Moderna will seek emergency authorization of its vaccine for young children.

Moderna said on Wednesday that it would seek emergency authorization of 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 years old. The company is studying the effectiveness of a booster shot, and one of its top officials said she expects a booster will be necessary for that age group, just as it is for adults.

The low level of protection demonstrated the ability of the Omicron variant to evade the vaccine's shield. She said in an interview that what they 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.

Pfizer and BioNTech are expected to release results from a three-shot study for children under five, using doses one-tenth as strong as those for people 12 and older. The results are expected in April.

Pfizer-BioNTech submitted preliminary results from their trial to the FDA last month in order to get the vaccine campaign started.

The F.D.A. changed its mind after new data showed that two doses failed to protect adequately against infections. In December, Pfizer and BioNTech reported that two doses of their vaccine produced a strong immune response in children 6 months to 2 years old, but not in children 2 through 4 years old.

18 million children under the age of 5 are not eligible for shots.

Moderna's chief executive officer said in a statement that the company will request emergency authorization for its two-shot vaccine for children 6 to 11. The request was put on hold because the FDA wanted to see more data on myocarditis.

Pfizer-BioNTech's shot is the only one authorized for Americans 5 through 17.

The trial was for children under the age of 6 in the United States and Canada. Moderna said there were no cases of severe illness, hospitalization or death in either the placebo or vaccine arm. Mild infections were the majority.

The firm said that the children who received the vaccine demonstrated a comparable response in the level of neutralizing antibodies as adults.

Moderna said there were enough infections to evaluate the vaccine's efficacy against disease. It said that the relatively low efficacy is related to research on how well vaccines have protected adults from the Omicron variant. Moderna is studying booster shots for children.

We intend to give a booster dose to these children.

The majority of side effects were mild or moderate. The firm said that a few children in the study group developed a high temperature.