In a sign that the highly contagious omicron variant is sparing no one, a report shows that new COVID-19 cases among U.S. children spiked to a high of more than 580,000 for the week ending Jan. 6, a 78 percent increase from the week before.

"Since many children remain unvaccinated -- or are too young to be vaccinated -- children are bearing a disproportionate burden of this illness," said Moira Szilagyi, M.D., president of the American Academy of Pediatrics. "These are challenging times."

Since the start of the pandemic, nearly 8.5 million U.S. children have tested positive for COVID-19, and nearly 11 percent of those cases have occurred in the past two weeks, according to the report that uses state-by-state data. Just two weeks ago, 325,000 cases were reported.

"The good news is that we know much more about this virus than we did two years ago, and we have more tools at our disposal," Szilagyi said in an AAP news release. "We understand the disease and how best to isolate and care for infected patients. We have access to masks and other personal protective equipment. And we have a vaccine that offers strong protection against severe illness, even with omicron."

While omicron has infected both vaccinated and unvaccinated people, those who are vaccinated are much less likely to become severely sick or be hospitalized. Millions of children and teens have been vaccinated, and the safety data are reassuring, according to the AAP. The academy said the vaccine does a good job of protecting children from the worst effects of the disease.

The AAP noted that the report's data on new case numbers are limited because the data rely on how each state reports its cases.