Being vaccinated after having had COVID-19 offers far better protection than prior infection alone, according to a study the Centers for Disease Control and Prevention released Friday.

The study, which appeared in the agency's Morbidity and Mortality Weekly Report, examined Kentucky residents who had previously had COVID-19. Some chose to get vaccinated, while others didn't. The result: Those who skipped the vaccine were more than twice as likely to be reinfected compared to those who were vaccinated.

"If you have had COVID-19 before, please still get vaccinated," said Dr. Rochelle Walensky, the CDC's director. "Getting the vaccine is the best way to protect yourself and others around you, especially as the more contagious delta variant spreads around the country."

-ABC News' Sony Salzman

As of Friday, half of Americans are now vaccinated for COVID-19, according to the White House.

"50% of Americans (all ages) are now fully vaccinated," Cyrus Shahpar, the White House's COVID-19 data director, wrote on Twitter Friday. "Keep going!"

-ABC News' William Gretsky

A real-world study of South African health care workers found that the single-shot Johnson & Johnson vaccine was highly effective against the delta and beta variants, according to the South African Medical Research Council, which presented the results Friday.

The vaccine was between 91% and 96% effective against death, according the study, which included more than 477,000 health care workers. J&J showed stronger protection against the delta variant, now dominant in the United States and South Africa, than it showed against the beta variant.

"The data reported today by South African investigators show that a single shot can protect against death due to COVID-19 in the most challenging epidemiologic setting: A massive delta surge in Africa," said Dr. Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center, who helped invent the J&J shot but was not directly involved in the South Africa study.

While the results have not yet been published in a peer-reviewed journal, they may serve as a reassuring data point as the delta variant sweeps the U.S.

Glenda Gray, president and CEO of the South African Medical Research Council, who led the study, said at the Friday press conference that the J&J vaccine showed good durability, an indication that there's no need for booster shots just yet.

-ABC News' Sony Salzman.

Thirteen theaters in Washington, D.C., have joined forces to require audiences to supply proof of vaccination in order to attend live performances at indoor venues.

In addition to the vaccination requirement, masks will be required inside the theaters. Medical and religious exemptions to the rule will be accepted, but those patrons will have to show proof of a recent negative COVID-19 test in order to gain entry. For children younger than 12, patrons should contact the theater directly for more information about individual policy.

More theaters are expected to join the cohort in comings weeks, according to Theatre Washington, an alliance of theater organizations and supporters.

Mississippi is facing "a phenomenal increase in daily reported cases of COVID, and this is entirely attributable to the delta variant, which is sweeping over Mississippi like a tsunami," state health officer Dr. Thomas Dobbs said Thursday.

Dobbs said 97% of new cases are among the unvaccinated, and that 89% of hospitalizations and 85% of deaths are unvaccinated.

Dobbs pleaded with the public to get vaccinated, stressing that the unvaccinated population is driving the current surge, but that vulnerable, vaccinated people are suffering the fallout.

"There is going to be some collateral damage, unfortunately, even folks who've done everything they can to protect themselves," Dobbs said.

"The minority of folks who are vaccinated and hospitalized are overrepresented by the older and those with weaker immune system, so we're seeing a pretty dramatic spillover effect from the transmission in the community to more vulnerable parts of our population," he said.

-ABC News' Soorin Kim