NEW YORK (AP) — Government researchers say three out of every four U.S. children have been infected with the coronavirus. Among Americans of all ages, more than half had signs of previous infection. The figures come from a Centers for Disease Control and Prevention study released Tuesday. It looked in the blood of more than 200,000 Americans for virus-fighting antibodies made from infections, not vaccines. They found that signs of past infection rose dramatically between December and February, when the omicron variant surged. Those antibodies may not protect people, though. So CDC officials stress that the previously infected should still get COVID-19 vaccines.