Children as young as 6 months old have received their initial shot of the Pfizer-BioNTech COVID-19 vaccine as part of a pediatric trial to assess the safety of the vaccine, Pfizer officials announced Thursday, according to NBC News.

The global study of 144 participants will also examine whether the vaccine can generate an immune response in children and determine the proper dosage for each age group in the trial: 6 months to 2 years; 2 years to 5 years; and 5 years to 11 years.

The trial, along with similar studies that Moderna, Johnson & Johnson and others are conducting, aims to guide how the shots should be administered to young children, who are not thought to be the most vulnerable to COVID-19 but can be infected and spread the virus.

During the trial's first phase, three different dosages of the vaccine — 10 micrograms, 20 micrograms and 30 micrograms — will be tested in each age group, beginning with the oldest, the company said. In people ages 16 and older, the Pfizer-BioNTech vaccine is given in two doses of 30 micrograms spaced three weeks apart.

The findings from Phase 1 will determine the dosage level used for later stages of the trial, according to NBC News.

In Phases 2 and 3, Pfizer will study whether the vaccine can induce an immune response, how well the shots are tolerated in each age group and monitor for side effects and other safety issues. The later stages of the pediatric trial will expand to include approximately 4,500 children in the United States and Europe, according to Pfizer. Trial participants will be randomly selected to receive either the Pfizer-BioNTech vaccine or a harmless placebo.

Company officials said the trial participants will be monitored for six months after vaccination.