The U.K. Health Security Agency said Friday that it appeared the BA.2 subvariant currently showed a “substantial” growth advantage over BA.1, which was first reported in southern Africa.

In a preliminary assessment, the agency said contact-tracing data suggested contacts of people harboring BA.2 had a 13.4% likelihood of getting infected themselves, whereas the likelihood is 10.3% with the earlier Omicron substrain. It said the early findings needed to be “interpreted with caution.” Nonetheless, these figures would indicate a 30% transmissibility advantage for BA.2 over BA.1.

“We now know that BA.2 has an increased growth rate which can be seen in all regions in England,” said UKHSA chief medical adviser Susan Hopkins in a statement. “We have also learnt that BA.2 has a slightly higher secondary attack rate than BA.1 in households.”

However, the British agency also stressed that its early findings suggested vaccines were as effective against BA.2 as they are against BA.1, in terms of warding off symptomatic disease. “There is currently no data on the severity of BA.2,” it added.

The British assessment is not far off what Danish scientists said a couple of days earlier. Again urging caution owing to the preliminary nature of its assessment, the Statens Serum Institut said Wednesday that it appeared BA.2 was 50% more contagious than BA.1.

Omicron became dominant in both countries relatively early on. While BA.2 has now outcompeted BA.1 in Denmark, that has yet to happen in the U.K.—although experts say it’s not far off.

“We might expect it to become dominant in the U.K. in the next few weeks, as it has done in Denmark recently,” commented John Edmunds, an epidemiology professor at the London School of Hygiene & Tropical Medicine. “It is difficult to say what the implications of this will be. It may well extend this wave of infection, or even lead to another peak.”

BA.2 is sometimes referred to as “stealth Omicron” because it is less easy to classify as Omicron when detected via a PCR test. Crucially, the nickname does not mean it is harder to detect Omicron infections than it is Delta infections.

Overall, the Omicron wave is pushing up death rates in the U.S. (where nearly half of states have confirmed BA.2 infections, though numbers remain low) and leading many unvaccinated people to reconsider their choices. Its rapid march, and the fact that it appears to be “milder” than previous variants, has also led some epidemiologists to back the idea of using hospitalization figures as a more meaningful tracking tool than case numbers.