Pfizer has begun testing in children under 12 for the COVID-19 vaccine Thursday.
That means children who are ages six-months-old to 11 years of age. At least 144 children will be enrolled in the first phase of the trial, according to CNBC.
The results from the trial, which included a pair of twins as the first participants, will be available later this year.
Both Pfizer and Moderna have already been testing participants over the age of 12 - those results will be available soon.
“Pfizer has deep experience in advancing clinical trials of vaccines in children and infants and is committed to improving the health and well-being of children through thoughtfully designed clinical trials,” the company said in a statement to CNBC.
Moderna, which is one of the vaccines approved to be used in the United States, also started testing the vaccine for those under age 12 earlier this month.
The twin girls were immunized at Duke University in North Carolina on Wednesday, according to The New York Times.
"AstraZeneca last month began testing its vaccine in children six months and older, and Johnson & Johnson has said it plans to extend trials of its vaccine to young children after assessing its performance in older children," the New York Times reported.