EU recommendations for seasonal flu vaccine composition

The winter flu season may be over in the northern hemisphere, but EU regulators are already looking ahead to the next outbreak in autumn 2024.

Influenza viruses continuously change and evolve. That is why it is necessary to review the virus strains contained in influenza vaccines and, if necessary, update them before the new flu season.

Every year, the European Medicines Agency (EMA) issues recommendations for the composition of seasonal influenza vaccines well in advance of flu season, giving manufacturers time to begin production. Some years, there is not change; other years, one or two of the strains are replaced.

The regulator advises companies which strains of flu viruses they should protect against when formulating the revamped vaccine. This is based on WHO data which draws on the experience of the southern hemisphere, where flu season is just beginning.

Based on this data, the EMA’s Emergency Task Force recommends moving from a four-strain vaccine to a three-strain vaccine which does not include the B/Yamagata component featured in recent years. This is because that flu virus has not been very common recent flu seasons, north or south.

In fact, it has not been detected since March 2020, when COVID-19 was declared a global pandemic. This is thought to be due in part to the public health measures put in place to limit the spread of COVID. Influenza B viruses are responsible for a quarter of annual influenza infections.

Given that the B-Yamagata virus strain no longer seems to pose a threat to public health, it is not necessary to include it in the formulation of influenza vaccines, the EMA said.

The ETF recommends that this strain should ideally be removed from all live-attenuated vaccines from the 2024/2025 season. In the interest of guaranteeing vaccine supplies for the coming vaccination campaign, the transition to a trivalent composition for all other influenza vaccines should be completed for the 2025/2026 season.

On foot of this, the following strains are now the priority for the 2024/2025 season.

Manufacturers of live-attenuated vaccines, or egg-based trivalent vaccines should include:

• an A/Victoria/4897/2022 (H1N1)pdm09-like virus;
• an A/Thailand/8/2022 (H3N2)-like virus;
• a B/Austria/1359417/2021 (B/Victoria lineage)-like virus.

Manufacturers of cell-based trivalent vaccines should include:

• an A/Wisconsin/67/2022 (H1N1)pdm09-like virus;
• an A/Massachusetts/18/2022 (H3N2)-like virus;
• a B/Austria/1359417/2021 (B/Victoria lineage)-like virus.