The Disappearance of Influenza B/Yamagata and Its Impact on Flu Vaccines

Seasonal influenza remains a significant public health challenge, causing thousands of deaths and hospitalizations annually. However, the COVID-19 pandemic unexpectedly and inadvertently reshaped the landscape of influenza viruses. Since March 2020, one lineage of influenza B—B/Yamagata—has not been detected globally, leading scientists to conclude it may have gone extinct. This unprecedented shift, driven by pandemic-era measures like masking, distancing, and reduced travel, marks the first time human behavior has potentially eradicated a circulating flu strain.

How COVID-19 Measures Altered Influenza Dynamics

Non-pharmaceutical interventions (NPIs) designed to curb SARS-CoV-2 transmission drastically suppressed the spread of other respiratory viruses, including influenza. Data from the World Health Organization’s Global Influenza Surveillance and Response System (GISRS) revealed a 99% drop in flu cases during the pandemic’s peak. While influenza A (H1N1, H3N2) and B/Victoria persisted at lower levels, B/Yamagata—already circulating less frequently before 2020—disappeared entirely. Genetic surveillance confirmed no confirmed cases since April 2020, suggesting its elimination.

Dr. Rebecca Wurtz, an infectious disease expert at the University of Minnesota, called this development “an interesting and unique story,” noting that without COVID-19 measures, B/Yamagata would likely still circulate. The strain’s lower pre-pandemic prevalence and existing population immunity made it vulnerable to extinction when transmission opportunities vanished.

Implications for Vaccines and Public Health

The disappearance of B/Yamagata has direct consequences for flu vaccine formulation. For over a decade, quadrivalent vaccines targeted two influenza A strains (H1N1 and H3N2) and both B lineages (Victoria and Yamagata). In 2024, regulators like the FDA and WHO recommended removing B/Yamagata and shifting to trivalent vaccines. This change simplifies production, allowing manufacturers to allocate resources toward the remaining active strains.

The benefits are twofold:

1. Increased Vaccine Supply: Producing trivalent vaccines could boost global output by up to 40%, addressing chronic shortages—especially critical in low-income countries with limited access.

2. Reduced Risks: Including an extinct strain offers no public health benefit and carries minimal but avoidable risks, such as accidental exposure during vaccine manufacturing.

Why Vaccination Still Matters

While B/Yamagata’s absence is a scientific milestone, influenza remains a threat. B/Victoria and influenza A viruses continue to cause seasonal outbreaks, with the CDC estimating 12,000–51,000 annual U.S. deaths linked to flu. Dr. Kawsar Talaat of Johns Hopkins emphasizes that this year’s trivalent vaccine remains essential: “The burden of flu hasn’t lessened—it’s just shifted.”

A Lesson in Global Collaboration

The eradication of B/Yamagata underscores the power of coordinated public health action. Scientists worldwide, including in countries with strained diplomatic ties, shared data to monitor flu’s decline. This collaboration highlights how robust public health efforts can track pathogens and adapt real-time strategies.

Looking Ahead

While B/Yamagata’s extinction is a success story, vigilance is key. Flu viruses evolve rapidly, and immunity gaps from reduced exposure during the pandemic could fuel future surges. Researchers also stress the need for universal flu vaccines, which target conserved viral regions rather than chasing evolving strains—a lesson accelerated by mRNA technology developed during COVID-19.

For now, the takeaway is the same message: Get your flu shot. This year’s vaccine protects against active threats, and its streamlined production ensures more doses reach those who need them. As global travel resumes, maintaining surveillance and equitable vaccine access will be critical to preventing another lineage from filling the void left by B/Yamagata.

Sources:

1. Boden, S. The Flu Shot Is Different This Year, Thanks to Covid. https://www.npr.org/sections/shots-health-news/2024/10/17/nx-s1-5155104/flu-shot-vaccine-b-yamagata-extinct.

2. Dhanasekaran, V., Sullivan, S., Edwards, K.M. et al. Human seasonal influenza under COVID-19 and the potential consequences of influenza lineage elimination. Nat Commun 13, 1721 (2022). https://doi.org/10.1038/s41467-022-29402-5

3. Trivalent Influenza Vaccines. https://www.cdc.gov/flu/vaccine-types/trivalent.html#:~:text=B%2FYamagata%20virus%20detections,lineage%20viruses%20after%20March%202020.

4. Vajo, Zoltan, and Peter Torzsa. “Extinction of the Influenza B Yamagata Line during the COVID Pandemic-Implications for Vaccine Composition.” Viruses vol. 14,8 1745. 9 Aug. 2022, doi:10.3390/v14081745