JN.1 was not descended from what came before it

The CDC's national SARS-CoV-2 genomic surveillance programme has tracked the
virus's evolution since Delta and Omicron. Between **May 2023 and September
2024**, the lineages circulating were **descendants of Omicron XBB and of
JN.1** — and the relationship between those two is the interesting part.

What happened

Multiple XBB descendants circulated in summer and autumn 2023, with
immune escape characteristics — each incrementally better at evading the
immunity people had built.

Then JN.1, which was not an XBB descendant, and **contained
substantial genetic differences**. It became predominant by January 2024,
and its own descendants emerged afterwards.

Why that matters

Variant evolution is usually incremental: a dominant lineage accumulates
mutations, and its children replace it. That is trackable and, to a degree,
anticipatable.

JN.1 did not do that. It came from elsewhere in the family tree, arriving with
a substantial set of differences at once rather than by steps — and took over
within months.

**Increases in COVID-19 cases occurred during both variant predominance
periods and periods when variants were co-circulating**, so the case pattern
did not map neatly onto which lineage was winning either.

The conclusion

**Given the unpredictable nature of SARS-CoV-2 evolution, continued
monitoring for genetic changes and their impact on disease severity and
medical countermeasure effectiveness remains essential.**

"Unpredictable" is the operative word, and JN.1 is the evidence for it. A
surveillance programme that only watched the descendants of the current
dominant lineage would have seen JN.1 late.

*Source: Ma KC, Castro J, Lambrou AS, et al. Genomic Surveillance for
SARS-CoV-2 Variants: Circulation of Omicron XBB and JN.1 Lineages — United
States, May 2023–September 2024. MMWR, Centers for Disease Control and
Prevention.*