Keep Factually independent
Whether you agree or disagree with our analysis, these conversations matter for democracy. We don't take money from political groups - even a $5 donation helps us keep it that way.
How do WHO 2025 vaccine strain recommendations differ between northern and southern hemisphere formulations?
Executive Summary
The WHO recommended different strain compositions for Northern Hemisphere 2025–26 and Southern Hemisphere 2026 influenza vaccines in the materials provided: the Northern Hemisphere egg-based trivalent recommendation lists A/Victoria/4897/2022 (H1N1)pdm09-like, A/Croatia/10136RV/2023 (H3N2)-like, and B/Austria/1359417/2021 (B/Victoria lineage)-like viruses, while Southern Hemisphere 2026 recommendations replace the H1N1 and H3N2 components with A/Missouri/11/2025 (H1N1)pdm09-like and A/Singapore/GP20238/2024 (H3N2)-like (with cell-culture alternatives noted) and retain the same B/Victoria component where used [1] [2]. Reporting on these recommendations in the supplied analyses varies: some summaries say hemisphere lists differ but omit southern details, while others explicitly present both sets, and one source notes equivalence between 2025–26 Northern and 2025 Southern formulations, creating an apparent tension among the documents [3] [4] [5] [6].
1. Why the headlines show a switch in H1N1 and H3N2 — and what that means for vaccine makers
WHO documents in the dataset indicate active replacement of both H1N1 and H3N2 seed strains between the Northern Hemisphere 2025–26 recommendations and the Southern Hemisphere 2026 recommendations. The Northern Hemisphere egg-based trivalent set listed A/Victoria/4897/2022 (H1N1)pdm09-like and A/Croatia/10136RV/2023 (H3N2)-like viruses, while the Southern Hemisphere 2026 set specifies A/Missouri/11/2025 (H1N1)pdm09-like and A/Singapore/GP20238/2024 (H3N2)-like for egg-based vaccines and alternative H3N2 choices for cell-culture or recombinant platforms [1] [2] [5]. The clinical implication is that manufacturers must adapt seed strains and production methods seasonally, and public health programs must track which product types (egg-based vs cell/recombinant) are available locally. The WHO analyses provided also emphasize the removal or non-recommendation of a B/Yamagata lineage antigen in trivalent formulations, with quadrivalent vaccines retaining a B/Yamagata component where the transition to trivalent use is incomplete [2] [5].
2. Conflicting summaries: some reporting says the seasons are the same — how to reconcile that
One analysis in the dataset asserts that the 2025–26 Northern Hemisphere vaccine composition is the same as the 2025 Southern Hemisphere composition, concluding likely similar effectiveness if those same viruses circulate [6]. This contrasts with other WHO-derived summaries that explicitly list different H1N1 and H3N2 components between the Northern 2025–26 and Southern 2026 recommendations [1] [2]. The discrepancy arises because the dataset mixes reports about different seasons (2025 Southern, 2025–26 Northern, 2026 Southern) and the WHO’s separate, twice-yearly recommendation cycles; interpretation errors can occur if one assumes immediate continuity between seasons. The mixed messaging in the supplied analyses underscores the importance of noting the exact season and vaccine formulation platform when comparing recommendations [3] [6].
3. Platform-specific differences: egg-based vs cell/recombinant recommendations matter
WHO materials in the supplied analyses show distinct recommendations by vaccine production platform. The Northern Hemisphere 2025–26 recommendations list one set for egg-based trivalent vaccines and alternative strain choices for cell culture-, recombinant protein-, or nucleic acid-based vaccines; Southern Hemisphere 2026 guidance similarly specifies alternative H3N2 choices for cell culture/recombinant production [1] [2] [5]. That means two manufacturers producing nominally the “same season” vaccine might use different seed strains depending on platform, which affects antigenic match and regulatory review. Public health interpretation must therefore consider not just hemisphere and year but the vaccine technology used, because effectiveness and supply logistics can differ accordingly [1] [5].
4. What’s consistent across the documents — and what’s changing for B-lineage guidance
Across the provided analyses, the B/Victoria lineage component (B/Austria/1359417/2021-like) remains a common recommendation in both Northern and Southern lists referenced, indicating continuity for the Victoria lineage antigen [1] [2]. Simultaneously, the WHO materials referenced state that a B/Yamagata lineage antigen is no longer recommended for inclusion in trivalent vaccines, though quadrivalent vaccines will include a Yamagata component where jurisdictions have not transitioned to trivalent formulations [2] [5]. This signals a strategic simplification in strain selection driven by observed circulation patterns and production choices, and it highlights an ongoing operational shift that affects vaccine composition while maintaining at least one stable B-lineage across the seasons in these documents [2].
5. Takeaways for practitioners, policymakers and the public — and why careful reading matters
The key practical takeaway from the supplied analyses is that WHO’s twice-yearly recommendations can and do change H1N1 and H3N2 seeds between hemispheres and production platforms, while maintaining some stability in B-lineage choice; but ambiguous reporting in secondary summaries can conflate different seasons and formulations, producing conflicting impressions [1] [2] [6]. Policymakers and clinicians should consult the specific WHO recommendation document for the exact season and for platform-specific language when planning procurement and communication. Readers should treat summaries cautiously: some analyses in the set omitted Southern Hemisphere specifics or misstated equivalence between seasons, so direct reference to the WHO recommended composition notices is necessary to resolve apparent contradictions [3] [4] [7].