An emerging omicron subvariant called CH.1.1 or "orthrus" only accounted for around 1.5% of new COVID-19 cases in January — but the subvariant, which shares a mutation seen in the highly contagious delta, BA.4, and BA.5 strains, has the potential to become more transmissible.

What you need to know about CH.1.1

The CH.1.1 subvariant was first detected this fall in Southeast Asia. It is a descendant of the BA.2.75 subvariant, which circulated during summer 2022 before fizzling out.

Since November, the strain's prevalence has increased sharply, now accounting for an estimated 10% of COVID-19 samples sequenced each day around the world, according to outbreak.info.

Currently, CH.1.1 is responsible for over a quarter of new COVID-19 infections in parts of the United Kingdom and New Zealand, according to a preprint study published last week in bioRxiv by researchers at Ohio State University (OSU). Data from outbreak.info, a community database of COVID-19 information, indicate that the majority of cases are currently in New Zealand, making up roughly one in three sequenced cases. In Hong Kong and Papua New Guinea CH.1.1 accounts for roughly 25% of cases, respectively.

The variant is among those being tracked by the World Health Organization (WHO). In WHO's February 2023 COVID-19 epidemiological update, the agency lists CH.1.1 among the top three most prevalent variants in Europe, with 12.3%, just slightly behind BQ.1 at 13% and BQ.1.1 at 31.3%.

"WHO is currently prioritizing the tracking of four Omicron descendent lineages," the agency said. "These variants are included on the basis of signals of an increase in prevalence or signs of growth rate advantage in some countries relative to other circulating variants, and additional amino acid changes that are known or suspected to confer fitness advantage."

According to Shan-Lu Liu, and colleagues, who authored the bioRxiv study, CH.1.1, and another emerging variant, called CA.3.1, have a "consistently stronger neutralization resistance than XBB, XBB.1, and XBB.1.5, which is astonishing and warrants continuous monitoring and further investigations."

Could CH.1.1 become a dominant variant?

While researchers still do not know much about CH.1.1, many have noted that it could become transmissible, evade immunity from vaccine and infection, and result in more severe illness.

In the United States, CH.1.1 only accounted for an estimated 1.5% of new COVID-19 cases in January. However, it shares a mutation previously seen in the delta, BA.4, and BA.5 variants, called L452R, according to the OSU researchers.

In addition, the subvariant features a mutation previously seen in the deadly delta variant that is not typically seen in omicrons subvariants. Notably, this mutation could make CH.1.1 even more daunting.

Ultimately, Michael Osterholm, director of the University of Minnesota's Center for Infectious Disease Research and Policy said it is anyone's guess how CH.1.1 will develop in different countries around the world, including the United States. 

"I don't think we have a real sense of what variants to be concerned about and which not to be," Osterholm said. 

"We've seen this before: What might appear to be a challenging variant turns out not to be a real challenge," he added.

For example, XBB.1.5, which "started out looking like it was going to be a very serious challenge, in terms of COVID" in the United States faded after becoming the dominant variant in the Northeast. "[I]t just began to peter out throughout the rest of the country," where it has not risen as quickly, Osterholm said.

According to Osterholm, anyone who claims they can definitively say what the future of the pandemic will look like "probably has a bridge to sell you." (Henderson, MedPage Today, 2/9; Prater, Fortune Well, 1/27)