Wis. (WLAX/WEUX) – The most contagious variant of COVID, so far, is spreading across the country. Levels are especially high in the northeast.

XBB1.5, responsible for less than three percent of active COVID cases at the beginning of December, is now the second leading variant in the US, causing nearly 28 percent of infections, according to new CDC estimates.

Epidemiologists say it’s more transmissible than previous versions of the virus but does not necessarily cause more severe disease.

“Right now, as we’re looking at preliminary data, you don’t see a disproportionate rise of hospitalizations and deaths associated with this particular subvariant, which is good news.”

Researchers say Americans are better prepared for a post-holiday COVID surge than last winter because there’s more immunity from natural infection and vaccination and even though new “omicron” targeting boosters offer little protection against infection. Experts say they continue to do what’s most important.

“Early laboratory studies would indicate that prevention against severe disease is still pretty good.”

Researchers believe XBB1.5 originated in the US, but they’re also worried about the potential for new mutations in China, which is experiencing a surge in cases as it ends lockdowns.

“It’s the question of having so much transmission in China that that could be the seedbed for a new variant.”

This week, the US began requiring a negative COVID test for air passengers traveling from China. Beijing says these measures are unnecessary, but, international health officials believe China is underreporting its data on infections, hospitalizations, and especially deaths after seeing independent reports of overwhelmed hospitals and crematories.