Enlarge / A CDC COVID-19 variant testing site inside Tom Bradley International Terminal at Los Angeles International Airport on Monday. The airport testing is part of the government's early warning system for detecting new variants, which began expanding recently in the wake of a COVID-19 surge in China. (credit: Getty | Jill Connelly/Bloomberg )

Amid a winter wave of COVID-19 in the US, the latest coronavirus omicron subvariant, XBB.1.5, has grabbed headlines due to its swift rise, raising fears of another towering spike in the disease. But the spotlight is revealing more questions than answers in the early days of the subvariant, which has ominously been described as one of the most immune-evasive omicron subvariants to date.

Last week, the US Centers for Disease Control and Prevention quietly downgraded estimates of its prevalence. As Ars and other outlets reported, the CDC previously estimated that XBB.1.5 accounted for 40.5 percent of COVID-19 cases throughout the country in the week ending on December 31, with the highest prevalence in the Northeast. But last Friday, the agency updated the estimates with a backlog of sequencing data from over the holidays, which indicated XBB.1.5 accounted for 18 percent of cases nationwide that week—not 40.5 percent. Currently, the CDC estimates that XBB.1.5 accounted for 27.6 percent of cases nationwide in the week ending on January 7. But the 95 percent prediction interval is wide, spanning 14 percent to 46.5 percent).

The updated estimate still indicates that the variant, first detected in New York in October, is on the rise. But the uncertainty throws a wrench in estimates of its transmission advantage over other omicron subvarianty, BQ.1.1 is still the most prevalent omicron subvariant, accounting for an estimated 34 percent of cases in the US.

Enlarge / Members of the public queue outside a pharmacy to receive COVID-19 antigen tests in Paris on January 6, 2022. (credit: Getty | LUDOVIC MARIN )

Omicron subvariants BA.4 and BA.5 are on the rise in the European Union, spurring officials there to warn that a surge of COVID-19 cases will likely follow in the coming weeks.

In an alert Monday , the European Center for Disease Prevention and Control cautioned that various factors would influence how bad the expected BA.4/BA.5 wave will be. Those factors include the extent of vaccination and past infection in the population, as well as timing since those events because protection from both wanes over time.

BA.4 and BA.5 are clumped together because they share the same mutations in the genetic coding for their spike proteins, though they have differing mutations elsewhere in their genome. Both have a transmission advantage over the initial omicron subvariant, BA.1, as well as subvariants BA.2 and BA.2.12.1.

Enlarge / Adenoviruses remain the leading suspect, though no cause has been identified. (credit: Getty | BSIP )

The global tally of unexplained hepatitis cases in children has reached about 450, including 11 reported deaths, according to an update from the European Centre for Disease Prevention and Control .

The cases come from more than two dozen countries around the world, with about 14 countries reporting more than five cases. The countries with the largest case counts so far are the United Kingdom and the United States.

In the UK, officials have identified 163 cases in children under the age of 16, 11 of whom required liver transplants. Last week, the US Centers for Disease Control reported 109 cases under investigation in children under the age of 10 from 25 states. Of those cases, 14 percent required liver transplants, and five children died.