Moderna said on Wednesday that it would seek emergency authorization for its coronaviruses 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.

The vaccine proved to be only 44 percent effective in preventing illness in children 6 months to 2 years old, and 37 percent effective in children 2 through 5.

The low level of protection demonstrated the ability of the Omicron variant to evade the vaccine. She said in an interview that what we have seen is a successful trial.

She said that the percentage was higher than zero and that she wanted there to be some protection on board.

The Biden administration's effort to protect people against an ever-changing virus enters a new period of change. Officials are debating whether the oldest Americans should be offered a second booster shot. Various studies are trying to determine if the existing vaccines can be changed to provide more protection against Omicron and the subvariant of the virus known as BA.2.

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

According to people familiar with the situation, Pfizer and its German partner, BioNTech, delayed seeking emergency authorization for their coronaviruses vaccine in young children after data gathered during the Omicron surge showed two doses were less than 50 percent effective.

Moderna has an efficacy rate of about 40 percent for children under the age of 6.

Moderna's and Pfizer's results for young children do not meet the minimum standard for effectiveness against Covid-19, despite the fact that Americans have been hearing for months that the vaccines are less potent against Omicron in all age groups. The F.D.A. initially set for 50 percent effectiveness for adult coronaviruses vaccines.

The authorized vaccines from Moderna, Pfizer and Johnson & Johnson vastly surpassed that at first. Omicron's ability to dodge the immune system is changing the equation, 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. When the agency authorized Pfizer's vaccine for children 5 to 11, it relied on immune response data, which showed that the vaccine triggered a powerful surge in antibodies. The trials of younger children were also set with that in mind.

Regulators were unwilling to wait for Covid infections to accumulate in enough trial participants to gauge efficacy because of the antibody response. Moderna and Pfizer are able to gauge efficacy in their youngest trial participants because of the spread of the Omicron variant.

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

Moderna is asking for authorization because of weak efficacy data. 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 years old produced levels comparable to older teenagers and young adults.

60 percent of the sought-after response was produced by children ages 2 through 4. The trial's primary criterion for success was met after two doses of Moderna's vaccine.

Monica Gandhi is an infectious disease doctor and professor of medicine at the University of California, San Francisco.

Moderna's shot proved to be safe for young children, according to Dr. Fauci, the chief medical adviser to the White House. He said that the efficacy rate in the era of Omicron was comparable to what we are seeing now.

The data looks good right now, he said.

Pfizer will have data on how well three doses work in April. The F.D.A. could compare Moderna's two-dose and Pfizer's three-dose results before authorizing either vaccine.

Half of those eligible for booster shots have received them, and nearly two-thirds of the U.S. population is fully vaccineed. Only about one in four children between the ages of 5 and 11 are fully vaccine free.

The Pfizer-BioNTech vaccine is the only one authorized for those 5 to 18. Moderna's chief executive said in a statement on Wednesday that the company would request emergency authorization for its shot in children 6 to 11 and would update a request it made last year for those 12 to 17.

The request was put on hold because the F.D.A. wanted to see more data on myocarditis. At least some federal health officials agree with Moderna's claims.

Phoebee Curtis, 9, receiving a Covid-19 vaccine in Albuquerque last year. Pfizer currently has the only shot approved for younger Americans.
ImagePhoebee Curtis, 9, receiving a Covid-19 vaccine in Albuquerque last year. Pfizer currently has the only shot approved for younger Americans.
Phoebee Curtis, 9, receiving a Covid-19 vaccine in Albuquerque last year. Pfizer currently has the only shot approved for younger Americans.Credit...Paul Ratje for The New York Times

Moderna's shots for babies through teenagers all at once may be authorized by the F.D.A. because they don't skip age groups.

The trial was for children under the age of 6 in the United States and Canada. There were no cases of severe illness, hospitalization or death. Mild infections were the norm in the trial group.

Moderna was studying whether a booster shot was needed for children under 18.

Most of the side effects among trial volunteers were mild or moderate. The firm said that fourteen children in the study group developed a high temperature.

Young children don't get very sick from Covid. 355 children younger than 5 in the United States have died of Covid since the start of the Pandemic, according to one data set released by the C.D.C. The most reliable data is based on death certificates, according to C.D.C. officials.

As more of the country is revealed, more parents return to work and more normal patterns of life resume, there is a demand to protect the youngest children.

Dr. Elgarten enrolls her two children in Moderna's trial. She was waiting for the results.

She was concerned about the long-term health consequences of long Covid if her children got sick. She said the 10-day quark that many day care centers enforce for children with infections is a huge strain on families.

She said that it feels like the world is moving on, while we either have to accept the risk or limit ourselves.