EXPLAINER: What to know about COVID vaccines for little kids

COVID-19 vaccinations for the youngest children just might be a step closer.

Moderna intends to seek U.S. authorization for kid-sized shots, releasing early study results Wednesday that suggest the two small doses work in tots younger than 6. Within weeks, competitor Pfizer hopes to learn if three of its even lower-dose shots do, too.

Here’s what is known so far, and what's next before the nation’s 18 million children under 5 can become eligible for vaccination.

MODERNA’S RESULTS

Moderna says tots as young as 6 months developed high levels of virus-fighting antibodies from shots containing a quarter of the dose given to adults. Full study results are yet to come but the early findings suggest the vaccine may protect against severe illness in kids just like it does in adults.

One complication: Moderna's study was conducted during the omicron surge, and none of the COVID-19 vaccines protect as well against infection with that super-contagious mutant — at any age — as they do against earlier variants.

There were no severe illnesses in the kid study. But sure enough, the vaccine proved just under 44% effective at preventing milder infections in tots up to age 2, and nearly 38% effective in the preschoolers.

PFIZER’S STUDY

The vaccine made by Pfizer and its partner BioNTech is the only one currently available for children in the U.S. Those 12 and older get adult-strength shots and 5- to 11-year-olds receive a third of that dose.

Pfizer is testing even smaller shots, a tenth of the adult dose, for kids under 5. Early results showed two shots produced enough antibodies to protect babies and toddlers but fell short for preschoolers.

Children’s immature immune systems often require multiple doses of vaccines to properly protect against other diseases. So rather than testing a...