Another new coronavirus variant, named EG.5, is spreading fast in Japan, though experts say there is little evidence so far that it causes more severe symptoms compared with previous strains that also belong to the omicron family.

EG.5, a mutated version of omicron subvariant XBB.1.9.2, was first discovered in February and has been reported in 51 countries, including the U.S., South Korea, China, Australia, Singapore and Britain, according to the World Health Organization.

Last week, the WHO designated EG.5 and its sublineages as “a variant of interest,” upgrading them from the “variant under monitoring” label it gave in July.

"Collectively, available evidence does not suggest that EG.5 has additional public health risks relative to the other currently circulating (omicron) descendent lineages," the WHO said in its initial risk evaluation.

In Japan and the rest of the world, XBB strains — part of the omicron group of variants — are the most dominant type of the coronavirus. In Tokyo, XBB.1.16 is the most common variant, accounting for 28% of all genomically sequenced cases, according to the latest data from the Tokyo Metropolitan Government.

But the prevalence of EG.5, called Eris by health experts, nearly doubled in July to account for 22% of cases in the capital.

The ratio of EG.5 cases has grown nationally as well. According to projections made by the National Institute of Infectious Diseases, it is estimated that EG.5.1, a subtype of EG.5, will account for 46% of all cases in Japan in the week through Sunday.

In the U.S., where EG.5 is now the most common coronavirus strain, making up 17.3% of sequenced cases in the two weeks through Aug. 5, authorities said updated COVID-19 vaccines targeting XBB should offer protection.

"Right now, what we're seeing with the changes in the viruses, they're still susceptible to our vaccine, they're still susceptible to our medicines, they're still picked up by the tests," Mandy Cohen, director of the U.S. Centers for Disease Control and Prevention, said in an interview with Reuters about EG.5. "We're seeing small changes that are what I would call subtypes of what we've seen before."

Japan is currently in the midst of what some experts call a ninth wave of coronavirus infections. The latest COVID-19 statistics released Monday show that the number of new cases is leveling off, while the number of new hospitalizations has kept rising, reaching 11,801 people in the week through Aug. 6.

The government plans to start a new vaccine drive on Sept. 20 using XBB shots. The shots will be available for free to everyone who is at least six months old, but the health ministry has decided to actively promote them only to people age 65 and over and those with underlying health conditions.