Omicron splinters into fast-spreading lineages, highlighting coronavirus’s evolution

Scientists around the world are discovering and tracking newer forms of the Omicron coronavirus variant, showing how even when a strain becomes globally dominant, it continues to evolve and can splinter into different lineages.

Case in point: Updated data released Tuesday showed that a burgeoning form of Omicron, called BA.2.12.1 —  itself a sublineage of the BA.2 branch of Omicron — now accounts for nearly one in five infections in the United States. It’s eating into the prevalence of the ancestral BA.2, highlighting the emergent virus’s transmission advantage over its parent. BA.2 now accounts for about 74% of cases, while the remaining 6% or so are from the BA.1 branch of Omicron, the first form of the variant that took over globally and whose prevalence has been falling as BA.2 became dominant.

The menagerie can be dizzying to track, especially because all these cases technically fall under the Omicron umbrella. But even as scientists closely monitor the divergence of Omicron, early signs suggest the different lineages don’t substantially differ in terms of how virulent they are or in their ability to evade the protection generated by immunizations. While some of the newer forms of the virus might be better spreaders than others, their emergence doesn’t necessarily result in huge increases in cases.

Already, the growth of BA.2 was helping drive up cases in parts of the United States, particularly in the Northeast. As BA.2.12.1 snowballs, it could give the virus an extra transmission boost. Last week, New York state officials noted that the sublineage was contributing to comparatively high infection rates in certain parts of the state and estimated that it had a 25% growth advantage over BA.2.

At the same time, countervailing factors like the warmer weather and huge levels of population immunity are acting as a drag on transmission. Because of all the layers of protection people have — both from vaccinations and past infections — new cases are far less likely to result in severe outcomes.

Even when Omicron was first identified late last year, the South African team that characterized it reported that it had different forms, BA.1, BA.2, and BA.3. Now, researchers in several African and European countries have identified newer forms, dubbed BA.4 and BA.5.

Experts have noted that this “drift” in evolution is more similar to how other respiratory viruses like influenza behave. Previous SARS-CoV-2 variants that became dominant were not always closely related to the strains that were circulating when they emerged.