Top scientists at the World Health Organization pushed back against the Biden administration's intention to supply booster shots to Americans beginning next month.

"We’re planning to hand out extra life jackets to people who already have life jackets, while we’re leaving other people to drown without a single life jacket," Dr. Mike Ryan, executive director of the WHO’s health emergencies program, told reporters Wednesday. "That’s the reality."

As of Wednesday, 60% of Americans had received at least one dose and 51% were fully vaccinated, compared with 32% of people worldwide who've gotten at least one shot and 24% who are fully vaccinated, according to data from the CDC and Our World in Data.

-ABC News' Zoe Magee

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 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