This morning, Pfizer/BioNTech submitted data to the Food and Drug Administration from its clinical trial of a Covid vaccine for children, launching a process that could deliver doses to kids between 5 and 11 within the next month or two.

With child cases rising—last week, those under age 18 made up almost 27 percent of newly-reported cases, according to the American Academy of Pediatrics, although they only represent 22 percent of the US population—there’s a lot of pressure to move through regulation quickly. But parents who have been waiting tensely for this moment may find more hurdles in their way than they expect. As complex as it has been, getting the vaccine authorized was just part of the challenge of protecting kids. Getting it distributed and administered will be complex, too.

For one thing, the new shots will arrive in a system that’s already overwhelmed. “Immunization programs haven’t had a second to think about this group yet, because they have been dealing with adult vaccination, and additional doses for the immunocompromised, and influenza vaccination season, which can’t be postponed,” says Kelly Moore, a physician and president and CEO of the nonprofit Immunization Action Coalition. “They haven’t been able to catch their breath, which is a problem, because it’s coming up fast.”

But those shots also will be delivered to kids in different locations, likely by different personnel, than the adult vaccines released last winter. They are arriving in a national landscape that is even more politicized than it was before. And it’s not even clear how many Americans welcome them: A Gallup poll released just this morning finds that 45 percent of parents do not plan to vaccinate their young kids.

A recap of where we are on shots and kids. Let’s stipulate, first, that all these plans address just Pfizer/BioNTech, the only manufacturer whose trials have moved far enough ahead to allow for child vaccination. Their vaccine has had full FDA new drug approval since August, but only for ages 16 and up; for kids 12 to 15, the vaccine remains under emergency use, and kids younger than 12 have not been able to receive it at all.

Pfizer has been pursuing clinical trials aimed at clearing the vaccine for younger kids since March. The approximately 4,500 children in those trials are split into three cohorts: 6 months to 23 months; 2 years to just under 5; and 5 to 11. In each group, the vaccine being tested is some fraction of the adult dose, from one-third down to one-tenth. Last week, the company made its first comment on its pediatric trial results, saying that two rounds of those one-third doses achieved the same antibody levels in 5- to 11-year-olds as two full-sized doses did in 16- to 25-year-olds. It is those results, with a lot more data, that the company brought to the FDA today. Data for the under-5s is supposed to follow.

(The other manufacturers haven’t gotten as far in their trials. Moderna’s vaccine has an emergency use authorization for ages 18 and over, and is awaiting full approval for adults and authorization for younger teens. The company began studies in children younger than 12 in August. J&J, the single-dose vaccine, is behind the other two companies in its research; it still has only an emergency authorization that just covers adults.)

Since last summer, Pfizer, regulators, and business analysts have all been saying that they hope the first authorization would arrive in late October or November. That estimate still looks solid, if optimistic. The timeline is wiggly, because no one outside the FDA knows how rapidly the agency will review what are likely to be complex data sets, covering efficacy, safety and side effects. Two independent committees, advisors to the FDA and the Centers for Disease Control and Prevention, must weigh in afterward, and no one outside their ranks can say how thoughtfully they will scrutinize that data. (Those are the two committees that met in the past two weeks to discuss and ultimately approve booster shots for a wide swath of Americans.)