Hospitalisations of US children under 5 with Covid-19 soared in recent weeks to their highest level since the pandemic began, according to government data released on Friday on the only age group not yet eligible for the vaccine.
The worrisome trend in children too young to be vaccinated underscored the need for older kids and adults to get their shots to help protect those around them, said Dr Rochelle Walensky, director of the Centres for Disease Control and Prevention.
Since mid-December, with the highly contagious Omicron variant spreading furiously around the country, the hospitalisation rate in these youngest kids has surged to more than 4 in 100,000 children, up from 2.5 per 100,000.
The rate among children ages 5 to 17 is about 1 per 100,000, according to the CDC data, which is drawn from more than 250 hospitals in 14 states.
Overall, "paediatric hospitalisations are at their highest rate compared to any prior point in the pandemic," Walensky said.
The overall hospitalisation rate among children and teens under 18 is still lower than that of any other age group. And they account for less than 5 per cent of average new daily hospital admissions, according to the CDC.
As of Tuesday, the average number of under-18 patients admitted to the hospital per day with Covid-19 was 766, double the figure reported just two weeks ago.
The trend among the very youngest kids is being driven by high hospitalisation rates in five states: Georgia, Connecticut, Tennessee, California and Oregon, with the steepest increases in Georgia, the CDC said.
At a briefing, Walensky said the numbers include children hospitalised because of Covid-19 and those admitted for other reasons but found to be infected.
The CDC also said the surge could be partially attributable to how Covid-19 hospitalisations in this age group are defined: a positive virus test within 14 days of hospitalisation for any reason.
The severity of illness among children during the Omicron wave seems lower than it was with the Delta variant, said Seattle Children's Hospital critical care chief Dr John McGuire.
"Most of the Covid+ kids in the hospital are actually not here for Covid-19 disease," McGuire said in an email. "They are here for other issues but happen to have tested positive."
The nation's top infectious-disease expert, Dr Anthony Fauci, said earlier this week that Omicron appears to cause less-severe disease across the board, but that the sheer number of infections because of its extreme contagiousness will mean many more children will get infected, and a certain share of them will wind up in hospital.
Fauci also said many children hospitalised with Covid-19 have other health conditions that make them more susceptible to complications from the virus. That includes obesity, diabetes and lung disease.
Fauci and Walensky have emphasised one of the best ways to protect the youngest children is to vaccinate everyone else.
The surge in hospitalisations only heightens some parents' worries.
Emily Hojara and Eli Zilke of Sawyer, Michigan, are being extra protective of their daughter Flora, who turns 2 in May. They limit her contact with other children, and no visitors are allowed in the house unless masked, not even grandparents.
"It's been a struggle, and now with this new variant, I feel it's knocked us back," Hojara said.
"It's scary that she can't be vaccinated," she said of her daughter.
Dr Jennifer Kusma, a paediatrician with Chicago's Lurie Children's Hospital, said she has seen increasing numbers of kids hospitalised with Omicron, and while most aren't severely ill, she understands parents' worries.
"I really wish we already had that vaccine for these young kids," Kusma said. But she added that what may seem like a long wait should reassure parents that vaccine testing is not being rushed.
Many had hoped the new year might bring a vaccine for young children, but Pfizer announced last month that two doses didn't offer as much protection as hoped for in youngsters 2 to 4.
Pfizer's study has been updated to give everyone under 5 a third dose, and data is expected in early spring.
Also on Friday, the CDC issued a report showing Pfizer shots seem to protect older children who develop a serious but rare Covid-19-linked condition that involves inflammation of multiple organs.
Among 102 kids ages 12 to 18 who were hospitalised with the condition, none who had received two Pfizer shots at least 28 days earlier needed ventilators or other advanced life support. By contrast, 40 per cent of unvaccinated children required such treatment.
The condition, multisystem inflammatory syndrome, causes symptoms that may include persistent fever, abdominal pain and rashes. Most children recover, but 55 deaths have been reported.
A separate CDC report found that children who had Covid-19 were more than twice as likely to be diagnosed with diabetes as youngsters who had not had the virus. Scientists are investigating why but say the virus seems to attack insulin-producing cells in the pancreas.