The New York State Department of Health said Friday a fourfold increase in admissions that began the week of Dec. 5 is concentrated in New York City and the surrounding area, where the highly contagious omicron variant was spreading rapidly.
Pediatric hospitalizations are up 395% in New York City since the week ending Dec. 11, jumping from 22 to 109 the week ending Dec. 23. Statewide pediatric hospitalizations jumped from 70 to 184.
"The risks of COVID-19 for children are real," said acting State Health Commissioner Dr. Mary T. Bassett. "We are alerting New Yorkers to this recent striking increase in pediatric COVID-19 admissions so that pediatricians, parents and guardians can take urgent action to protect our youngest New Yorkers."
The state's trend is having a particularly disturbing impact on unvaccinated children -- even as the state touts a high vaccination rate among adults. Of the 5- to 11-year-old children admitted to New York City hospitals with COVID from the week of Dec. 5 until the current week, none were fully vaccinated.
"There's just no reason. We have the supply. We have the capacity. We have the staff in place for every child to be vaccinated who is eligible," Bassett said Monday.
Across the country, nearly 2,000 children are currently hospitalized with confirmed or suspected cases of COVID-19 -- up by approximately 700 pediatric patients compared to a month ago, according to federal data.
On average, more than 260 children are being admitted to the hospital each day, an 80% increase over four weeks.
While New York has the highest number of pediatric hospitalizations, Ohio and Texas also reported more than 150 children hospitalized with COVID-19.
Last week, Philadelphia officials revealed a "sharp increase in test positivity" in school staff and students: One in 20 kids and teens who took COVID-19 tests had positive results for coronavirus last week, a nearly fivefold increase in children in the last two weeks.
Similar trends were reported overseas.
A Dec. 23 U.K. analysis found that unvaccinated children ages 5 to 11 comprised the "highest prevalence" of positive COVID-19 tests by age group.
And preliminary data from South Africa estimates that children had a 20% higher risk of hospitalization in the country's omicron-driven fourth wave, given the fact that so many children were still unvaccinated, and therefore, unprotected.
"If you have a child 5 to 11, please get that child vaccinated to prevent them from getting anything that even resembles a serious illness," Dr. Anthony Fauci said in a "Good Morning America" interview Monday.
WATCH: Dr. Fauci talks why omicron cases are expected to keep rising
Millions of children remain unvaccinated
As of Dec. 5, fewer than 17% of 5-to-11-year-old children had received at least one COVID-19 vaccine dose, according to a KFF analysis.
This comes even though in the U.S., anyone over the age of 5 is eligible for the vaccine. Pfizer shots were authorized by the Food and Drug Administration for those 5 to 11 at the end of October.
Now, given the potential for the highly transmissible omicron variant to cause an even greater wave of infections, experts say it is more critical than ever for children to be vaccinated.
Although young people have largely been spared from acute COVID-19 illness, experts stress that children are not immune from the virus. According to the Centers for Disease Control and Prevention, children are as likely to be infected with COVID-19 as adults, and the virus is now one of the top 10 causes of death for children ages 5 through 11 years.
MORE: Early indicators suggest omicron symptoms are mild, flu-like; more data needed to be sure
"[Omicron's] increased transmissibility makes it possible that we'll see very high case numbers in children, especially if they remain unvaccinated," Dr. Kristin Moffitt, an infectious disease specialist at Boston Children's Hospital told ABC News. "Even if severe infection remains relatively uncommon in children, if case numbers in children skyrocket, we'll see many more pediatric hospitalizations."
However, about two-thirds of parents of elementary school-aged children are either holding off on getting their younger children vaccinated or refuse to do so, according to another recent KFF poll, conducted before the discovery of omicron.
The safety profile of the Pfizer vaccine for eligible children remains "very reassuring," added Moffit.
The CDC has yet to identify any concerns with the temporary heart inflammation known as myocarditis, a potential side effect of mRNA vaccines seen in rare circumstances in teenagers and young adults.
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How can I protect a child too young for a COVID-19 vaccine?
Children younger than 5 can't get COVID-19 vaccines in the U.S. yet, but there are steps you can take to protect them from infection over the holidays.
"Surround them with adults and siblings who are vaccinated, boosted if eligible," advises Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention.
She also encourages taking COVID-19 home tests before gatherings.
The CDC recommends that anyone who's not vaccinated - including children ages 2 and older - wear masks indoors in public. If your child is younger than 2 -- or cannot wear a mask for other reasons -- the agency suggests limiting visits with unvaccinated people. And it says to keep a distance between the child and others in public places.
Adults might also opt to wear a mask indoors in public to set an example for young children, the CDC says. But in virus hot spots, it says everyone should wear masks in those settings, regardless of whether they're vaccinated.
The Associated Press and ABC News contributed to this report.