# Influenza B virus
**Influenza B virus** is one of the four types of influenza viruses (A, B, C, and D) that cause seasonal epidemics of respiratory disease in humans almost every winter, alongside influenza A[1](https://www.cdc.gov/flu/about/viruses-types.html). Unlike influenza A, influenza B viruses are not divided into subtypes; they are classified into two antigenically and phylogenetically distinct lineages, B/Victoria and B/Yamagata, named after the reference viruses B/Victoria/2/87 and B/Yamagata/16/88[1](https://www.cdc.gov/flu/about/viruses-types.html) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5790164/). Influenza B viruses contribute roughly one third of the global influenza disease burden and are associated particularly with severe disease in children[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5790164/).
| Key fact | Detail |
|---|---|
| Classification | No subtypes; two lineages, B/Victoria and B/Yamagata, further divided into HA clades and sub-clades[1](https://www.cdc.gov/flu/about/viruses-types.html) |
| Divergence | The two lineages diverged in the 1970s and have co-circulated globally since at least 2002[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5790164/) |
| Disease burden | Approximately one third of the global influenza disease burden; hospitalization and mortality lower than A(H3N2) but higher than seasonal A(H1N1)[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5790164/) |
| Evolutionary pace | Genetic and antigenic change is slower than in influenza A, especially A(H3N2)[1](https://www.cdc.gov/flu/about/viruses-types.html) |
| B/Yamagata status | Not detected by global surveillance after March 2020; WHO no longer recommends a Yamagata vaccine component[1](https://www.cdc.gov/flu/about/viruses-types.html) • [3](https://www.who.int/publications/m/item/recommended-composition-of-influenza-virus-vaccines-for-use-in-the-2026-southern-hemisphere-influenza-season) |
| Current vaccines | Seasonal vaccine recommendations now specify one A(H1N1), one A(H3N2), and one B/Victoria lineage virus[1](https://www.cdc.gov/flu/about/viruses-types.html) |
## Classification and structure
Influenza viruses are RNA viruses of the family [Orthomyxoviridae](https://www.edgepedia.org/orthomyxoviridae). Types A and B, the predominant types causing human disease, both have genomes of eight separate RNA segments, and reassortment of these segments contributes to antigenic variability[4](https://www.ecdc.europa.eu/en/seasonal-influenza/facts/factsheet). Influenza A viruses are subdivided by their hemagglutinin (HA) and neuraminidase (NA) surface proteins into 18 HA and 11 NA subtypes; influenza B viruses lack this subtype system and are instead distinguished by lineage[1](https://www.cdc.gov/flu/about/viruses-types.html) • [4](https://www.ecdc.europa.eu/en/seasonal-influenza/facts/factsheet).
Both lineages are further classified into HA genetic clades and sub-clades, such as clades V1A for Victoria and Y1, Y2, and Y3 for Yamagata, which public health experts use to track the proportions of viruses in circulation[1](https://www.cdc.gov/flu/about/viruses-types.html).
The virus was first isolated in New York State in 1940 and was then traced back to a California epidemic in 1936[5](https://www.theatlantic.com/magazine/archive/1944/03/how-bad-is-the-flu/303636/).
## The two lineages
The B/Victoria and B/Yamagata lineages diverged in the 1970s and have co-circulated globally since at least 2002, often alternating in regional dominance[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5790164/). Because the predominant lineage in a given year was difficult to predict at the time of vaccine virus selection, and mismatches between the trivalent vaccine's single B component and the circulating lineage occurred, quadrivalent vaccines containing representatives of both lineages were developed and recommended[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5790164/) • [6](https://pmc.ncbi.nlm.nih.gov/articles/PMC11569178/).
**Different evolutionary styles.** Genome-wide analysis of more than 2,500 influenza B genomes sampled worldwide between 1987 and 2015 showed that the lineages maintain their diversity in different ways. Yamagata-lineage diversity results from co-circulation of two antigenically distinct groups that also segregate genetically across the whole genome, with alternating dominance between the antigenic groups; Victoria-lineage diversity stems from geographic segregation of different genetic clades, with antigenic drift of a single lineage[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5790164/).
Reassortment between the lineages is frequent but selective. Of the eight genome segments, only PB1, PB2, and HA have maintained separate Victoria and Yamagata lineages; circulating strains carry PB1, PB2, and HA segments derived entirely from one lineage or the other, while the other segments have repeatedly reassorted between lineages, reducing genetic diversity[7](https://arxiv.org/abs/1403.1094). This pattern is attributed to genomic incompatibility among viral genome segments[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5790164/).
The hemagglutinin gene of both lineages evolves at a slower pace than in influenza A virus lineages, but the lineages differ in turnover: Victoria-lineage lineages turn over on average every 3.9 to 5.1 years, compared with 6.3 to 7.2 years for Yamagata[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC6955377/).
## Epidemiology and severity
Influenza B viruses cause epidemics worldwide each year. Rates of hospitalization and mortality attributed to influenza B are lower than for influenza A subtype A(H3N2) but higher than for the seasonal A(H1N1) subtype[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC5790164/). Type B infections are generally less common and usually milder than influenza A(H3N2)[4](https://www.ecdc.europa.eu/en/seasonal-influenza/facts/factsheet).
A repeated cross-sectional study of US influenza-associated hospitalizations from 2010 to 2019 found that, after adjustment for season, vaccination status, age, and surveillance site, influenza B infection was associated with slightly higher odds of severe outcomes than influenza A(H3N2): an adjusted odds ratio of 1.06 (95% CI 1.01 to 1.12) for ICU admission, 1.14 (1.05 to 1.24) for mechanical ventilation or ECMO use, and 1.18 (1.07 to 1.31) for death[9](https://www.thelancet.com/article/S2666524723001878/pdf).
The two lineages also show contrasting age distributions of cases, which immune imprinting and lineage-specific protection help shape. A study of lineage-specific protection found evidence of protection against B/Yamagata in people for whom it was the first influenza B infection, while a similar effect for B/Victoria could not be estimated from the data[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC8280188/).
## Extinction of B/Yamagata and the return to trivalent vaccines
B/Yamagata-lineage viruses have not been detected by global influenza surveillance after March 2020[1](https://www.cdc.gov/flu/about/viruses-types.html). The continued absence of confirmed naturally occurring B/Yamagata detections after that date is indicative, in the WHO's assessment, of a very low risk of infection by these viruses[11](https://cdn.who.int/media/docs/default-source/influenza/who-influenza-recommendations/vcm-sh-2025/a.-26-september-2025-recommended-composition-of-influenza-virus-vaccines-for-use-in-the-2026-southern-hemisphere-influenza-season---full-report.pdf?download=true&sfvrsn=eba6ad63_5).
The WHO influenza vaccine composition advisory committee concluded in September 2023 that inclusion of a B/Yamagata antigen in influenza vaccines is no longer warranted, and every effort should be made to exclude the component as soon as practically possible[12](https://www.cdc.gov/acip/downloads/slides-2023-10-25-26/07-influenza-kondor-508.pdf). There will no longer be updated recommendations for the B/Yamagata lineage component[3](https://www.who.int/publications/m/item/recommended-composition-of-influenza-virus-vaccines-for-use-in-the-2026-southern-hemisphere-influenza-season). The change has been implemented across regulatory systems: US flu vaccines dropped the B/Yamagata component beginning with the 2024-2025 season[13](https://www.cdc.gov/flu/vaccine-types/trivalent.html), the UK live attenuated influenza vaccine moved to trivalent for 2024 to 2025[14](https://www.gov.uk/government/publications/flu-vaccines-2025-to-2026-jcvi-advice/jcvi-statement-on-influenza-vaccines-for-2025-to-2026), and all 2026 southern hemisphere seasonal influenza vaccines available in Australia are trivalent[15](https://www.tga.gov.au/resources/publication/corporate-reports/2026-seasonal-influenza-vaccines). Where the transition to trivalent vaccines is not yet complete, quadrivalent vaccines retain a fourth component, a B/Phuket/3073/2013-like (B/Yamagata lineage) virus[3](https://www.who.int/publications/m/item/recommended-composition-of-influenza-virus-vaccines-for-use-in-the-2026-southern-hemisphere-influenza-season).
Manufacture and use of vaccines containing B/Yamagata lineage viruses also pose a theoretical risk of reintroducing the lineage into the population, a consideration in the WHO's recommendation[12](https://www.cdc.gov/acip/downloads/slides-2023-10-25-26/07-influenza-kondor-508.pdf).
A modeling study combining mechanistic transmission modeling with worldwide surveillance data concluded that Yamagata's extinction is most consistent with a lower intrinsic transmissibility than that of the surviving Victoria lineage[16](https://www.medrxiv.org/content/10.64898/2026.07.22.26358639v1.full).
## Influenza B in recent seasons
Influenza B continues to circulate at lower levels than influenza A. In the 2025-26 US season, 88% of subtyped influenza A-positive specimens were A(H3N2), and among sequenced influenza B viruses, 64% were clade C.3.1, which differed antigenically from that season's influenza B vaccine virus[17](https://cdc.gov/mmwr/volumes/75/wr/mm7509a2.htm). Interim estimates of 2025-26 vaccine effectiveness against influenza B outpatient visits ranged from 45% to 71% among children and adolescents and was 63% among adults[17](https://cdc.gov/mmwr/volumes/75/wr/mm7509a2.htm).
## References
1. [Types of Influenza Viruses, CDC](https://www.cdc.gov/flu/about/viruses-types.html)
2. [Langat et al. (2017). Genome-wide evolutionary dynamics of influenza B viruses on a global scale. PLoS Pathogens.](https://pmc.ncbi.nlm.nih.gov/articles/PMC5790164/)
3. [Recommended composition of influenza virus vaccines for use in the 2026 southern hemisphere influenza season, WHO](https://www.who.int/publications/m/item/recommended-composition-of-influenza-virus-vaccines-for-use-in-the-2026-southern-hemisphere-influenza-season)
4. [Factsheet about seasonal influenza, ECDC](https://www.ecdc.europa.eu/en/seasonal-influenza/facts/factsheet)
5. [How Bad Is the Flu? The Atlantic, March 1944](https://www.theatlantic.com/magazine/archive/1944/03/how-bad-is-the-flu/303636/)
6. [Implications of the apparent extinction of B/Yamagata-lineage human influenza viruses, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC11569178/)
7. [Reassortment between influenza B lineages and the emergence of a co-adapted PB1-PB2-HA gene complex, arXiv](https://arxiv.org/abs/1403.1094)
8. [Divergent evolutionary trajectories of influenza B viruses underlie their contemporaneous epidemic activity, PNAS/PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC6955377/)
9. [Severity of influenza-associated hospitalisations by influenza virus type and subtype in the USA, 2010-19, The Lancet](https://www.thelancet.com/article/S2666524723001878/pdf)
10. [Lineage-specific protection and immune imprinting shape the age distributions of influenza B cases, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC8280188/)
11. [WHO full report, recommended composition of influenza virus vaccines, 2026 southern hemisphere season](https://cdn.who.int/media/docs/default-source/influenza/who-influenza-recommendations/vcm-sh-2025/a.-26-september-2025-recommended-composition-of-influenza-virus-vaccines-for-use-in-the-2026-southern-hemisphere-influenza-season---full-report.pdf?download=true&sfvrsn=eba6ad63_5)
12. [B Yamagata Lineage: Report from WHO Collaborating Centres, July 13, 2023, CDC ACIP slides](https://www.cdc.gov/acip/downloads/slides-2023-10-25-26/07-influenza-kondor-508.pdf)
13. [Trivalent Influenza Vaccines, CDC](https://www.cdc.gov/flu/vaccine-types/trivalent.html)
14. [JCVI statement on influenza vaccines for 2025 to 2026, GOV.UK](https://www.gov.uk/government/publications/flu-vaccines-2025-to-2026-jcvi-advice/jcvi-statement-on-influenza-vaccines-for-2025-to-2026)
15. [2026 Seasonal Influenza Vaccines, Therapeutic Goods Administration](https://www.tga.gov.au/resources/publication/corporate-reports/2026-seasonal-influenza-vaccines)
16. [Identifying the viral and epidemiological factors behind the apparent global extinction of influenza B/Yamagata, medRxiv](https://www.medrxiv.org/content/10.64898/2026.07.22.26358639v1.full)
17. [Interim Estimates of 2025-26 Seasonal Influenza Vaccine Effectiveness, MMWR](https://cdc.gov/mmwr/volumes/75/wr/mm7509a2.htm)
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*Topic: Encyclopedia › Life and health › Microorganisms and fungi › Viruses and acellular agents › Viruses of animals and humans › Influenza viruses › Influenza B*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license