As of Friday, CDC data estimates indicate that the quickly spreading variant is now causing almost half of infections nationwide, when just days ago it was estimated that JN.1 caused only about 20% of infections.
JN.1 makes up nearly 57% of new COVID cases in the Northeast, according to the data. Earlier this week, the variant was already dominant in the region, causing about a third of new infections at the time.
Globally, JN.1 continues to be reported in multiple countries, and its prevalence has been rapidly increasing. The World Health Organization named it a variant of interest on Tuesday because of its "rapidly increasing spread" but noted that the additional public health risk remains low.
The CDC estimates that prevalence of JN.1 more than doubled in the US between late November and mid-December. It seems to be getting an assist from holiday travel and waning immunity.
Variant trackers say they expect JN.1 to become the leading coronavirus variant around the world in a matter of weeks.
Vaccine immunity is expected to remain "cross-reactive" to JN.1, a descendant of the variant BA.2.86, itself a subvariant that came to the world's attention over the summer because of the large number of changes to its spike proteins.
COVID caused 7 hospitalizations for every 100,000 people in the week ending December 9, accounting for a 3% increase, CDC data shows.
More than three-quarters of US hospital beds are currently in use due to COVID hospitalizations, which is largely in line with trends over the past three years. However, the CDC warns that rising respiratory virus hospitalizations could strain health care resources in the coming weeks.
Overall COVID data shows weekly hospitalizations have been trending down in recent weeks.
Meanwhile, flu hospitalizations are rising but remain steady compared with recent weeks, according to CDC data. Flu-like activity is very high in five states and high in 15 states.
CNN Health's Brenda Goodman contributed to this report.
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