna in the cell triggers apoptosis, i.e. programmed cell death, which is initiated by cellular factors to restrict viral replication. antigenic drift and shift antigenic shift, or reassortment, can result in novel and highly pathogenic strains of human influenza. two key processes that influenza viruses evolve through are antigenic drift and antigenic shift. antigenic drift is when an influenza virus's antigens change due to the gradual accumulation of mutations in the antigen's (ha or na) gene. this can occur in response to evolutionary pressure exerted by the host immune response. antigenic drift is especially common for the ha protein, in which just a few amino acid changes in the head region can constitute antigenic drift. the result is the production of novel strains that can evade pre-existing antibody-mediated immunity. antigenic drift occurs in all influenza species but is slower in b than a and slowest in c and d. antigenic drift is a major cause of seasonal influenza, and requires that flu vaccines be updated annually. ha is the main component of inactivated vaccines, so surveillance monitors antigenic drift of this antigen among circulating strains. antigenic evolution of influenza viruses of humans appears to be faster than influenza viruses in swine and equines. in wild birds, within-subtype antigenic variation appears to be limited but has been observed in poultry. antigenic shift is a sudden, drastic change in an influenza virus's antigen, usually ha. during antigenic shift, antigenically different strains that infect the same cell can reassort genome segments with each other, producing hybrid progeny. since all influenza viruses have segmented genomes, all are capable of reassortment. antigenic shift, however, only occurs among influenza viruses of the same genus and most commonly occurs among iavs. in particular, reassortment is very common in aivs, creating a large diversity of influenza viruses in birds, but is uncommon in human, equine, and canine lineages. pigs, bats, and quails have receptors for both mammalian and avian iavs, so they are potential "mixing vessels" for reassortment. if an animal strain reassorts with a human strain, then a novel strain can emerge that is capable of human-to-human transmission. this has caused pandemics, but only a limited number have occurred, so it is difficult to predict when the next will happen. mechanism transmission people who are infected can transmit influenza viruses through breathing, talking, coughing, and sneezing, which spread respiratory droplets and aerosols that contain virus particles into the air. a person susceptible to infection can then contract influenza by coming into contact with these particles. respiratory droplets are relatively large and travel less than two meters before falling onto nearby surfaces. aerosols are smaller and remain suspended in the air longer, so they take longer to settle and can travel further than respiratory droplets. inhalation of aerosols can lead to infection, but most transmission is in the area about two meters around an infected person via respiratory droplets that come into contact with mucosa of the upper respiratory tract. transmission through contact with a person, bodily fluids, or intermediate objects (fomites) can also occur, such as through contaminated hands and surfaces since influenza viruses can survive for hours on non-porous surfaces. if one's hands are contaminated, then touching one's face can cause infection. influenza is usually transmissible from one day before the onset of symptoms to 5–7 days after. in healthy adults, the virus is shed for up to 3–5 days. in children and the immunocompromised, the virus may be transmissible for several weeks. children ages 2–17 are considered to be the primary and most efficient spreaders of influenza. children who have not had multiple prior exposures to influenza viruses shed the virus at greater quantities and for a longer duration than other children. people who are at risk of exposure to influenza include health care workers, social care workers, and those who live with or care for people vulnerable to influenza. in long-term care facilities, the flu can spread rapidly after it is introduced. a variety of factors likely encourage influenza transmission, including lower temperature, lower absolute and relative humidity, less ultraviolet radiation from the sun, and crowding. influenza viruses that infect the upper respiratory tract like h1n1 tend to be more mild but more transmissible, whereas those that infect the lower respiratory tract like h5n1 tend to cause more severe illness but are less contagious. pathophysiology how the different sites of infection (shown in red) of h1n1 and h5n1 influences their transmission and lethality in humans, influenza viruses first cause infection by infecting epithelial cells in the respiratory tract. illness during infection is primarily the result of lung inflammation and compromise caused by epithelial cell infection and death, combined with inflammation caused by the immune system's response to infection. non-respiratory organs can become involved, but the mechanisms by which influenza is involved in these cases are unknown. severe respiratory illness can be caused by multiple, non-exclusive mechanisms, including obstruction of the airways, loss of alveolar structure, loss of lung epithelial integrity due to epithelial cell infection and death, and degradation of the extracellular matrix that maintains lung structure. in particular, alveolar cell infection appears to drive severe symptoms since this results in impaired gas exchange and enables viruses to infect endothelial cells, which produce large quantities of pro-inflammatory cytokines. pneumonia caused by influenza viruses is characterized by high levels of viral replication in the lower respiratory tract, accompanied by a strong pro-inflammatory response called a cytokine storm. infection with h5n1 or h7n9 especially produces high levels of pro-inflammatory cytokines. in bacterial infections, early depletion of macrophages during influenza creates a favorable environment in the lungs for bacterial growth since these white blood cells are important in responding to bacterial infection. host mechanisms to encourage tissue repair may inadvertently allow bacterial infection. infection also induces production of systemic glucocorticoids that can reduce inflammation to preserve tissue integrity but allow increased bacterial growth. the pathophysiology of influenza is significantly influenced by which receptors influenza viruses bind to during entry into cells. mammalian influenza viruses preferentially bind to sialic acids connected to the rest of the oligosaccharide by an α-2,6 link, most commonly found in various respiratory cells, such as respiratory and retinal epithelial cells. aivs prefer sialic acids with an α-2,3 linkage, which are most common in birds in gastrointestinal epithelial cells and in humans in the lower respiratory tract. furthermore, cleavage of the ha protein into ha1, the binding subunit, and ha2, the fusion subunit, is performed by different proteases, affecting which cells can be infected. for mammalian influenza viruses and low pathogenic aivs, cleavage is extracellular, which limits infection to cells that have the appropriate proteases, whereas for highly pathogenic aivs, cleavage is intracellular and performed by ubiquitous proteases, which allows for infection of a greater variety of cells, thereby contributing to more severe disease. immunology cells possess sensors to detect viral rna, which can then induce interferon production. interferons mediate expression of antiviral proteins and proteins that recruit immune cells to the infection site, and they also notify nearby uninfected cells of infection. some infected cells release pro-inflammatory cytokines that recruit immune cells to the site of infection. immune cells control viral infection by killing infected cells and phagocytizing viral particles and apoptotic cells. an exacerbated immune response, however, can harm the host organism through a cytokine storm. to counter the immune response, influenza viruses encode various non-structural proteins, including ns1, nep, pb1-f2, and pa-x, that are involved in curtailing the host immune response by suppressing interferon production and host gene expression. b cells, a type of white blood cell, produce antibodies that bind to influenza antigens ha and na (or hef) and other proteins to a lesser degree. once bound to these proteins, antibodies block virions from binding to cellular receptors, neutralizing the virus. in humans, a sizeable antibody response occurs ~1 week after viral exposure. this antibody response is typically robust and long-lasting, especially for icv and idv. 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 main article: influenza vaccine giving an influenza 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. young infants cannot receive flu vaccines for safety reasons, but they can inherit passive immunity from their mother if inactivated vaccines are administered to the mother during pregnancy. influenza vaccination also helps to reduce the probability of reassortment. in general, influenza vaccines are only effective if there is an antigenic match between vaccine strains and circulating strains. additionally, most commercially available flu vaccines are manufactured by propagation of influenza viruses in embryonated chicken eggs, taking 6–8 months. flu seasons are different in the northern and southern hemisphere, so the who meets twice a year, one for each hemisphere, to discuss which strains should be included in flu vaccines based on observation from ha inhibition assays. other manufacturing methods include an mdck cell culture-based inactivated vaccine and a recombinant subunit vaccine manufactured from baculovirus overexpression in insect cells. antiviral chemoprophylaxis influenza can be prevented or reduced in severity by post-exposure prophylaxis with the antiviral drugs oseltamivir, which can be taken orally by those at least three months old, and zanamivir, which can be inhaled by those above seven years of age. chemoprophylaxis is most useful for individuals at high-risk of developing complications and those who cannot receive the flu vaccine due to contraindications or lack of effectiveness. post-exposure chemoprophylaxis is only recommended if oseltamivir is taken within 48 hours of contact with a confirmed or suspected influenza case and zanamivir within 36 hours. it is recommended that it be offered to people who have yet to receive a vaccine for the current flu season, who have been vaccinated less than two week since contact, if there is a significant mismatch between