A new study by the U.S. Centers for Disease Control and Prevention estimates that roughly 83% of blood donors in the nation have either been vaccinated against COVID-19 or were previously infected with the virus.

This, however, does not mean that more than 80% of Americans are immune from infection. That's because neither vaccination nor prior infection provides 100% protection -- antibodies are just one part of the overall immune response, and immunity wanes over time. The analysis also may overestimate the portion of people with antibodies because blood donors may be more likely to be vaccinated or have previously been infected.

The study, posted online Tuesday by JAMA Network Open, a monthly open access medical journal published by the American Medical Association, took a snapshot of the presence of antibodies from COVID-19 vaccination or prior infection in about 1.4 million donated blood samples from across the United States. The repeated cross-sectional analysis was conducted each month during July 2020 through May 2021, before delta became the predominant variant of the novel coronavirus in the U.S.

The study shows that the number of blood donors who tested positive for antibodies, indicating either vaccination or prior infection, has gone up over time, from 3.5% in July 2020 to 20.2% for infection-induced antibodies and 83.3% for both infection- and vaccine-induced antibodies in May 2021.

Being vaccinated offers better protection compared to prior infection, and it's recommended that people who have previously contracted COVID-19 should still get inoculated.

-ABC News' Katie Bosland and Sony Salzman

The European Union will be returning some 20 million doses of Johnson & Johnson's COVID-19 vaccine that were imported from a plant in South Africa, and the shots filled and finished there will no longer leave the African continent.

African Union special envoy Strive Masiyiwa, who heads the regional bloc's COVID-19 Vaccine Acquisition Task Team, told reporters Thursday that the decision was made at a meeting last week between South African President Cyril Ramaphosa and European Commission President Ursula von der Leyen.

Johnson & Johnson's South African partner, Aspen Pharmacare, has a contract to import the drug substance for the one-dose vaccine from the American pharmaceutical giant and then package them -- the so-called fill-and-finish process -- at its facility in Port Elizabeth, South Africa.

"All the vaccines produced at Aspen will stay in Africa and will be distributed to Africa," Masiyiwa said at a press conference Thursday.

The decision came amid criticism of the arrangement, with the World Health Organisation's director-general, Dr. Tedros Adhanom Ghebreyesus, who is Ethiopian, saying last month that he was "stunned" that vaccines will be shipped from Africa to Europe. Just 3% of people in Africa, the world's second-largest, second-most populous continent, are fully vaccinated against COVID-19. In comparison, 57% of people are fully vaccinated in the European Union and 52% in the United States, according to the WHO.

Nearly 300 children are currently hospitalized with COVID-19 in Texas, state data shows.

According to the Texas Department of State Health Services' online COVID-19 dashboard, which was last updated on Thursday afternoon, there are 282 pediatric patients in hospitals across the Lone Star State.

The data also shows there are 81 staffed pediatric intensive care unit beds available in all of Texas.

-ABC News' Gina Sunseri

Dr. Paul Offit, a member of the U.S. Food and Drug Administration's vaccine advisory committee, said a two-dose COVID-19 vaccine "appears to be enough" to curb infection, rather than adding a booster shot.

"You look at states in the United States that have high immunization rates with a two-dose vaccine, it appears the two doses appears to be enough to be able to control this infection," Offit, who is also the director of the Vaccine Education Center at Children's Hospital of Philadelphia, told ABC News on Thursday night. "I think the critical issue here is not going to be boosting the vaccinated. I think if we really want to get on top of this pandemic, it's going to be about vaccinating the unvaccinated."

The FDA's vaccine advisory committee is set to hold a key meeting on COVID-19 vaccine booster shots on Sept. 17, just three days before the Biden administration plans to begin offering the shots to Americans.

"If the companies or the FDA can make a case that there has been an erosion in protection against severe critical disease and that that erosion in protection against severe disease would be mediated or eliminated by a third dose, then we could move forward," Offit said. "But to date, we really need to see those data to be able to make that decision."