The quadrivalent influenza vaccine is designed to provide protection against both co-circulating B lineages. Since 2016, it has shown increased effectiveness in preventing influenza caused by the Influenza B virus compared to the previous trivalent vaccine.
Interestingly, the B/Yamagata lineage might have been eradicated during the 2020/2021 period due to measures implemented during the COVID-19 pandemic. Consequently, in October 2023, the World Health Organization concluded that protection against the Yamagata lineage was no longer necessary in the seasonal flu vaccine, reducing the number of targeted lineages from four to three. In February 2023, they stated that a trivalent vaccine is considered sufficient to provide full protection against circulating lineages. Recommended composition of influenza virus vaccines for use in the 2024-2025 northern hemisphere influenza season
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Recently, the FDA has discussed the selection of influenza vaccines for the 2024–2025 US influenza season and they strongly recommend trivalent influenza vaccine. In fact, since 2020, there have been no confirmed detections of circulating B/Yamagata lineage viruses worldwide.
It seems that the FDA is acting very quickly, working with the manufacturers and being very determined to move from a quadrivalent to a trivalent vaccine. I am curious what will be the statment of other countries? I assume not all will be able to make changes as quickly as the US.
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Recommendations for the 2024/25 influenza vaccination season are anticipated in April by the EMA and in May by Canada. Indeed, it will be interesting to see how regulatory authorities respond to the new circumstances, but this change also has significant implications for manufacturers.
They now face the decision of whether to concentrate solely on trivalent vaccines or to update quadrivalent vaccines to incorporate another influenza lineage in place of B/Yamagata. However, this would require conducting clinical trials to assess the efficacy of the new quadrivalent vaccines and updating the manufacturing process.
Additionally, numerous trials for quadrivalent vaccines are currently underway. For example, Moderna and Pfizer are conducting phase 3 clinical trials for quadrivalent mRNA vaccines that include B/Yamagata. Novavax is also conducting trials with recombinant hemagglutinin proteins from four influenza strains, including B/Yamagata. Additionally, they are exploring a combined quadrivalent influenza and SARS-CoV-2 vaccine. How these manufacturers respond to statements from the WHO, FDA, and other authorities will be an interesting case study 
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As anticipated, in line with the World Health Organization’s observations indicating the absence of the B/Yamagata strain of the influenza B virus in circulation since March 2020, the Emergency Task Force of the European Medicines Agency (EMA) has issued a statement. They recommend transitioning from quadrivalent to trivalent vaccines that exclude the B/Yamagata component. Since the B/Yamagata virus strain no longer appears to pose a public health threat, the ETF suggests removing this strain ideally from all live-attenuated vaccines for the 2024/2025 season. For more information, you can visit: EU recommendations for 2024/2025 seasonal flu vaccine composition | European Medicines Agency
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