Researchers examined blood samples taken from more than 200 000 Americans to search for viruses-fighting antibodies. The researchers found that past infections rose rapidly between December and February as the more contagious Omicron variant surged in the U.S.

Children saw the most dramatic increase. The number of children 17 years and younger with antibodies increased from 45% in December, to 75% in February.

About 34% of Americans aged all over had symptoms of an infection prior to December. Two months later, 58% had the same symptoms.

"I expected it to increase. "I didn't expect it to grow quite this much," Dr. Kristie Clarke said, co-leader of a CDC group that monitors the severity of coronavirus infection.

Study results showed that older people are less likely to have evidence of previous infections. The study found that 19% of 65-year-olds had evidence of previous infection in December, while 33% showed signs in February. Clarke explained that older adults may have higher vaccination rates, and may be more inclined to take COVID-19 precautions such as wearing masks or avoiding crowds.

Clarke stated that the tests can detect antibodies up to two years after infection and may even last longer.

Although studies have shown that previous infections can help protect some individuals from severe illness and hospitalizations, CDC officials insist that those who are already infected with COVID-19 should still be vaccinated.

The study was able to detect any antibodies, but it didn't distinguish between those with protective antibody levels. Scientists continue to investigate the role of these antibodies in protecting against future viruses exposures.

Officials are still urging Americans to get boosters and vaccines to protect against COVID-19. These offer additional protection for everyone, even those already infected.