Influenza/B Yamagata Strain and Influenza Vaccines

Influenza, a highly contagious viral infection that affects millions of people worldwide each year, can lead to severe illnesses and even fatalities. Vaccines developed to establish immunity against annually changing influenza virus strains are highly effective preventive measures aimed at curbing the spread of the disease. The continuous genetic variation of the influenza virus necessitates the annual updating of strains to be used in vaccine formulations. The utilization of strains such as influenza/A H1N1 and H3N2, along with influenza B/Yamagata and Victoria-like strains found in quadrivalent vaccines, is known to reduce the risk of contracting the disease by 40-60% each year. According to data presented by the World Health Organization (WHO) and the Global Initiative on Sharing All Influenza Data (GISAID) since March 2020, the Yamagata lineage of the influenza B virus has not been detected in circulation. It is known that the protective efficacy of currently used influenza vaccines against influenza B and influenza A/H1N1 strains is dual. Besides vaccine effectiveness, measures taken in response to the SARS-CoV-2 pandemic, such as mask usage and travel restrictions, are believed to have played a significant role in reducing the global influenza outbreak rate and the elimination of the influenza B/Yamagata lineage. The 2023-2024 seasonal influenza vaccines identified by the WHO still include strains similar to influenza B/Yamagata. However, considering the strains not detected in circulation, a reassessment of the strains for influenza vaccines is necessary, taking into account the potential risks associated with the inclusion of influenza B/Yamagata-like strains.