BA.2.86, yet another new coronavirus subvariant that is a heavily mutated version of the omicron variant, has been detected in Japan for the first time.

While there’s no evidence so far that it causes more severe illness, experts say it may be more capable of infecting people who have been previously infected with the coronavirus or vaccinated with older versions of COVID-19 shots.

The news comes as the number of new cases continues to grow in Japan amid what is widely considered a ninth wave of coronavirus infections.

According to the latest weekly statistics, released by the health ministry Friday, the average number of new patients reported by around 5,000 designated hospitals stood at 20.5 in the week through Sunday, up from 19.07 a week before and 17.84 from two weeks before. It is at the highest level since May, when the nation entered a post-pandemic phase and did away with the daily counting and reporting of new infections and other special measures.

On Thursday, the Tokyo Metropolitan Government said one person tested positive for BA.2.86 on Aug. 24. The person’s symptoms have been mild.

Globally, BA.2.86 has not yet spread widely, with only about 40 cases reported from countries including Denmark, Thailand, Sweden and the U.S., according to the metropolitan government. But it has raised concerns, as it features over 35 amino acid mutations compared with the more recently circulating XBB.1.5 — the same magnitude of change seen between the initial omicron variant (BA.1) and previous variants such as delta, according to the U.S. Centers for Disease Control and Prevention.

On Wednesday, Moderna said its updated COVID-19 vaccine was confirmed as boosting the amount of neutralizing antibodies against currently circulating variants, including BA.2.86, EG.5 and FL.1.5.1. Pfizer has also reportedly said that its updated vaccine elicited a strong antibody response against BA.2.86 in a preclinical study involving mice, although the U.S. pharmaceutical giant has yet to release an official statement.

The health ministry plans to roll out these XBB-adapted COVID-19 vaccines in a new round of free booster shots that starts Sept. 20, having agreed to buy 20 million doses from Pfizer and 5 million doses from Moderna.

In Japan and the rest of the world, XBB strains — part of the omicron group of variants — have been the most dominant type of the coronavirus. In Tokyo, EG.5, which is a mutated version of omicron subvariant XBB.1.9.2 and called Eris by health experts, has grown to become the most prevalent strain in recent months, accounting for 36% of all sequenced cases in the capital in the week through Aug. 20.