NEW YORK (AP) — Three out of every four US children have been infected with the coronavirus, 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 US.

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

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

“I did expect it to increase. I did not expect it to increase quite this much,” said Dr Kristie Clarke, co-leader of a CDC team that tracks the extent of coronavirus infections.

The older people were, the less likely they had evidence of past infections, the study found. For those 65 and older, 19 per cent had signs of prior infection in December and 33 per cent did in February. That may be because older adults have higher vaccination rates and they may be more likely to take other COVID-19 precautions, such as wearing masks and avoiding crowds, Clarke said.

Clarke said the tests can detect antibodies for one to two years after infection, and possibly longer.

Studies have shown previous infection can protect some people against severe disease and hospitalisation, but CDC officials stressed that the previously infected should still get COVID-19 vaccines.

The study looked for any detectable level of antibodies; it did not distinguish how many people had antibody levels that might be protective. Scientists are still trying to understand what role these kinds of antibodies play in protection from future virus exposures.

Officials continue to urge Americans to get vaccines and boosters, which offer additional protection against COVID-19 for all, including those who were previously infected.