The XBB.1.5 omicron subvariant, that’s currently dominating the U.S. is the most contagious version of Covid-19 yet, but it doesn’t appear to make people sicker, according to the World Health Organization, CNBC reports. 

Maria Van Kerkhove, the WHO’s Covid-19 technical lead, said global health officials are worried about how quickly the subvariant is spreading in the northeastern U.S. The number of people infected with XBB.1.5 has been doubling in the U.S. about every two weeks, making it the most common variant circulating in the country.

“It is the most transmissible subvariant that has been detected yet,” Van Kerkhove told reporters during a press conference in Geneva on Wednesday. “The reason for this are the mutations that are within this subvariant of omicron allowing this virus to adhere to the cell and replicate easily.”

It has been detected in 29 countries so far but it could be even more widespread, Van Kerkhove said. Tracking Covid variants has become difficult as genomic sequencing declines across the world, she said.

The WHO doesn’t have any data yet on the severity of XBB.1.5, but there’s no indication at the moment that it makes people sicker than previous versions of omicron, Van Kerkhove said. The WHO’s advisory group that tracks Covid variants is conducting a risk assessment on XBB.1.5 that it will publish in the coming days, she said.

“The more this virus circulates the more opportunities it will have to change,” Van Kerkhove said. “We do expect further waves of infection around the world but that doesn’t have to translate into further waves of death because our countermeasures continue to work.”