Flu Shots in the US to Exclude Yamagata Strain Starting This Fall Due to Covid-19 Impact
For the past decade, Americans have had access to flu shots that protect against four strains of the virus. However, starting this fall, there will be a change in the flu shots distributed in the United States. All flu shots will now contain only three strains, excluding any viruses from the B strains known as Yamagata. This change is partly due to the impact of Covid-19.
Yamagata viruses were already in decline 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. Since March 2020, there have been no detections of Yamagata strains in testing. Previously, 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 or keep it in?
In the 1990s, Yamagata was prevalent, while another branch of B-strain flu viruses called Victoria was sporadic. However, Victoria experienced a resurgence in the 2000s. The concern was that if Yamagata came back after a lengthy absence, it would be challenging to change how flu vaccines are manufactured. Changes require regulatory review and approval.
In September, the World Health Organization stated 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 (VRBPAC), mentioned that they had been discussing this for four years.
Surprisingly, the change is happening faster than expected. Dr. Arnold Monto, another VRBPAC member, noted that they were initially told it might be impossible to exclude Yamagata due to regulatory red tape. However, the FDA has been working with manufacturers to remove the Yamagata strain from US vaccines in time for the 2024-25 flu season. The US was able to move faster than other countries because all vaccine manufacturers already had approvals for trivalent flu vaccines.
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. The logistics of making the switch seem to be the main challenge in other parts of the world.
There are valid reasons for dropping the Yamagata strain. Dr. Offit explained that it is unnecessary to vaccinate people for something they don’t need. Additionally, there may be potential harms in continuing to include it. Dr. Jodie Guest, senior vice chair of the Department of Epidemiology at Emory University’s Rollins School of Public Health, mentioned that growing the virus in a lab could pose a small risk of it escaping and reintroducing it into the population through the vaccine.
Dropping the Yamagata strain would also free up production capacity to increase the number of flu vaccine doses globally, benefiting countries facing shortages. This change opens the door to considering new vaccine formulas. Some experts have suggested doubling the dose of the A/H3N2 component or introducing a second member of that family since B/Victoria and A/H1N1 strains are often more effective. However, any such change would require testing and regulatory approval, making it unlikely for four-strain flu shots to return quickly. It is considered a long-term goal for improving vaccine effectiveness.
In the meantime, scientists will continue testing flu viruses to ensure that Yamagata has indeed been eradicated. The exclusion of Yamagata from flu shots marks a significant step in adapting to the changing landscape of infectious diseases and improving vaccine strategies.
