(NewsNation) — By the end of last year, about one in four U.S. adults and older teenagers had still not been infected with COVID-19, according to reports.

New figures from the Centers for Disease Control and Prevention looked into seroprevalence, the percentage of people with antibodies against a virus in their blood. The study was conducted with 143,000 blood donors.

The CDC estimates that 96.7% of Americans ages 16 and older had COVID-19 antibodies and that 77.5% had some immunity from an earlier infection.

According to the Food and Drug Administration, antibodies are proteins made by the body’s immune system to help fight off infections.

“Some antibodies in your body may protect you from getting those infections. Your immune system can also safely learn to make antibodies through vaccination,” the FDA said.

An antibody test cannot be used to diagnose current COVID-19 infections since it can take anywhere from days to weeks after a sickness for the body to make detectable antibodies, the FDA said.

The CDC gathered data from 47 states on the prevalence of past COVID-19 infections.

Vermont had the lowest number of reported past cases with 64.4% having antibodies from a past infection. Iowa came in with the highest number of residents with a past infection at 90.6%.

Within the study, men had a slightly higher percentage of people with antibodies against COVID-19 than women at 79.3%. People ages 16 to 29 had the highest amount of infection-induced COVID-19 antibodies, while those 65 and older had the lowest at 56.5%.

The CDC reported that the amount of people with COVID-19 immunity from infection or a blend of having the virus and being vaccinated has increased, but immunity wanes over time.