The vaccine partners announced plans on Tuesday to kick-start the trial of the shot within weeks. It will enroll as many as 4,500 participants at more than 90 sites spanning the U.S., Finland, Poland and Spain.

Expanding the vaccine to more populations promises to further reduce the spread of the pandemic that has already begun waning in Western countries where shots are used widely. More than 2.15 billion shots have been administered worldwide, according to Bloomberg’s vaccine tracker, including more than 300 million in the U.S.

Based on findings from an early-stage trial that evaluated safety, tolerability and the immune response prompted by the vaccine, the companies selected new doses for children. Those ages 5 to 11 will receive a 10-microgram dose—a third of the size given to those 12 years old and older. Children 6 months to 5 years old will be given 3 micrograms—a tenth of the amount given to adolescents and adults.

New York-based Pfizer’s shares fell 0.2% at 9:45 a.m. in New York.

“Kids are not little adults,” said Michael Joseph Smith, a pediatric infectious disease specialist and the co-principal investigator of a Duke University-based trial site. “The younger you are, the more robust your immune system is and the more of a reaction you have, so essentially, you can get by with less antigen but get the same immune response without high fever or swollen arms.”

In an interview from the site in Durham, North Carolina, Smith said parents shouldn’t be worried that the trials are moving too quickly.

“In the face of a public health pandemic, some of the red tape isn’t there as it used to be,” Smith said. “There’s nothing about the rollout of these vaccine trials that’s different from other vaccine trials.”

On May 11, the two-shot messenger RNA vaccine was cleared for use in those 12 to 15 in the U.S., paving the way for the mass vaccination of middle- and high-school students. That trial in more than 2,000 young teens found the vaccine was 100% effective against symptomatic disease.

As of Monday, 22% of U.S. children ages 12 to 15 years old had received their first dose of the Pfizer-BioNTech vaccine, according to the Centers for Disease Control and Prevention.

The study of those under 12 isn’t designed to determine efficacy, William Gruber, Pfizer’s senior vice president of clinical research and development, said in an interview in May. That’s because it will be difficult to accumulate sufficient symptomatic COVID-19 cases as the pandemic subsides in the U.S., and since many young children don’t show symptoms of infection.

Instead, Pfizer and BioNTech will see whether the vaccine produces an immune response, Gruber said. The approach has been used to validate other pediatric vaccines.

Gruber said he expects the drugmakers to report safety and immune response data on the vaccine in those aged 2 to 12 in September or October, paving the way for U.S. regulators to clear the shot for that group before the end of the year. For those 6 months to 2 years old, data is likely to become available in the first quarter of 2022, he said.