Researchers are monitoring the behavior of BA.2, a subvariant of SARS-CoV-2 that is responsible for a number of outbreaks ongoing in Europe and some parts of Asia. First described in November, the strain is a subtype of the Omicron variant, and preliminary data hint that it might spread slightly faster than the better-studied BA.1, the subtype responsible for most Omicron outbreaks to date.

“Among all the lineages of Omicron, this is the one showing a higher increase of cases,” Ramon Lorenzo-Redondo, assistant professor of medicine for infectious diseases at Northwestern University Feinberg School of Medicine in Chicago, tells CNN. “But we have to be careful in interpreting that, because higher increases from a very low number are easier to observe.”

The BA.1 version of Omicron has been the dominant strain in many countries for weeks, and nearly all Omicron infections in the US at the moment are due to this subtype, according to the Centers for Disease Control and Prevention (CDC). However, BA.2 has been spreading rapidly in European countries such as Denmark—where almost half of all COVID-19 cases reported in mid-January were attributed to this subtype—and has recently been detected in several US states, including California and Texas.

Scientists already have documented some of the differences between the variants. At the end of last year, researchers in India reported that BA.1 and BA.2 showed divergence in sequences coding for the spike protein, for example, although overall the two strains were still too similar to be classed as different variants. 

“You could say they’re like brothers in the same family,” infectious disease expert Cameron Wolfe of the Duke University School of Medicine tells NBC News. “There are some subtle differences, but most of the genetics are the same in both.”

How such differences might influence the behavior of the strain is still unclear, but researchers are working on comparing the two lineages.

Scientists in Denmark, for example, have noted that BA.2 seems to be taking off even in geographical regions that had high numbers of BA.1 cases, NBC reports, hinting that BA.2 might have some kind of advantage over its predecessor. This “suggests that maybe BA.2 is displacing BA.1, like it’s really competing,” Stuart Ray, a professor of medicine at Johns Hopkins University, tells the network.

Early data have also prompted speculation that BA.2 is faster-spreading. Gabriel Leung, dean of medicine at the University of Hong Kong, tells STAT he thinks that BA.2 could be up to about 35% more transmissible than the original Omicron variant, although he adds that it’s not clear if the difference reflects an inherently more infectious variant or perhaps just one that is better at bypassing natural or vaccine-induced immunity.

Other researchers have offered more conservative estimates. Emma Hodcroft, an epidemiologist at the University of Bern who has been tracking SARS-CoV-2 variants, tells NPR that “BA.2 might well be, like, 1% to 3% more transmissible, or something like that.”

As for the severity of disease the subvariant causes, scientists “have found that there was no increased risk in going to the hospital if you have BA.2 compared to if you have BA.1,” Peter Chin-Hong of the University of California, San Francisco, tells NPR. “That could change, but that’s what we know so far.”