Background and aim: The majority of seasonal influenza vaccines are trivalent, containing two A virus strains (H1N1 and H3N2) and one B virus strain. The co-circulation of two distinct lineages of B viruses can lead to mismatch between the influenza B virus strain recommended for the trivalent seasonal vaccine and the circulating B virus. This has led some manufacturers to produce quadrivalent influenza vaccines containing one strain from each B lineage in addition to H1N1 and H3N2 strains. However, it is also important to know whether vaccines containing a single influenza B strain can provid

Trivalent Influenza Vaccines | Influenza (Flu) | CDC Skip directly to site content Skip directly to search An official website of the United States government Here's how you know Official websites use .gov A .gov website belongs to an official government organization in the United States. Secure .gov websites use HTTPS A lock ( ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites. Trivalent Influenza Vaccines For Everyone May 11, 2026 At a glance All flu vaccines in the United States are trivalent (three component) vaccines since the 2024–2025 flu season. Learn about the switch from quadrivalent (four component) flu vaccines to trivalent flu vaccines. Everyone ages 6 months and up should get an annual flu vaccine. About trivalent flu vaccines Trivalent flu vaccines protect against three different influenza viruses. Trivalent flu vaccines in the United States include vaccine viruses or viral proteins from one influenza A(H1N1) virus, one influenza A(H3N2) virus, and one influenza B/Victoria lineage virus. Quadrivalent to trivalent flu vaccine transition Quadrivalent flu vaccines include vaccine viruses or viral proteins from four viruses: one influenza A(H1N1) virus, one influenza A(H3N2) virus, one influenza B/Victoria lineage virus and one B/Yamagata lineage virus. They differ from trivalent flu vaccines because two influenza B viruses are represented in the vaccine (one from each lineage). Trivalent flu vaccines include three vaccine viruses, with only one influenza B virus. They do not include an influenza B/Yamagata lineage vaccine virus or viral protein component. Beginning with the 2024-2025 flu season, the influenza B/Yamagata vaccine component in U.S. flu vaccines was removed because influenza B/Yamagata viruses have not been detected by global flu surveillance after March 2020. Because B/Yamagata viruses are not actively circulating in people, the risk of infection with B/Yamaga

s from the two circulating A viruses and either B Yamagata or B Victoria – however, selecting the B strain for inclusion in these trivalent vaccines has variable success. The alternative approach is to include both B strains in a quadrivalent vaccine. Immunological studies of such vaccines show non-inferiority with a trivalent vaccine comparator, and significant superiority to the additional B strain. Quadrivalent vaccines are more expensive than trivalent preparations but theoretical evidence would suggest they are likely to be more effective and therefore play a much greater role in national immunisation programmes in the future. KEYWORDS: Trivalent Quadrivalent Influenza vaccine Introduction Influenza disease is caused by a small number of co-circulating A subtypes and B types and so the usual approach by manufacturers has been to develop inter-pandemic vaccines based on the 2 most common A subtypes and the commonest B virus – i.e. a trivalent vaccine. 1,2 More recently, influenza vaccines that protect against an additional B type have been produced – i.e., a four component or quadrivalent vaccine, and this paper explores the rationale and merits of this development. Influenza virus The influenza virus was first isolated in 1933 – this was subsequently classified as influenza A virus. A separate, serologically distinct virus was isolated in 1940 and designated influenza B. Influenza C virus was isolated in 1950. 2 Influenza viruses belong to the family orthomyxoviridae and are widely distributed among the animal kingdom. They are divided into different types based on variation in their expressed matrix and nucleoproteins – only types A, B and C affect humans with the latter 2 being unique human pathogens. 2,3 Type A viruses are further divided into subtypes based on differences in their haemagglutinin (HA) and neuraminidase (NA) surface glycoproteins: types B and C are not subtyped. For type A there are 16 different HA (1-16) and 9 different NA (1-9) proteins, wi

groups. Vaccines against the flu are trivalent or quadrivalent, providing protection against an H1N1 strain, an H3N2 strain, and one or two influenza B virus Influenza, commonly known as the flu, is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptoms begin one to four (typically two) days after exposure to the virus and last for about two to eight days. Diarrhea and vomiting can occur, particularly in children 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 influenza B virus strains…