For weeks, the school principal had been imploring Kemika Cosey: Would she please allow her children, ages 7 and 11, to get COVID shots?

Cosey remained firm. A hard no. But “Mr. Kip” — Brigham Kiplinger, principal of Garrison Elementary School in Washington, D.C. — swatted away the “no.”

Since the federal government authorized the coronavirus vaccine for children ages 5-11 nearly three months ago, Kiplinger has been calling the school’s parents, texting, nagging and cajoling daily. “The vaccine is the most important thing happening this year to keep kids in school,” Kiplinger said.

As the omicron variant has stormed through U.S. classrooms, sending students home and, in some cases, to the hospital, the rate of vaccination overall for America’s 28 million children ages 5-11 remains even lower than health experts had feared. According to a new analysis by the Kaiser Family Foundation based on federal data, only 18.8% are fully vaccinated, and 28.1% have received one dose.

The disparity of rates among states is stark. In Vermont, the share of children who are fully vaccinated is 52%; in Mississippi, it is 6%.

After the Pfizer-BioNTech vaccine was authorized for younger children in late October, the out-of-the-gate surge in demand lasted a scant few weeks. It peaked just before Thanksgiving, then dropped precipitously and has since stalled. It hovers at 50,000 to 75,000 new doses a day.

Public health officials say persuading parents to get younger children vaccinated is crucial not only to sustaining in-person education but also to containing the pandemic overall. With adult vaccination hitting a ceiling — 74% of Americans ages 18 and older are fully vaccinated — unvaccinated elementary school children remain a large, turbulent source of spread.

As caseloads of children in whom COVID has been diagnosed keep rising, proponents of COVID shots are redoubling their efforts to convince parents. The American Academy of Pediatrics has put together talking points for pediatricians and parents, Kaiser has a parent-friendly vaccine-information site, and Johns Hopkins Bloomberg School of Public Health has posted a free online training course to help give pro-vaccine parents language and ways to approach resistant friends.

This article originally appeared in The New York Times.