Hospitalisations of US children under five with COVID-19 soared in recent weeks to their highest level since the pandemic began, according to government data on the only age group not yet eligible for the vaccine.

The worrisome trend in children too young to be vaccinated underscores the need for older kids and adults to get their shots to protect those around them, said Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention.

Since mid-December, as the highly contagious Omicron variant has spread furiously around the country, the hospitalisation rate in these youngest children has surged to more than four in 100,000 youngsters, up from 2.5 per 100,000.

That compares with a current rate of about one per 100,000 for children ages 5 to 17, according to CDC data released on Friday.

In a statement, Walensky said that while children still have the lowest rate of hospitalisation of any age group, "pediatric hospitalisations are at their highest rate compared to any prior point in the pandemic."

At a briefing, she said the numbers include children hospitalised because of COVID-19 and those admitted for other reasons but found to be infected.

She noted that just over 50 per cent of children ages 12 to 18 are fully vaccinated and only 16 per cent of those 5 to 11 are fully vaccinated.

As of Tuesday, the average number of children and teens admitted to the hospital per day with COVID-19 was 766, double the figure reported just two weeks ago.

At a White House briefing this week, Dr Anthony Fauci, the top US infectious-disease expert, 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 that one of the best ways to protect the youngest children is to vaccinate everyone else.

Data suggest booster shots offer the best protection against Omicron, and CDC this week recommended them for kids as young as 12. Among older ages already eligible, just 34 per cent have received them.