CDC Finds Over 83 Percent of Americans Had COVID Antibodies Before Delta Surge

More than 83 percent of Americans had COVID-19 antibodies before the surge of the Delta variant began, according to a new study of blood sample specimens by the Centers for Disease Control and Prevention (CDC).

The CDC released the findings of the study on Thursday and noted that the percentage of the U.S. population with COVID antibodies increased significantly after the vaccine rollout began.

The Delta variant has seen a surge in cases in the U.S. and though it is difficult to know with certainty when the first Delta cases hit the country, reports indicate the variant was in the U.S. by May at the latest.

The CDC tweeted on Thursday: "Data from blood donations show Americans w/ COVID-19 antibodies increased from 20.5% to 83.3% after the rollout of #COVID19 vaccines."

"CDC is learning more about how many people need antibodies before the population can be considered protected," the agency said.

The CDC shared a graphic showing that the percentage of Americans with antibodies from vaccination has increased from 4.6 percent in January to 63.1 percent in May. By contrast, the percentage of people without antibodies has fallen from 79.5 percent to 16.7 percent.

The study examined 1,443,519 blood donation specimens "from a catchment area representing 74% of the US population" between July 2020 and May 2021.

The study was investigating seroprevalence of the novel coronavirus SARS-CoV-2. Seroprevalence is defined as the "overall occurrence of a disease or condition within a defined population at one time, as measured by blood tests," according to the National Institutes of Health (NIH).

The authors wrote that "estimated SARS-CoV-2 seroprevalence weighted for differences between the study sample and general population increased from 3.5% in July 2020 to 20.2% for infection-induced antibodies and 83.3% for combined infection- and vaccine-induced antibodies in May 2021."

The study also noted that "the findings in this analysis predate the surge in SARS-CoV-2 infection in the US related to transmission of the Delta variant."

"Despite weighting to adjust for demographic differences, these findings from a national sample of blood donors may not be representative of the entire US population," the study's authors cautioned.

Administration of the COVID-19 vaccine began on December 14, 2020 and is ongoing with 74.5 percent of the population aged over 18 having received at least one shot. However, vaccination rates vary from the state to state and vaccine hesitancy has proven a problem during the rollout.

"Estimates show the Delta variant causing more than 99% of recent #COVID19 cases in the United States," the CDC tweeted on Thursday. "Help protect yourself against Delta & other variants by getting vaccinated."