SARS-CoV-2 lineage TB.1
SARS-CoV-2 lineage TB.1 is a Pango lineage of the SARS-CoV-2 virus, designated on 23 June 2026 as a descendant within the XFG.5.1.7 branch of the XFG recombinant lineage.1 • 2
| Key fact | Detail |
|---|---|
| Nomenclature | Pango lineage, named under the Phylogenetic Assignment of Named Global Outbreak LINeages system used since 20203 |
| Designation date | 23 June 2026, among 114 Pango lineages newly designated between 9 June and 6 September 20261 |
| Parent branch | XFG.5.1.7, a sublineage of the XFG recombinant2 |
| Broader background | XFG, a JN.1-derived recombinant classified by WHO as a variant under monitoring on 25 June 2025, Nextstrain clade 25C4 |
| Same-day relatives | TB.2 and XHA were designated on the same date, 23 June 20261 |
Position in the SARS-CoV-2 family tree
Pango lineages are named branches of the SARS-CoV-2 evolutionary tree, each representing a cluster of genetically related genomes descended from a common ancestor that constitutes an epidemiologically meaningful line of transmission. A new lineage must be genetically distinct from its ancestor by one or more mutations, show evidence of onward community transmission, and be supported by a sufficient number of high-quality genomes.3
TB.1 was added to the Nextclade reference datasets in the release covering lineages designated between 9 June and 6 September 2026, with its designation date recorded as 23 June 2026.1 Lineage-proposal records place it on the XFG.5.1.7 branch; related descendants of that branch carry the spike mutation S:A435S together with Orf7a:A3V and Orf8 changes including a 66-67 deletion and K68E, and one related lineage carries a frameshifting Orf7b:17-18 deletion.2
The XFG background
XFG, the recombinant lineage from which the TB.1 branch descends, is a JN.1-derived lineage whose spike carries mutations including S:T22N, S:S31P, S:K182R, S:R190S, S:R346T, S:K444R, S:V445R, S:F456L, S:N487D, S:Q493E, and S:T572I relative to JN.1. Its earliest documented samples date to 27 January 2025, and WHO designated it a variant under monitoring with an initial risk evaluation on 25 June 2025, assigning it Nextstrain clade 25C.4
By mid-2026 XFG and its descendants were among the dominant circulating variants: the European Centre for Disease Prevention and Control reported a median XFG proportion of 53.8% in EU/EEA sequencing data for weeks 34-35 of 2026, and the World Health Organization recorded XFG at 17% of submitted sequences globally in the 28-day period ending 7 June 2026, behind NB.1.8.1 at 50%.5 • 6 WHO stated that none of the variants under monitoring circulating at that time had shown evidence of increased public health risk.6
Related lineages
Several lineages designated on the same day as TB.1 belong to the same period of diversification. TB.2 received its designation on 23 June 2026, as did XHA, with both added in the same Nextclade data release.1 The concentration of new names in June 2026, including TA.1, TB.1, TB.2, RV.1.1.1, and XHA on 23 June alone, reflects the fine-grained naming of sublineages within the XFG and related JN.1-descended backgrounds as sequencing continued.1
References
- Releases, nextstrain/nextclade_data, GitHub
- sars-cov-2-variants/lineage-proposals, GitHub Explorer
- What Is a Pango Lineage? Pango Nomenclature Explained, Pango Network
- Tracking SARS-CoV-2 variants, World Health Organization
- Weekly Communicable Disease Threats Report, Week 40 2026, ECDC
- WER 101-28, Weekly Epidemiological Record, World Health Organization
Initially written Oct 21, 2026
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