Enlarge / A pharmacist prepares to administer COVID-19 vaccine booster shots during an event hosted by the Chicago Department of Public Health at the Southwest Senior Center on September 9 in Chicago. The recently authorized booster vaccine protects against the original SARS-CoV-2 virus and the more recent omicron variants, BA.4 and BA.5. (credit: Getty | Scott Olson )

In mice, the BA.5-targeting bivalent booster now rolling out nationwide did an equally good job at thwarting the BA.5 omicron subvariant as the bivalent booster targeting its predecessor, BA.1, which US regulators passed on.

That's according to a pre-print study —which hasn't been peer-reviewed or formally published—authored by researchers at Moderna and Washington University School of Medicine.

Although the study is still a preprint and only involved mice, it provides some of the first head-to-head data comparing the two omicron-targeting booster options considered for this fall—one of which is currently going into arms across the US. And the findings may raise questions about the US booster strategy.

Enlarge / A person holds a positive SARS-CoV-2 rapid test on February 17, 2022 in Berlin, Germany. (credit: Getty | Thomas Trutschel )

Omicron subvariant BA.2.12.1 has overtaken BA.2 as the dominant version of the pandemic coronavirus in the US, now accounting for an estimated 59 percent of cases nationwide . But BA.2.12.1's reign may end as quickly as it began, with yet another batch of omicron subvariants gaining ground—BA.4 and BA.5—and threatening to cause more breakthrough infections.

BA.2.12.1 has a transmission advantage over BA.2, which itself has an edge over the initial omicron subvariant, BA.1, that caused a towering surge of US cases in mid-January. BA.2 peaked in mid-April, accounting for 76 percent of US cases at its height. But then came BA.2.12.1 , which is named for being the 12th lineage stemming from BA.2 and the first branch of that BA.2.12 lineage.

When BA.2 peaked in mid-April, BA.2.12.1 accounted for about 18 percent of cases. It reached about 43 percent prevalence by mid-May and has since overtaken BA.2, which currently accounts for only about 35 percent of cases. BA.2.12.1 is dominant in every region of the country, except for the Northwest, according to the Centers for Disease Control and Prevention.

Enlarge / A medical worker arranges nucleic acid samples at a makeshift nucleic acid testing site on May 3, 2022 in Beijing, China. (credit: Getty | Pang Songgang )

The omicron subvariant BA.2.12.1 is poised to become dominant in the US, currently accounting for an estimated 36.5 percent of all US SARS-CoV-2 cases, according to the latest estimates released Tuesday by the Centers for Disease Control and Prevention.

The subvariant's ascent is the latest rapid succession of omicron subvariants, from the sky-scraping peak of cases from the initial omicron subvariant BA.1 in January, to the current bump driven by the subvariant BA.2, which achieved dominance in March. As before, the reason for the viral usurping is that omicron subvariants continue to evolve advantages: BA.2.12.1 has a transmission advantage over BA.2, which had a transmission advantage over BA.1, which had a significant advantage over delta.

The imminent reign of BA.2.12.1 raises concern for yet another wave of infections and poses questions about how effective future omicron-specific vaccines could be against symptomatic infections.