Three out of every four U.S. children have been infected with the coronavirus and more than half of all Americans had signs of previous infections, Centers for Disease Control and Prevention researchers estimated in a report Tuesday.

The researchers examined blood samples from more than 200,000 Americans and looked for virus-fighting antibodies made from infections, not vaccines. They found that signs of past infection rose dramatically between December and February, when the more contagious omicron variant surged through the U.S.

The most striking increase was in children. The percentage of those 17 and under with antibodies rose from about 45% in December to about 75% in February.

For Americans of all ages, about 34% had signs of prior infection in December. Just two months later, 58% did.