For more than a decade, the annual flu shot has protected against four types of influenza viruses, but this year, things have changed.
In the United States, vaccines for the 2024-2025 influenza season provide protection against only three subtypes of influenza A viruses, two called H1N1 and H3N2. The other is the influenza B virus known as the “Victoria strain.” A fourth type of influenza virus, the Yamagata strain, was excluded from the formulation. This will be the first time since 2012 that a U.S. influenza vaccine will be trivalent, or three-component, rather than quadrivalent or four-component.
But why the change?
Based on all available evidence, the Yamagata lineage is now extinct. Therefore, in March 2024, the U.S. Food and Drug Administration (FDA) announced that this year’s influenza vaccine would not contain this subtype of the virus, as no cases of the virus had been reported in recent years. We know this because there is a worldwide consortium of scientists who routinely isolate influenza viruses from sick patients and analyze the virus’ genetic material.
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The FDA made this decision following a recommendation from the Vaccines and Related Biological Products Advisory Committee, and shortly after, the World Health Organization issued the same recommendation globally. However, it may take time for global manufacturers to switch to new formulations, STAT News reported.
Flu vaccines are designed to protect people from the most common influenza subtypes that plague people each season. As flu seasons alternate in the northern and southern hemispheres, scientists can determine which strains of these subtypes will be prevalent in the next flu season by tracking infections in the opposite hemisphere. Predicting what is most likely.
The flu shot works by stimulating the immune system to produce antibodies that are ready to attack the selected influenza strain if the vaccinated person becomes infected.
Scientists have reported early signs that the Yamagata strain went extinct in 2021, when they began to notice that no one around the world was sick with that subtype of influenza. Experts speculated at the time that the most likely explanation was that the yamagata disappeared due to public health measures aimed at curbing the spread of the coronavirus. Early in the pandemic, many people wore masks, socially distanced, and stayed home when sick, which had a knock-on effect in suppressing other respiratory infections such as the flu. Ta.
From 2013 through the 2023-2024 influenza season, all influenza vaccines in the United States were quadrivalent. This means it provides protection against four subtypes of the influenza virus. (Image credit: Houston Chronicle/Hearst News via Getty Images / Contributor via Getty Images)
Kevin McCarthy, assistant professor of microbiology and molecular genetics at the University of Pittsburgh, told Live Science that influenza B viruses were circulating at much lower levels than in Victoria before the pandemic began. He said Yamagata Prefecture was probably the hardest hit. He also suggested that Yamagata virus may be inherently more perishable than other virus subtypes. Its ability to infect large numbers of people at once is relatively weak.
Since there have been no confirmed cases in Yamagata Prefecture since March 2020, it makes sense to exclude Yamagata Prefecture from future influenza vaccinations.
Hanover Matz, a postdoctoral fellow at Washington University School of Medicine in St. Louis, told Live Science that “there is no reason to include this virus strain if we don’t expect people to actually be exposed to the virus and become infected.” spoke.
CNN reported in March that Yamagata Prefecture’s exclusion may allow manufacturers to produce more doses of the flu vaccine. Additionally, this change could reduce the potential risks associated with this production step, since it would no longer be necessary to grow so many viruses in the lab. Currently, many influenza vaccines require scientists to grow the influenza virus inside eggs or cells, which poses a very small risk of virus leakage. Scientists follow strict safety protocols to avoid such accidents.
Importantly, discontinuing Yamagata will not reduce the effectiveness of future flu shots because the vaccine is designed to provide protection only against the virus strains it contains, McCarthy said. said. Therefore, removing chevrons will not affect the effectiveness of vaccines against H1N1 or Victoria strains.
Despite evidence that the Yamagata lineage is extinct, public health officials will continue to monitor it for traces of the Yamagata lineage. Once Yamagata returns, McCarthy said scientists will simply need to make the vaccine quadrivalent again.
This article is for informational purposes only and does not provide medical advice.
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