Nurse Lydia Holly prepares a child's COVID-19 vaccine dose, on Nov. 3, 2021, at Children's National Hospital in Washington. (AP Photo / Carolyn Kaster, File)

The nation’s 19 million children under 5 are the only group not yet eligible for vaccination against the coronavirus. Many parents have been pushing for an expansion of shots to toddlers and preschoolers, especially as the omicron wave sent record numbers of youngsters to the hospital.

Dr. Rochelle Walensky, Director of the Centers for Disease Control and Prevention (CDC) speaks during an interview with The Associated Press on Dec. 8, 2021, in Atlanta. (AP Photo / Brynn Anderson, File)

One year into Dr. Rochelle Walensky’s tenure as director, her bid to make the Centers for Disease Control and Prevention more agile is being challenged by political pressures, vocal scientists and the changing virus itself.

People wait to be vaccinated by a member of the Western Cape Metro EMS (Emergency Medical Services) at a mobile "Vaxi Taxi" which is an ambulance converted into a mobile COVID-19 vaccination site in Blackheath in Cape Town, South Africa, Tuesday, Dec. 14, 2021. (AP Photo/Nardus Engelbrecht)

The findings released Tuesday are preliminary and have not been peer-reviewed — the gold standard in scientific research — but they line up with other early data about omicron's behavior, including that it seems to be more easily transmitted.