Getting the youngest child vaccinated against COVID-19 may be one step closer.

Moderna intends to seek U.S. authorization for the child-sized injections, and on Wednesday released results from early studies showing the two small doses work in children younger than 6 years old. In a few weeks, rival Pfizer hopes to see if three of its lower-dose injections will work, too.

Here’s what’s known so far, and what’s next before the nation’s 18 million children under the age of 5 become eligible for the vaccine.

Results in Modena

Moderna said that only 6-month-olds developed high levels of antiviral antibodies when given the vaccine, which contained a quarter of the adult dose. Full study results are yet to be released, but early findings suggest the vaccine can prevent serious illness in children as well as in adults.

One complication: Moderna’s research was conducted during the omicron surge, and a COVID-19 vaccine at no age does not protect against infection by this super-infectious mutant as well as against earlier variants.

There were no serious illnesses in the children’s study. But to be sure, the vaccine proved to be slightly less than 44 percent effective at preventing mild infections in children under the age of 2, and nearly 38 percent effective in preschoolers.

Pfizer Research

The vaccine produced by Pfizer and its partner BioNTech is currently the only vaccine available for children in the United States. Those aged 12 and older received the adult-strength vaccine, while those aged 5 to 11 received a third of the dose.

Pfizer is testing smaller injections, one-tenth the adult dose, for children under 5. Early results showed that two injections produced enough antibodies to protect infants and toddlers, but not enough for preschoolers.

Children’s immature immune systems often require multiple doses of vaccines to properly protect against other diseases. So instead of testing a higher dose, Pfizer gave the kids a third shot. Results are expected in early April.

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What’s next?

Once the FDA receives an application from one or both companies, it is expected to debate the evidence publicly with its scientific advisors. If the FDA authorizes the vaccine for the youngest child, the Centers for Disease Control and Prevention will call in its own experts before recommending whether all teens need the vaccine — or only those at higher risk of contracting COVID-19.

While cases in the United States are currently declining, cases in other countries are increasing. Dr. Bill Mueller of Northwestern University, a Pediatrics research fellow at Moderna, said the lull period may be “the best time to get vaccinated because by the time the next surge starts, you’ll be protected.”

Why do children need vaccines?

While COVID-19 is generally not as dangerous in teens as adults, some people do get very sick and even die. About 400 children under the age of 5 have died from COVID-19 since the pandemic began, according to the CDC. The omicron variant hit children especially hard, with hospitalization rates in children under 5 higher than at the peak of the previous delta surge, the CDC found.

So far, about 57% of 12- to 17-year-olds have received two Pfizer vaccines, compared with 27% of 5- to 11-year-olds.

What about Moderna lenses for older kids?

The FDA has restricted Moderna’s vaccine to adults, although it is used in children under 6 in certain other countries. The FDA is studying a very rare side effect — inflammation of the heart — that sometimes occurs in teens and young adults, mostly men.

Moderna is providing updated safety information to the FDA, saying it supports an adult dose in children ages 12 to 17 and is seeking authorization to use half that dose in children ages 6 to 11. Regulators should consider all three age groups simultaneously.