Pfizer and BioNTech on Friday said they were delaying their request for the Food and Drug Administration to authorize the vaccine for children under 5 until at least April, as they wait for data on the efficacy of a third dose.

“Cases continue to accumulate according to the study protocol and more data are being generated because rates of infection and illness remain high in children of this age, especially due to the recent Omicron surge,” Pfizer said in a statement.

The FDA said it’s postponing a meeting Tuesday that was scheduled to look at the kids’ data.

The Centers for Disease Control and Prevention was already laying the groundwork to distribute the shots, telling state and local health officials they could receive their first shipments by Feb. 21.

The CDC plans to roll out 10 million doses in three phases as soon as the Food and Drug Administration authorizes the lower-dose, 3-microgram Pfizer and BioNTech shot for children 6 months to 4 years old, according to a new planning document quietly issued Sunday. State and local health officials could start preordering the first doses Monday and will start receiving vaccine shipments on Presidents Day, according to the CDC.

Pfizer and BioNTech asked the FDA last week to expand the authorization for their vaccine to most kids under 5. The FDA had asked the companies to start submitting data for authorization of the first two-doses of the three-dose vaccine for children six months through 4-years-old.

Pfizer and BioNTech expect kids under 5 will ultimately need three doses for the highest level of protection against the omicron Covid variant and future strains of the virus. The companies said data on the third dose will be finished and submitted to the FDA in the coming months.

Pfizer amended its clinical trial for younger kids in December to study a third shot after the first two doses did not produce an adequate immune response in children 2- to 4-years-old. Younger kids will receive a smaller, three microgram dose compared with the 30 microgram shots that are approved for adults.

Children under 5-years-old are the last age group left in the U.S. that isn’t eligible for vaccination. There has been growing public pressure from many parents and doctors for the FDA to speed up authorization of the vaccine as the omicron variant has lead to an increase in children hospitalized with Covid.

Pfizer and BioNTech said last week that their application for emergency approval came in response to an “urgent public health need” for younger children as omicron has caused an unprecedented wave of infection across the country.