WASHINGTON — Moderna said on Wednesday that it would seek emergency authorization of its coronavirus vaccine for children younger than 6, after interim results from its clinical trial showed that volunteers in that age group had a similar immune response to young adults when given a dose one-fourth as strong.

But the company said the vaccine proved only about 44 percent effective in preventing symptomatic illness among children 6 months to 2 years old, and 37 percent effective in children 2 through 5.

Dr. Jacqueline Miller, the firm’s senior vice president for infectious diseases, said the relatively low level of protection demonstrated the ability of the Omicron variant to evade the vaccine’s shield. Nonetheless, she said in an interview, “what we have seen is a successful trial.”

“What I will say is 37.5 percent and 43.7 percent are higher than zero,” she said. “If I were the parent of a young child, I would want there to be some protection on board, especially if we see another wave of infections.”

The firm’s announcement comes as the Biden administration’s effort to protect people against an ever-mutating virus enters a new period of flux. Officials are debating whether the oldest Americans, at least, should be offered a second booster shot this spring. Meanwhile, various studies are seeking to determine whether the existing vaccines can be reconfigured to provide more protection against Omicron and the subvariant of the virus known as BA.2.

Now, Moderna’s findings about how well its vaccine works in the nation’s youngest children — the only Americans not yet eligible for shots — are bringing another question to the forefront: What level of effectiveness is good enough for a pediatric vaccine?

Just last month, Pfizer and its German partner, BioNTech, delayed seeking emergency authorization for their coronavirus vaccine in young children after data gathered during the Omicron surge showed two doses were less than 50 percent effective against symptomatic disease, according to people familiar with the situation.

Whether the Food and Drug Administration, the Centers for Disease Control and Prevention, and the public will be willing to accept Moderna’s efficacy rate of about 40 percent on average for children under 6 is unclear.

While Americans have been hearing for months that the vaccines are less potent against Omicron in all age groups, neither Moderna’s nor Pfizer’s results in young children meet what many vaccine experts consider the minimum standard for effectiveness against Covid-19. The guidance that the F.D.A. initially set for adult coronavirus vaccines was at least 50 percent effectiveness against symptomatic infection.

All three authorized vaccines — from Moderna, Pfizer and Johnson & Johnson — vastly surpassed that at first. But Omicron’s uncanny ability to dodge the immune system’s defenses is changing the calculus, and so far, no new standard has been set.

The F.D.A. authorized the Pfizer-BioNTech vaccine for those aged 12 to 15 in May after a trial showed zero infections in the vaccine arm and 18 in the placebo group — a ratio Pfizer described as 100 percent efficacy. But in October, when the agency authorized Pfizer’s vaccine for children 5 to 11, it relied solely on immune response data, which showed that the vaccine triggered a powerful surge in antibodies. That was also the criterion set for the trials of younger children.

At the time, given the antibody response, regulators were unwilling to wait for Covid infections to accrue in enough trial participants to gauge efficacy. But because the Omicron variant spread through so much of the population over the winter, both Moderna and Pfizer are now able to gauge efficacy in their youngest trial participants.

Pfizer decided last month to hold off on a request for emergency authorization of two doses after seeing efficacy data from the Omicron surge.

Moderna is asking for similar authorization, citing efficacy data that is just as weak. On the other hand, Moderna’s two-dose regimen achieved better results than Pfizer’s in boosting the immune systems of young children. Pfizer said in December that after two doses of its vaccine, children aged 6 months to 2 produced antibody levels comparable to those of older teenagers and young adults.

But children ages 2 through 4 produced only 60 percent of the sought-after antibody response. After two doses of Moderna’s vaccine, the antibody response of children in both subsets compared favorably to that of people 18 to 25, meeting the trial’s primary criterion for success.

“I’m heartened by the strong neutralizing antibody response indicating long-lasting cellular immunity,” said Dr. Monica Gandhi, an infectious disease doctor and professor of medicine at the University of California, San Francisco.

Dr. Anthony S. Fauci, the chief medical adviser to the White House, said at a briefing on Wednesday that Moderna’s shot proved safe for young children. While the efficacy rate “may seem like a low number,” he said, “in the era of Omicron, this is actually quite comparable to the efficacy against infection in what we’re seeing now with other vaccines.”

“So the data look pretty good right now,” he said.

Pfizer has said it will have data on how well three doses work in early April. That might allow the F.D.A. to compare Moderna’s two-dose and Pfizer’s three-dose results before authorizing either vaccine.

Nearly two-thirds of the U.S. population, or more than 217 million people, are fully vaccinated, and about half of those eligible for booster shots have received them. But progress among children has been dragged: Only about one in four of those between 5 and 11 are fully vaccinated.

So far, the Pfizer-BioNTech vaccine has been the only one authorized for those 5 to 18. But Stéphane Bancel, Moderna’s chief executive, said in a statement on Wednesday that the company would also request emergency authorization for its shot in children 6 to 11, and would update a similar request that it filed last year for those 12 to 17.

That request was put on hold because the F.D.A. wanted to see more data on a rare side effect, myocarditis, which involves inflammation of the heart. Moderna says those concerns have now abated, and at least some federal health officials agree.

Regulators typically do not skip age groups in authorizing vaccines, so the F.D.A. may consider whether to authorize Moderna’s shots for babies through teenagers all at once.

About 6,900 children in the United States and Canada were enrolled in Moderna’s trial for those under 6. Moderna said there were no cases of severe illness, hospitalization or death. Most infections in the trial group were mild.

Dr. Miller said Moderna was studying whether a booster shot may be needed for all those under 18. “We certainly intend to administer a booster dose to these children,” she said.

A majority of side effects among trial volunteers were mild or moderate, and fever rates were typical of existing pediatric vaccines. Fourteen children — 0.2 percent of the study group — developed fevers greater than 104 degrees, the firm said.

Young children generally do not get severely ill from Covid. But according to one data set released by the C.D.C., 355 children younger than 5 in the United States have died of Covid since the start of the pandemic. C.D.C. officials say that data is the most reliable because it is based on death certificates; other estimates are higher.

There is clearly a demand to protect the youngest children, especially as more of the country unmasks, more parents return to workplaces and more normal patterns of life resume.

Dr. Caitlin Elgarten, a pediatrician in Philadelphia, enrolled her 2-year-old son and 4-year-old daughter in Moderna’s trial. She has been anxiously awaiting the results.

She said she was concerned about the potential of long Covid, with its lasting health consequences, should her children get sick. She also said the 10-day quarantine that many day care centers enforce for infected children “is a huge strain on life and families.”

“It does feel like the world is moving on,” she said, “while we either have to just accept the risk or keep limiting ourselves when no one else is.”