"About 18 million children under the age of 5 in the United States could soon be eligible to get a Covid-19 vaccine -- but first, the child-sized shot must go through a rigorous authorization process that's taking a different path than the Covid-19 vaccines that came before."

In December, Pfizer announced that it decided to add a third dose to the primary vaccine regimen for young children. It said the original two-dose series did not provide the expected immunity in 2- to 5-year-olds.