Vaccine maker Moderna said Thursday that its coronavirus shots remain 93 percent effective six months after full vaccination and that people will likely need a COVID-19 vaccine booster dose in the fall to contend with the Delta variant.

The company said that it has tested three potential booster shots, which it said have demonstrated "robust antibody responses'', bringing antibodies back to the protective levels triggered by full vaccination.

France and Germany on Thursday reiterated plans to administer booster doses of COVID-19 vaccines to older people or those with compromised immune systems or both, despite calls from the World Health Organization to halt such shots and send more doses to poorer nations.

A consensus hasn't been reached by US health experts on whether booster shots are needed to boost immunity in fully vaccinated people.

Dr Paul Offit, a member of the Food and Drug Administration's vaccine advisory committee, said that the data presented by Moderna and other vaccine manufacturers so far didn't justify rolling out boosters in the next few months. That wouldn't be needed, he said, unless there was evidence that the vaccines are no longer protecting people against severe disease.

"You want this vaccine to protect against the kind of illness to cause you to seek medical attention, or be hospitalized," he said. "Until you see any evidence that that isn't true, then you don't need a booster dose."

The Biden administration is developing a plan to require nearly all foreign visitors to the US to be fully vaccinated against COVID-19 as part of eventually lifting travel restrictions that bar much of the world from entering the country, a White House official told Reuters on Wednesday.

The White House wants to reopen travel, which would boost business for airlines and the tourism industry but isn't ready to immediately lift restrictions because of the rising COVID-19 caseload and highly transmissible COVID-19 Delta variant, the official said.

That stance was reiterated on Wednesday evening by White House officials who said that there was no timetable yet for requiring foreign travelers to be inoculated, The New York Times reported.

Daily new cases, hospitalizations and deaths from the Delta variant of the coronavirus are all surging, federal health officials said Thursday.

"Across the board, we are seeing increases in cases and hospitalizations in all age groups," Centers for Disease Control and Prevention Director Dr Rochelle Walensky said at a White House coronavirus briefing where she and other health leaders reiterated pleas for people to get immunized.

Daily cases are passing 100,000, numbers not seen since February. A seven-day average of hospital admissions is up by more than 40 percent from the week before, and deaths trending up by roughly the same rate.

White House coronavirus coordinator Jeff Zients said seven Southern states — Florida, Texas, Missouri, Arkansas, Louisiana, Alabama and Mississippi —accounted for about half of all new infections and hospitalizations over the past week, although these states represent less than a quarter of the US population.

Across Florida, more than 12,000 patients were hospitalized with COVID-19 on Wednesday, and nearly 2,500 of them were in ICU beds. On Tuesday, the Centers for Disease Control and Prevention reported more than 50,000 new coronavirus cases in Florida during a three-day span, raising the seven-day average to one of the highest counts since the pandemic began. In total, the state has seen more than 2.6 million cases and 39,179 deaths.

A South Florida hospital chain is suspending elective surgeries and putting beds in conference rooms, an auditorium and a cafeteria as many more patients seek treatment for COVID-19.

"We are seeing a surge like we've not seen before in terms of the patients coming," Memorial Healthcare System's Chief Medical Officer Dr Marc Napp said Wednesday during a news conference.

Napp said they have opened up an additional 250 beds at Memorial's six hospitals in Broward County.

The Biden administration also announced a new push Thursday to vaccinate young people as they head back to school. It will hold pop-up clinics, send pediatricians to back-to-school nights to discuss shots with parents and incorporate vaccination against COVID-19 into physicals for student athletes.

"The resources are there and the urgency is there," Secretary of Education Miguel Cardona said at a news briefing where he said the administration is partnering with groups such as the American Academy of Pediatrics and the National Parent Teacher Association.

Nearly 650 colleges and universities are now requiring shots for students and staff on campus, and more than 100 healthcare systems are embracing similar mandates.

Amazon said Thursday it would delay corporate employees' return to offices both in the US and other countries until at least Jan 3 as conditions around the COVID-19 pandemic evolve.

The online retail giant previously had targeted early September for regular office work to resume.