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Vaccine Effectiveness Studies plus icon.
Why CDC Estimates Vaccine Effectiveness.How Vaccine Effectiveness and Efficacy are Measured.Randomized Assessment of Influenza Vaccine Efficacy Network (RAIVEN).Respiratory Virus Transmission Network (RVTN).Influenza and Other Viruses in the Acutely Ill (IVY).Hospitalized Adult Influenza Vaccine Effectiveness Network (HAIVEN).CDC’s Vaccine Effectiveness Networks plus icon.Historical Reference of Vaccine Doses Distributed.Frequently Asked Questions on Vaccine Supply.Vaccine Supply & Distribution plus icon.Flu Vaccine and People with Egg Allergies.Selecting Viruses for the Seasonal Influenza Vaccine.Live Attenuated Influenza Vaccine (LAIV)/Nasal Spray Vaccine.Who Should & Who Should NOT Get Vaccinated.Flu and COVID-19 Vaccine Coadministration.Mixing Oseltamivir Capsules for Children.Adults with Chronic Conditions plus icon.Who is at Higher Risk of Flu Complications plus icon.2022-2023 Preliminary In-Season Burden Estimate.Reconstruction of the 1918 Influenza Pandemic Virus.Understanding Influenza Viruses plus icon.Type A viruses undergo both antigenic drift and shift and are the only flu viruses known to cause pandemics, while flu type B viruses change only by the more gradual process of antigenic drift. Flu pandemics occur rarely there have been four flu pandemics in the past 100 years. While flu viruses change all the time due to antigenic drift, antigenic shift happens less frequently. When shift happens, most people have little or no immunity against the new virus. Such a “shift” occurred in the spring of 2009, when an H1N1 virus with genes from North American Swine, Eurasian Swine, humans and birds emerged to infect people and quickly spread, causing a pandemic.
Such animal-origin viruses can contain HA or HA/NA combinations that are different enough from human viruses that most people do not have immunity to the new (e.g., novel) virus. Shift can happen if a flu virus from an animal population gains the ability to infect humans. Antigenic shift can result in a new flu A subtype. Antigenic ShiftĪnother type of change is called “antigenic shift.” Shift is an abrupt, major change in a flu A virus, resulting in new HA and/or new HA and NA proteins in flu viruses that infect humans. Drift is also a primary reason why the composition of flu vaccines for use in the Northern and Southern Hemispheres is reviewed annually and updated as needed to keep up with evolving flu viruses. As a result, a person becomes susceptible to flu infection again, as antigenic drift has changed the virus’ antigenic properties enough that a person’s existing antibodies won’t recognize and neutralize the newer flu viruses.Īntigenic drift is an important reason why people can get flu more than one time. When antigenic drift occurs, the body’s immune system may not recognize and prevent sickness caused by the newer flu viruses. It also is possible for a single change in a particularly important location on the HA to result in antigenic drift. However, the small changes associated with antigenic drift can accumulate over time and result in viruses that are antigenically different (further away on the phylogenetic tree). This means that antibodies your immune system creates against one flu virus will likely recognize and respond to antigenically similar flu viruses (this is called “cross-protection”). Flu viruses that are closely related to each other usually have similar antigenic properties. The small changes that occur from antigenic drift usually produce viruses that are closely related to one another, which can be illustrated by their location close together on a phylogenetic tree. The nasal spray flu vaccine (LAIV) may target both the HA and NA of a flu virus. Most flu shots are designed to target the HA surface proteins/antigens of flu viruses.
The changes associated with antigenic drift happen continually over time as flu viruses replicate (i.e., infect a host and make copies of themselves). The HA and NA surface proteins of influenza viruses are “antigens,” which means they are recognized by the immune system and are capable of triggering an immune response, including production of antibodies that can block infection. One way flu viruses change is called “antigenic drift.” Drift consists of small changes (or mutations) in the genes of influenza viruses that can lead to changes in the surface proteins of the virus, HA (hemagglutinin) and NA (neuraminidase). Influenza (flu) viruses are constantly changing.
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