WASHINGTON — Pfizer said it has started a clinical trial testing its COVID-19 vaccine on healthy 6-month to 11-year old children, a crucial step in turning back the pandemic. The first participants in the study have already gotten their shots, which were developed in partnership with German drugmaker BioNTech, New York-based Pfizer announced Thursday. It intends to enroll 144 children in the first phase. Results from the trial are expected in the second half of the year, and the company hopes to vaccinate younger children early next year, the company said. Researchers will evaluate the safety and effectiveness of the selected dose levels in the next phase of the trial, with participants being randomly selected to receive the vaccine or a placebo, the company said. After a six-month follow-up, kids who received a placebo will have the opportunity to receive the vaccine, it said. “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. Pfizer’s vaccine has already been authorized for the use in the US in Americans who are 16 and older. Clinical trial studies testing the vaccine in kids, whose immune systems can respond differently than adults, still need to be completed. Moderna also is beginning a trial of its vaccine in children six months to 12 years of age. Both companies have been testing their vaccines in children 12 and older, and expect those results in the next few weeks. — Agencies