The increase in hospitalizations among children ages 4 and below, who are ineligible for vaccines, surged to a seven-day average of more than four in 100,000 as of Jan. 1, according to data from the Centers for Disease Control and Prevention. 

That’s double the rate it was a month ago, and about three times as high as the rate of hospitalizations for children 4 years old and younger at this time last year, according to the CDC.

While federal health officials are still considering the possibility that Omicron may cause a more severe reaction in young children, doctors and public health experts said the rise is more likely the result of Omicron’s high transmissibility rate and not its severity. 

“This is likely due to the sheer number of children who are catching the Omicron variant as compared to prior variants, and not because Omicron is particularly more dangerous than Delta [or other previous variants],” Gregg Miller, an ER doctor in Washington and chief medical officer at Vituity, a physician-owned health care company that specializes in acute care innovation, told Fortune.

At a press conference Friday, CDC director Dr. Rochelle Walensky said that while the CDC was “still learning” about the severity of Omicron in children, early signs were promising.

“We have not yet seen a signal that there is any increased severity in this age demographic,” Walensky said. 

Hospitalization rates among children under 18 remain lower than any age group overall, and the recent data includes children who were admitted to hospitals for reasons other than COVID, but ended up testing positive after they were admitted. 

“Many children are hospitalized with COVID as opposed to because of COVID,” said Dr. Anthony Fauci, the nation’s top infectious disease expert, during a Dec. 29 press briefing at the White House.

Walensky and other health officials have cited the inability for young children to get vaccinated as a major reason that adults should make sure those who are eligible receive the vaccine and booster shots as soon as possible. 

“Please, for our youngest children, those who are not yet eligible for vaccination, it’s critically important that we surround them with people who are vaccinated to provide them protection,” Walensky said Friday. 

However, even among children who are eligible, vaccination rates remain low. Children 5 and older remain unvaccinated at higher rates than the general population, despite the U.S. Food and Drug Administration authorizing the vaccine’s use in children ages 5 to 11 last October. Only 16% of children between the ages of 5 and 11 are fully vaccinated, and that number climbs to just over 50% of adolescents between ages 12 and 17, according to CDC data. 

Experts say they anticipate children younger than 5 could be eligible for the vaccine by the summer. 

“Hopefully, the Pfizer vaccine will be available this summer—but unfortunately, that won’t be in time to help with the current Omicron surge,” Miller said. “Once vaccines are available for this age group, parents should vaccinate their children against COVID, just like we do against many other diseases.”  

On Friday, President Joe Biden signaled there’s hope that Omicron’s peak may be arriving in the coming weeks. 

“I don’t think COVID is here to stay,” Biden told reporters at the White House. “The new normal is not going to be what it is now, it’s going to be better.”