A new CDC analysis found that Pfizer and Moderna vaccine efficacy dropped "significantly" among nursing home residents from March to July, as the delta variant became the predominant strain in the United States.

Researchers analyzed weekly reports from thousands of nursing home facilities in the U.S. and found that mRNA vaccines were roughly 75% effective against preventing any infection in late winter/early spring of 2021, early in the mass vaccination rollout and prior to the emergence of the delta variant. By summer of this year, effectiveness against any infection had dropped to 53%.

Crucially, this doesn't mean vaccines aren't working for nursing home residents, but the significant drop in effectiveness from March to July may support the use of booster doses for them, according to the CDC report.

A second analysis found that the mRNA vaccines are holding up well against hospitalizations for COVID-19. The research, which was conducted across 21 U.S. hospitals, found that the Pfizer and Moderna vaccines remained between 84% and 86% effective against potential hospitalizations from March to July of this year. A third analysis, conducted in New York State, found that all three authorized vaccines remained more than 90% effective at preventing hospitalization from early May to late June.

-ABC News' Sony Salzman, Eric Strauss

The U.S. is prepared to offer booster shots for all Americans beginning the week of Sept. 20, top health officials announced Wednesday. Starting eight months after a person's second dose, they are eligible for a booster.

"At that time, the individuals who were fully vaccinated earliest in the vaccination rollout, including many health care providers, nursing home residents, and other seniors, will likely be eligible," Rochelle Walensky, director of the CDC; Janet Woodcock, acting FDA commissioner; Dr. Vivek Murthy, surgeon general; Francis Collins, director of the NIH; and Anthony Fauci, director of the NIAID, said in a joint statement.

The data behind the decision is expected to be released at 11 a.m. EST during the White House COVID briefing, but public health officials said it's clear that vaccines are waning over time and "we are starting to see evidence of reduced protection against mild and moderate disease."

With regard to the Johnson & Johnson vaccine, officials stressed that more data will be released in the next few weeks. "We will keep the public informed with a timely plan for J&J booster shots," they said.

-ABC News' Cheyenne Haslett

A mask mandate in El Paso, Texas, which, took effect at 12:01 Wednesday, requires people 2 and older to cover their faces in indoor public spaces. Failure to comply with the new rule is a misdemeanor carrying a fine of $500.

The mandate comes after a judge blocked Republican Gov. Greg Abbott's executive order late Tuesday night, which had forbid masks mandates in the state of Texas. The news comes also within hours of Abbott, who is fully vaccinated, testing positive for COVID-19.

An advisory committee to the U.S. Centers for Disease Control and Prevention will meet next week to discuss additional doses of COVID-19 vaccines, including booster shots.

The meeting is scheduled for Aug. 24.

The Biden administration on Tuesday will ship the first 188,370 doses of a 500 million dose commitment of the Pfizer vaccine to Rwanda, according to a senior administration official. The 500 million dose pledge was announced at the G7 summit in June and the contract the U.S. negotiated with Pfizer identified late August as the goal for initial shipments.

The U.S. is also sending 300,000 additional U.S. surplus doses to Rwanda Tuesday, making this first total shipment 488,370. All the doses will be distributed through Covax, the World Health Organization's vaccine-sharing initiative.

-ABC News' Sarah Kolinovsky