Johnson & Johnson said Wednesday it is "engaging" with the Food and Drug Administration, the Centers for Disease Control and Prevention and other health authorities on a booster of its single-dose COVID-19 vaccine.

The company said it "will share new data shortly regarding boosting" with its vaccine, which one study suggests provides immunity for at least eight months.

The statement comes after the Biden administration said Wednesday it is preparing to roll out booster shots of Pfizer's and Moderna's vaccines to more Americans next month.

People attending large-scale indoor events in California soon will be required to show proof of COVID-19 vaccination or a recent negative COVID-19 test, state officials announced Wednesday.

The new rules apply to indoor events with more than 1,000 people beginning Sept. 20. Tests must be administered within 72 hours of the event.

Currently, attendees have to self-attest to either having the vaccine or a negative test to attend events with more than 5,000 people.

Health officials pointed to the highly transmissible nature of the delta variant in updating the rules.

President Joe Biden plans to issue a memo to Secretary of Education Miguel Cardona Wednesday to counter the Republican governors who have blocked mask mandates in their states.

"Some state governments have adopted policies and laws that interfere with the ability of schools and districts to keep our children safe during in-person learning, with some going as far as to try to block school officials from adopting safety protocols aligned with recommendations from the CDC," according to a fact sheet released by the White House Wednesday.

Biden's memo will ensure the department of education "is doing everything it can to prevent anything from standing in the way of local leaders and school leaders taking steps to keep all students safe in full-time, in-person learning, without compromising students' health or the health of their families or communities," the fact sheet continues.

In a press conference, the president had a blunt message for Republican governors who are banning school districts from requiring masks in schools: "If you aren't going to fight COVID-19, at least get out of the way of everyone else who is trying."

Biden said his administration is not going to "sit by as governors try to block and intimidate educators protecting children," and announced that he's directing Cardona to go as far as using legal action against these governors and that his administration will use COVID-19 relief funds to pay the salaries of educators who defy them.

-ABC News' Justin Gomez

The Centers for Medicare and Medicaid Services plans to require nursing homes participating in Medicare or Medicaid to have all workers be vaccinated for COVID-19, a Biden administration official confirmed to ABC News Wednesday.

The new rule, which will impact more than 15,000 nursing homes and 1.3 million workers, will go into effect in late September. Nursing homes that don't comply could lose federal funding.

-ABC News' Cheyenne Haslett

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