Pediatricians, health care centers and pharmacies across the United States have begun administering COVID-19 vaccines to infants and young children this week, after the U.S. Food and Drug Administration (FDA) and U.S. Centers for Disease Control and Prevention (CDC) authorized both Pfizer (PFE  )-BioNTech (BNTX  ) and Moderna's (MRNA  ) vaccines last week.

"Together, with science leading the charge, we have taken another important step forward in our nation's fight against COVID-19. We know millions of parents and caregivers are eager to get their young children vaccinated," said CDC Director Dr. Rochelle Walensky in a press statement.

The Biden administration has initially made 10 million vaccine doses available to local partners. Dr. Ashish Jha, who oversees the White House's COVID response, said that vaccine appoint available for children may be limited at first, but most children should be able to get vaccinated in the next few weeks. There are about 18 million children between the ages of 6 months and 5-years-old in the U.S. eligible for the shots.

Now, parents of infants, toddlers and preschoolers will be able to choose between a three-dose regimen from Pfizer-BioNTech that spans about three months, or a two-dose schedule from Moderna that takes about four weeks. Both vaccines rely on the same mRNA technology given to adults, they differ in dosage sizes, with Pfizer's shot about one-tenth and Moderna's being about one-fourth the size given to adults.

Pfizer's vaccine was found to be 75% to 85% effective, but its efficacy was based on preliminary data and was based on a small sample size. However, the company found that the three-shot regimen triggered a similar immune system response as that seen in adults. Moderna's vaccine efficacy was found to be between 36.8% and 50.6% in the infants to young children, and the company is currently testing a booster shot for this age group.

While COVID has typically been less severe in children compared to adults, COVID hospitalizations of children under age 5 spiked during the Omicron wave earlier this year. Moreover, COVID is currently the fifth leading cause of death for children ages 1- to 4-years-old, according to CDC data. Over 200 children under age 5 have died from COVID since January 2022, and more than 2 million in this age group have been infected throughout the pandemic and over 20,000 have been hospitalized, according to the CDC.