The drug company Moderna has begun a study that will test its Covid vaccine in children under 12, including babies as young as six months, the company said on Tuesday.

The study is expected to enroll 6,750 healthy children in the United States and Canada.

“There’s a huge demand to find out about vaccinating kids and what it does,” said Dr. David Wohl, the medical director of the vaccine clinic at the University of North Carolina, who is not involved the study.

In a separate study, Moderna is testing its vaccine in 3,000 children ages 12 to 17.

Many parents want protection for their children, and vaccinating children should help to produce the herd immunity considered crucial to stopping the pandemic. The American Academy of Pediatrics has called for expansion of vaccine trials to include children.

Each child in Moderna’s study will receive two shots, 28 days apart. The study will have two parts. In the first, children aged 2 years to less than 12 may receive two doses of 50 or 100 micrograms each. Those under 2 years may receive two shots of 25, 50 or 100 micrograms.

In each group, the first children inoculated will receive the lowest doses and will be monitored for reactions before later participants are given higher doses.

Then, researchers will perform an interim analysis to determine which dose is safest and most effective for each age group.

Children in part two of the study will receive the doses selected by the analysis — or placebo shots consisting of salt water.

The children will be followed for a year, to look for side effects and measure antibody levels that will help researchers determine whether the vaccine is effective. The antibody levels will be the main indicator, but the researchers will also look for coronavirus infections, with or without symptoms.

Dr. Wohl said the study appeared well designed and likely to be efficient, but he questioned why the children were to be followed for only one year, when adults in Moderna’s study are followed for two years. He also said he was somewhat surprised to see the vaccine being tested in children so young this soon.

“Should we learn first what happens in the older kids before we go to the really young kids?” Dr. Wohl asked. Most young children do not become very ill from Covid, he said, though some develop a severe inflammatory syndrome that can be life threatening.

Johnson & Johnson has also said it would test its coronavirus vaccine in babies and young children after testing it first in older children.

Pfizer-BioNTech is testing its vaccine in children ages 12 to 15, and has said it plans to move to younger groups; the product is already authorized for use in those 16 and up in the United States.

Last month, AstraZeneca began testing its vaccine in Britain in children 6 years and older.