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SummarySeasonal influenza (flu) vaccines are designed to protect against the four main groups of flu Type A and B viruses that research indicates are most likely to spread and cause illness among people during the upcoming flu season. All U.S. flu vaccines protect against a flu A(H1) virus, a flu A(H3) virus, a flu B/Yamagata lineage virus and a flu B/Victoria lineage virus. Each of these four vaccine virus components are selected based on the following:
There are currently 144 national influenza centers in over 114 countries that conduct year-round surveillance for flu viruses as part of the World Health Organization (WHO) Global Influenza Surveillance and Response System (GISRS).external icon This involves receiving and testing thousands of flu virus samples from patients. For human seasonal flu surveillance, the laboratories send representative viruses to five* of the seven WHO Collaborating Centers for Influenza, which are located in the following places:
*Note: Two of the WHO Collaborating Centers: the one in Memphis, Tennessee and the one in Koltsovo, Russian Federation only collect flu virus specimens from animals and do not participate in human seasonal flu surveillance. Twice a year, the WHO organizes a consultation with the Directors of the seven WHO Collaborating Centers, Essential Regulatory Laboratories and representatives of key national laboratories and academies. They review the results of surveillance, laboratory, and clinical studies, and the availability of flu vaccine viruses and make recommendations on the composition of flu vaccines. These meetings take place in February for selection of the upcoming Northern Hemisphere’s seasonal flu vaccines and in September for the Southern Hemisphere’s flu vaccines. The WHO vaccine composition committee, which is comprised of independent technical advisors, i.e., directors of each of the WHO collaborating centers and essential regulatory laboratories (ELRs), meets to present global flu data and recommend specific vaccine viruses for trivalent (three-virus component) and quadrivalent (four-virus component) flu vaccines. Next, each country makes its own decision about which viruses should be included in flu vaccines licensed in the country. In the United States, the Food and Drug Administration (FDA)’s Vaccines and Related Biological Products Advisory Committee (VRBPAC) makes the final decision about vaccine viruses for domestic flu vaccines. Information about the circulation of flu viruses and available vaccine viruses is summarized and presented to an advisory committee of the FDA in February or March of each year for the U.S. decision about which viruses to include in the upcoming season’s flu vaccine. Questions & AnswersWhat is CDC Influenza Division’s role in vaccine virus selection?As one of seven WHO Global Influenza Surveillance and Response System’s (GISRS) WHO Collaborating Centers, CDC’s Influenza Division receives and tests thousands of influenza viruses from around the world each year and collaborates with other WHO Collaborating Centersexternal icon and national influenza centers in the annual seasonal vaccine virus selection process for the northern and southern hemispheres. CDC plays a major role in identifying and testing the flu viruses that are spreading globally through its global surveillance activities and then preparing candidate vaccine viruses (CVVs). The Influenza Division provides this information during the WHO vaccine consultation meetings and participates in discussions regarding which viruses should be recommended by WHO and, in the United States, FDA VRBPAC decisions for inclusion in flu vaccines. What are the main factors that influence which viruses are selected for use in flu vaccine production each year?The flu viruses in seasonal flu vaccines are selected each year based on a variety of data, including the following:
Availability of good vaccine candidate viruses: Candidate vaccine viruses (CVVs) must be tested and available in time to allow for manufacturers to produce the large amount of vaccine virus needed to make flu vaccine. CVVs are chosen to protect against the viruses likely to circulate during the upcoming season. There are numerous factors that can make getting a good vaccine virus for vaccine production challenging, including both scientific issues and time constraints. For example, CVVs must be prepared and made available to vaccine manufacturers early enough for them to produce vaccine in time for flu season. In some years, certain flu viruses may not circulate until later in the flu season, making it difficult to prepare a candidate vaccine virus in time for vaccine production. This can make vaccine virus selection very challenging. Historically, vaccine viruses were required by FDA to be isolated and grown in chicken eggs, but now the FDA allows vaccine viruses to be grown in cells, too. Some flu viruses, like flu A(H3N2) viruses grow poorly in eggs, making it challenging to get good CVVs for vaccine production. CVVs grown in eggs can develop changes caused by virus adaptation to eggs and such changes can reduce antigenic similarity of vaccine virus to circulating viruses. Vaccines that use newer technologies, such as cell-based or recombinant flu vaccines, are not affected by egg-adapted changes. In some years certain influenza viruses may not circulate until later in the influenza season, making it difficult to prepare a candidate vaccine virus in time for vaccine production. This can make vaccine virus selection very challenging. How does CDC determine if the vaccine virus will protect against a circulating virus?CDC’s Flu Division collects and reports information on flu activity in the United States each week. Laboratory studies of circulating flu viruses allow CDC to evaluate the ability of vaccine viruses to target and neutralize circulating flu viruses each season. Genetic characterization can inform decision-making for vaccine virus selection based on similarities between the genomes of vaccine viruses and circulating flu viruses. Antigenic characterization can indicate if circulating viruses have evolved in such a way that the current flu vaccine may not produce an optimal immune response against them. Antigenic characterization plays an important role in early assessments of how well vaccine may work by looking at how well ferret antibodies can target and neutralize circulating flu viruses. Because the ferrets used in antigenic characterization testing have never been vaccinated or infected with influenza previously, they produce very specific and narrowly focused antibodies. However, one limitation of antigenic characterization data from ferrets is that it does not account for the human experience and how people’s prior flu infections and vaccinations can influence the way their immune systems respond to current flu vaccines. As a result, CDC also uses human serology studies that involve collecting blood samples from people. As part of these studies, people’s antibodies (found in their blood) are tested to determine whether they target and neutralize circulating flu viruses, both before and after vaccination. Such studies better account for the role prior infections and vaccinations play in the human immune response to vaccination. The most direct measure of the protection received from flu vaccination comes from vaccine effectiveness studies. For more information, see Vaccine Effectiveness – How Well Does the Flu Vaccine Work? For more information about CDC’s surveillance and to access the weekly reports, visit Flu Activity and Surveillance. What happens after a recommendation has been made about which viruses should be included in the seasonal flu vaccine?As soon as recommendations have been made about what viruses should be included in the vaccine, private sector manufacturers begin the process of producing vaccine. In fact, some manufacturers may start growing one or more viruses for use in production of vaccine even before a WHO recommendation or FDA decision is made on the vaccine viruses. This gives manufacturers more time to make vaccine ahead of the season; the more time a manufacturer has to make vaccine, the greater the number of doses that can be produced. How long does it take to manufacture seasonal flu vaccine?Currently, it takes at least six months to produce large quantities of flu vaccine. For information about flu vaccine production, see How Influenza (Flu) Vaccines Are Made. Top of Page |