Advisers to the U.S Food and Drug Administration on Wednesday will vote on whether to recommend authorization of two COVID-19 vaccines for the millions of children ages five years and under, an important step toward immunizing a group that has not been eligible for the shots during the pandemic.

The U.S. government is planning for a June 21 start to its under-5 vaccination campaign should the vaccines from Moderna Inc and Pfizer/BioNTech receive FDA authorization, White House COVID-19 response coordinator Ashish Jha said last week.

Once the FDA authorizes the vaccines for the age group - 6 months to 4 years old for Pfizer/BioNTech and 6 months to 5 years old for Moderna - the U.S. Centers for Disease Control and Prevention (CDC) will make its recommendations on use of the shots in young children.

The companies have presented data showing that their vaccines are safe and effective in that age group, but it remains unclear how many parents will vaccinate the youngest children.

The Pfizer/BioNTech vaccine was authorized for children ages 5 to 11 in October, but only about 29% of that group is fully vaccinated. About 76% of U.S. adults are fully vaccinated, and nearly 90% have received one dose.

Public health officials and experts say that even though a large portion of small children were infected during the winter surge in cases driven by the omicron variant of the coronavirus, natural immunity wanes over time and vaccinations should help prevent a hospitalizations and deaths when cases rise again.

"With omicron waves continuing to come, this approval will set in motion a long, long awaited reduction in hospitalizations among some kids, particularly since omicron," Andy Slavitt, former senior pandemic adviser to U.S. President Joe Biden said on Twitter this week.

"This 19 million person group is the last one that can’t access a vaccine. It will be a watershed moment that within a week vaccines will be approved for all ages," he added.

FDA panel members will consider data from separate trials conducted by Moderna and Pfizer.

For Moderna's vaccine, they have data from more than 6,000 children showing the vaccine is safe and that it generated a similar immune response compared to what was observed in adults in a previous study.

Pfizer's data from roughly 4,000 children also showed its vaccine was safe and had similar immune response compared to a trial in teens and adults.