The Centers for Disease Control and Prevention (CDC) found that hospitalizations of American children under 5 with COVID-19 has greatly increased in recent weeks. It is the only age group that cannot yet get the COVID-19 vaccine. The rates of hospitalized children are now at the highest level since the pandemic began.

Since mid-December, the hospitalization rate in the youngest children has increased from 2.5 in 100,000 to more than 4 in 100,000. The omicron variant has quickly spread during this period in the United States.

The CDC information is taken from over 250 hospitals in 14 states. It shows about 1 in 100,000 children ages 5 to 17 are hospitalized with COVID-19.

Dr. Rochelle Walensky is the director at the CDC. She said hospitalizations for children are at their highest rates compared to any former point in the pandemic. She noted that just over 50 percent of children ages 12 to 18 and only 16 percent of those 5 to 11 are fully vaccinated.

The CDC says the overall hospitalization rate among those under 18 is still lower than that of any other age group. And they make up less than five percent of average new daily hospital intakes.

As of January 4, the average number of under-18 patients admitted to the hospital per day with COVID-19 was 766, double the number reported just two weeks ago.

The CDC said increase among the very youngest children is being driven by high hospitalization rates in five states: Georgia, Connecticut, Tennessee, California, and Oregon. The greatest increases were in Georgia.

Walensky said the numbers include children hospitalized because of COVID-19 and those admitted for other reasons but found to be infected.

The CDC said the increase could partially be explained by how COVID-19 hospitalizations in this group are defined: a positive virus test within 14 days of hospitalization for any reason.

Dr. John McGuire is critical care chief at Seattle Children's Hospital. He said the severity of illness among children during the omicron wave seems lower than it was with the delta variant.

Most of the hospitalized children with COVID came in for other medical reasons, "but happened to have tested positive," McGuire said in an email.

The nation's top infectious-disease expert, Dr. Anthony Fauci, said that omicron appears to cause less-severe disease. But because of how quickly it spreads, it causes more infections, and more children will get the disease. And some of them will be hospitalized.

Fauci also said that many children hospitalized with COVID-19 have other health problems that make it easier for them to have other issues from the virus.

Fauci and Walensky have said that one of the best ways to protect the youngest of children is to vaccinate everyone else.

Dr. Jennifer Kusma is a children's doctor with Chicago's Lurie Children's Hospital. She said she has seen increasing numbers of children hospitalized with omicron. While most are not severely sick, she understands the concerns parents have for their young children.

"I really wish we already had that vaccine for these young kids,'' Kusma said. She added that what seems like a long wait should help parents' concern that the vaccine testing is not being hurried.

Many hoped the new year might bring a vaccine for young children. But Pfizer announced last month that the two shots did not offer as much protection as hoped for children ages two to four.

Pfizer's study has been renewed to give everyone under 5 a third shot and the information is expected in early spring.

Among 102 children ages 12 to 18 who were hospitalized with COVID-19, none who had received two Pfizer shots at least 28 days earlier needed ventilators or other advanced life support. Of unvaccinated children, 40 percent required such treatment.

I'm Gregory Stachel.

Raf Casert reported this story for The Associated Press. Gregory Stachel adapted it for VOA Learning English.

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variant – n. something that is different in some way from others of the same kind

positive – adj. showing the presence of a particular germ, condition, or substance

kid – n. a young person

ventilator – n. a device for helping a person to breathe