Influenza B virus
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Influenza B Virus: Epidemiology and Disease Burden
Influenza B virus (IBV) is a significant cause of seasonal flu outbreaks, leading to severe respiratory illnesses such as bronchitis and pneumonia, and can worsen chronic conditions like asthma, especially in children and the elderly 14. While often overshadowed by influenza A virus (IAV), IBV is responsible for a substantial proportion of global influenza cases—about 23%—and is the dominant strain in roughly one out of every seven flu seasons . Notably, IBV can be as virulent as IAV, with studies showing higher risks of hospitalization and mortality in certain populations, including children and immunocompromised individuals 24. For example, during the 2010–2011 flu season, IBV accounted for 38% of pediatric deaths, despite representing only 26% of circulating strains .
Influenza B Virus Lineages: Victoria and Yamagata
IBV is divided into two main lineages: B/Victoria and B/Yamagata. These lineages have distinct patterns of circulation and age predilection. B/Yamagata is more common in temperate regions and tends to infect older individuals, while B/Victoria is more prevalent in tropical areas and affects younger populations 389. Both lineages can co-circulate during a single season, but typically one is more dominant 78. The unpredictability of which lineage will dominate complicates vaccine formulation and public health planning .
Evolution and Genetic Diversity of Influenza B Virus
IBV evolves more slowly than IAV, with lower within-host genetic diversity and a tight genetic bottleneck during transmission 510. The two lineages, Victoria and Yamagata, have followed different evolutionary paths. Victoria viruses show frequent antigenic drift, especially near the hemagglutinin receptor-binding site, leading to periodic strain replacement every 1–3 years. In contrast, Yamagata viruses are more antigenically stable but have recently shown increased epidemic activity due to antigenic drift in neuraminidase 169. Reassortment between the two lineages is rare, but within-lineage reassortment and the emergence of hemagglutinin deletion variants have contributed to their continued circulation and diversity 610.
Clinical Impact and Management of Influenza B Virus
IBV causes significant morbidity and mortality, particularly in children and high-risk groups, but its impact is often underestimated due to its lack of pandemic potential . Both lineages can lead to severe disease and hospitalization 248. There has been uncertainty about the effectiveness of neuraminidase inhibitors (NAIs) for IBV treatment, but new antiviral agents like baloxavir show promise, especially for high-risk patients . Vaccination remains the primary preventive strategy, and the inclusion of both IBV lineages in quadrivalent vaccines has improved protection, though these vaccines are not universally available .
Vaccine Challenges and Future Directions
Frequent mismatches between circulating IBV lineages and vaccine strains occur in over 40% of seasons in temperate regions and 30% in the tropics, reducing vaccine effectiveness . The ongoing diversification of IBV, especially the emergence of new hemagglutinin variants, may eventually require more complex vaccine formulations or the development of universal influenza vaccines 67. Reverse genetics and other advanced molecular techniques are aiding in the development of broader and more effective vaccines 17.
Conclusion
Influenza B virus is a major contributor to seasonal influenza epidemics, with two distinct lineages that differ in their epidemiology, evolution, and clinical impact. Its slower evolution compared to influenza A, combined with frequent lineage mismatches in vaccines, presents ongoing challenges for public health. Continued surveillance, improved vaccine strategies, and new antiviral treatments are essential to reduce the disease burden associated with IBV 1234+6 MORE.
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