More than half of all Americans and about 75 percent of kids had been infected by COVID-19 by the end of February this year, according to a new study on antibodies by the U.S. Centers for Disease Control and Prevention (CDC).

The CDC examined blood samples nationwide for anti-nucleocapsid antibodies which are produced in response to COVID-19 infection but not in response to COVID-19 vaccines.

The researchers found that signs of past infection rose dramatically between December 2021 and February 2022, when the highly contagious Omicron variant surged across the country.

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

The reason leading to the greatest increase in this age group may be related to the lowest vaccination coverage, according to the CDC.

The older people were, the less likely they had evidence of past infections, the study found. That may be because older adults have higher vaccination rates and may be more likely to take other COVID-19 precautions, such as wearing masks and avoiding crowds, according to the CDC.