Pfizer now joins Moderna in testing its vaccine in younger children.

The company will enroll 4,500 children in a trial that will ultimately include children as young as 6 months old. Previous trials have studied children ages 12 years and older.

Experts say it’s important to specifically test vaccines in children to ensure they are safe and effective in the youngest members of society — so that ultimately everyone is eligible to receive a potentially life-saving vaccine.

The trial is also important for protecting children from severe COVID-19 infection or complications from an infection, said Dr. Emmanuel Walter, chief medical officer of the Duke Human Vaccine Institute. Duke University is one of several medical centers participating in the Pfizer trial.

“If you look at the number of pediatric deaths reported in the last year, it actually exceeds that for influenza in any influenza season that we generally see,” Walter said.

Three phases will study different aspects of the vaccine. The first phase will find an ideal dose for children, with the following two phases studying vaccine safety and efficacy.

“We always worry that side effects may be different in young children. They’re not just little adults,” said Walter.

On Wednesday, Marisol and Alejandra Gerardo were the first participants in the Pfizer trial.

“I think we’re super excited that this is an opportunity for us to both potentially get our kids access, but also to contribute to knowledge of how safe this is for kids,” said Dr. Susanna Naggie, mother of Marisol and Alejandra.

Other vaccine manufacturers have also been hard at work moving to offer their vaccines to children. Moderna announced on March 16 that it would enroll 6,750 children ages 6 months to 12 years in a clinical trial. Johnson & Johnson has announced its intentions to develop trials for children. Meanwhile, AstraZeneca, which announced encouraging phase III trial results this week in the United States, has been studying children ages 6 and older in the United Kingdom since February.

Naggie noted that her children did well. While getting the injection, Alejandra told her mother, “That feels good!”

Tarun Jain, MD, is a chief resident physician in internal medicine and pediatrics and a member of the ABC News Medical Unit.