Flu Shots in the US to Exclude Yamagata Strain, Driven by Covid-19
For the past decade, flu shots in the United States have provided protection against four strains of the virus. However, starting this fall, all flu shots distributed in the US will contain only three strains. This change is partly due to the impact of Covid-19. A panel of experts who advise the US Food and Drug Administration (FDA) on vaccines recently voted unanimously to recommend three-strain flu vaccines that exclude any viruses from the Yamagata branch of the flu’s family tree.
Yamagata viruses were already declining before the pandemic, and the precautions taken to avoid Covid-19, such as masking, staying at home, and better ventilation, seem to have eradicated them completely. These viruses have not been detected in testing since March 2020. In previous years, one Yamagata strain was typically included in each year’s flu shot recipe. However, vaccine designers faced a dilemma this year: should they drop the strain from the formula or keep it in, considering that B-viruses are known to be unpredictable?
In the 1990s, when Yamagata was prevalent, another branch of B-strain flu viruses called Victoria was sporadically seen in testing. However, it experienced a resurgence in the 2000s. The concern was that Yamagata might also make a comeback after a lengthy absence. Changing how flu vaccines are manufactured is a complex process that requires regulatory review and approval. Therefore, the decision to exclude Yamagata strains from flu vaccines was not taken lightly.
The World Health Organization (WHO) stated in September that including Yamagata-lineage antigens in influenza vaccines is no longer necessary. In October, the FDA’s vaccine experts also recommended dropping the Yamagata strains as quickly as possible. Dr. Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia, and a member of the FDA’s Vaccines and Biological Products Advisory Committee, emphasized that discussions about this change have been ongoing for four years.
The change is happening faster than expected, according to Dr. Arnold Monto, another member of the FDA committee. Initially, when the committee voted to recommend removing Yamagata from flu vaccines in October, they were informed that excluding it next season might be impossible due to regulatory red tape. However, Dr. Jerry Weir, director of the FDA’s Division of Viral Products, announced on Tuesday that the agency has been working with manufacturers to remove the Yamagata strain from US vaccines in time for the 2024-25 flu season.
The US has been able to move faster than some other countries in making this change because all vaccine manufacturers already had approvals for trivalent flu vaccines. However, it is taking longer in other parts of the world to switch to the new formula. For this reason, the committee also voted to recommend a four-strain formula, including one from B/Yamagata, for vaccines manufactured in the US but distributed in other countries.
There are valid reasons for dropping the Yamagata strain. Dr. Offit explains that vaccinating people for something they don’t need is unnecessary. Additionally, including the strain could potentially have some harms. Dr. Jodie Guest, senior vice chair of the Department of Epidemiology at Emory University’s Rollins School of Public Health, points out that flu vaccines are produced using live or attenuated virus, which poses a small risk of escape from the lab and reintroduction into the population through the vaccine.
Dropping the Yamagata strain would also free up production capacity to increase the number of flu vaccine doses made globally, benefiting countries facing shortages. The change opens the door to considering new vaccine formulas. Some experts suggest doubling the dose of the A/H3N2 component or introducing a second member of that family, as B/Victoria and A/H1N1 strains are often more effective. However, any such change would require testing and regulatory approval, making the return of four-strain flu shots unlikely in the near future. It remains a long-term goal for improving vaccine effectiveness.
Scientists will continue testing flu viruses to ensure that Yamagata has indeed been eradicated. The exclusion of this strain from flu vaccines marks a significant step in adapting vaccine formulas to better protect against prevalent strains and optimize global vaccine production.
