As flu season intensifies, infection preventionists remain central to protecting patients and healthcare teams. The CDC has classified this season as moderately severe, with more than 5,000 deaths, including nine pediatric deaths, one in Kentucky. Vaccination continues to be the strongest defense, and the CDC recommends that everyone aged ≥6 months, barring medical contraindications, receive the influenza vaccine.
Current influenza vaccines are produced using egg‑based, cell‑based, and recombinant technologies. Egg‑based production—still the most common—grows viruses in fertilized chicken eggs, while cell‑based vaccines use mammalian cell cultures, and recombinant vaccines use genetic sequencing rather than live virus growth. Since the 2023-2024 influenza season, the CDC no longer recommends additional safety measures for individuals with egg allergies as evidence shows severe allergic reactions are unlikely.
Each year global surveillance data, genetic and antigenic characteristics of circulating viruses, and vaccine performance is reviewed to determine which strains should be included in the upcoming season’s vaccine. This process ensures the vaccine targets the strains most likely to circulate. The influenza vaccine, currently available in the U.S., is trivalent containing two influenza A vaccine viruses and one influenza B.
Egg-based vaccines
- an A/Victoria/4897/2022 (H1N1)pdm09-like virus;
- an A/Croatia/10136RV/2023 (H3N2)-like virus; and (Updated)
- a B/Austria/1359417/2021 (B/Victoria lineage)-like virus
Cell- or recombinant-based vaccines
- an A/Wisconsin/67/2022 (H1N1)pdm09-like virus;
- an A/District of Columbia/27/2023 (H3N2)-like virus; and (Updated)
- a B/Austria/1359417/2021 (B/Victoria lineage)-like virus
A concern this year is influenza A (H3N2) virus subclade J.2.4.1 also referred to as “H3N2 subclade K”. Subclade K viruses have shown signs of being “antigenically drifted” compared to other A(H3N2) viruses, including those in the current vaccine. However, it is still too early in the season to determine the impact. As always, annual flu vaccination remains the best way to reduce the risk of severe illness, hospitalization, and complications CDC.
How Influenza (Flu) Vaccines Are Made | Influenza (Flu) | CDC
Selecting Viruses for the Seasonal Influenza Vaccine | Influenza (Flu) | CDC
Trivalent Influenza Vaccines | Influenza (Flu) | CDC
Influenza Virus Vaccine Strain Selection 2025 – 2026 Influenza Season in the U.S.
Key Facts About Seasonal Flu Vaccine | Influenza (Flu) | CDC
How CDC Classifies Flu Severity each Season in the United States | Influenza (Flu) | CDC
Flu Vaccines and People with Egg Allergies | Influenza (Flu) | CDC
2025–2026 Flu Season | Influenza (Flu) | CDC
Kentucky Reports 1st Pediatric Influenza Death This Respiratory Virus Season.pdf