Seasonal influenza is an acute respiratory infection caused by influenza viruses, and it circulates in seasonal patterns in all parts of the world. The virus causes annual outbreaks and has the potential for causing future pandemics. Annual influenza vaccination, before the predicted peak activity, is regarded as the most effective strategy to prevent disease and its severity, especially when the circulating strains of the influenza virus match with those contained in the vaccine. The vaccine is introduced twice every year for the Northern Hemisphere (NH) and the Southern Hemisphere (SH), each based on the predicted circulation of the strains in the respective geographical hemispheres, which is based on the global surveillance of influenza viruses. However, the virus circulation does not always respect the geographical distribution into NH and SH patterns, and areas in the geographical NH region have been shown to demonstrate seasonal influenza activity that mimics that in the SH geography.[1,2] Influenza vaccines are available as quadrivalent or trivalent vaccines. Typically, the quadrivalent vaccine contains antigens of two influenza A viruses (A/H1and A/H3) and two influenza B viruses (B/Yamagata and B/Victoria), whereas the trivalent vaccine contains only one of the influenza B strains. This is to cover for both the B influenza viruses as the circulation of one of the 2 lineages may be dominant even as a co-circulation of both the lineages has been reported.[3]
After the onset of the COVID-19 pandemic, there was a virtual elimination of the influenza circulation from the globe, which, however, reappeared in 2021 [Figures 1 and 2]. However, the recent global influenza surveillance has been notable in the virtual absence of the circulation of B/Yamagata lineage [Figures 1 and 2]. Despite this, the World Health Organization continues to recommend B/Yamagata as a component of the quadrivalent vaccine, both for the egg-based and cell-based vaccines,[4] with the most recent recommendation for the SH 2024 season also containing B/Phuket/3073/2013 (Yamagata lineage) for the quadrivalent vaccine. Given the virtual absence of B/Yamagata from the global circulation [Figure 3] after the onset of the COVID-19 pandemic in 2020, it would be apt to drop B/Yamagata totally from the vaccine and recommend only the trivalent vaccine with B/Victoria lineage. This would also prevent inadvertent reintroduction of the vaccine-contained B/Yamagata strain into the community circulation. WHO needs to take a call about this during the next strain selection meeting for the NH 2024-25 season in February 2024.
Figure 1.
Influenza virus detections reported to FluNet (all viruses) from January 2020 till July 2023. The solid line represents the positivity. Green depicts influenza A (not subtyped), deep turquoise represents influenza A/H3, pale blue represents influenza A/H1pdm09, deep orange represents influenza B (lineage not determined), whereas pale orange depicts B/Victoria samples. Data source: WHO[5]. WHO = World Health Organization
Figure 2.
Influenza virus detections reported to FluNet (influenza B) from January 2020 till July 2023. Deep orange depicts influenza B (lineage not determined), whereas pale orange depicts B/Victoria samples. The solid line represents the positivity. Data source: WHO[5]. WHO = World Health Organization
Figure 3.
Influenza virus detections reported to FluNet (influenza B/Yamagata) from January 2020 till July 2023. The solid line represents the positivity. Data source: WHO[5]. WHO = World Health Organization
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