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.

Nearly 72,000 children in the U.S. tested positive for COVID-19 last week, a massive jump from the approximately 39,000 cases among kids one week earlier, according to a new report from the American Academy of Pediatrics and the Children’s Hospital Association.

Children represented 19% of all COVID-19 cases for the week ending July 29.

Severe illness due to COVID-19 remains "uncommon" among children, the two organizations wrote in the report. According to the nearly two dozen states that reported pediatric hospitalizations, 0.1% to 1.9% of all child COVID-19 cases resulted in hospitalization.

However, the two organizations warned that there is an urgent need to collect more data on long-term impacts of the pandemic on children, "including ways the virus may harm the long-term physical health of infected children, as well as its emotional and mental health effects."

-ABC News’ Arielle Mitropoulos