The omicron XBB.1.5 subvariant does not have any mutations known to make people sicker when they catch the virus, according to a World Health Organization risk assessment published on Wednesday.

But the WHO noted in the report that it doesn’t have any real-world data on how XBB.1.5 is affecting patients’ health, so it cannot draw any conclusions at this time about the severity of the subvariant.

The WHO said XBB.1.5 is one of the Covid subvariants that is most adept at dodging immunity from vaccination or infection. It is just as immune evasive as another subvariant in its family, XBB.1., which was the Covid variant that best dodged antibodies that block infections.

The global health organization said XBB.1.5 has a growth advantage in the U.S., particularly in the Northeast where it has rapidly become dominant. XBB.1.5 could cause cases to increase globally, but it’s difficult to know for sure because almost all of the data is coming from the U.S., according to the WHO. The organization said it needs more data on how fast XBB.1.5 is spreading in other countries.

Maria van Kerkhove, the WHO’s Covid-19 technical lead, said last week that XBB.1.5 is the most transmissible Covid subvariant to date. Scientists believe it has a growth advantage because it is highly immune evasive and binds more tightly to human cells, making it more infectious.

“It is the most transmissible subvariant that has been detected yet,” Van Kerkhove told reporters during a press conference in Geneva last 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.”

In the U.S., XBB.1.5 is the only subvariant showing substantial growth right now. It rose from about 2% of cases in early to December nearly 28% in the first week of January, according to data from the Centers for Diesase Control and Prevention. It is causing more than 70% of new Covid cases in the Northeast.

Data on how well the vaccines hold up against XBB.1.5 is limited right now, though the subvariant is expected to cause more breakthrough infections. Lab studies have indicated other variants in the XBB family have proven adept at evading the antibodies generated by vaccination with the omicron boosters.

Although omicron subvariants have proven adept at causing breakthrough infections, Pfizer’s and Moderna’s vaccines are still generally preventing hospitalization and death. However, public health officials in the U.S. have emphasized that its particularly important for vulnerable groups, such as the elderly, to stay up to date on their shots to prevent severe illness.