WASHINGTON - Hit by a new wave of hospitalisations on the back of a stalling vaccination rate and the Delta variant ripping through the country, local governments and organisations in the United States are scrambling to impose vaccination mandates previously thought politically and socially controversial.

The surge is expected to accelerate through the summer and autumn, peaking in mid-October, with daily deaths potentially more than triple what they are now. On Sunday (July 25), 49 people died of Covid-19, according to the New York Times daily tracker.

"What's going on in the country with the virus is matching our most pessimistic scenarios," University of North Carolina epidemiologist Justin Lessler told National Public Radio.

Dr Lessler helps run the Covid-19 Scenario Modeling Hub, a consortium working in consultation with the Centres for Disease Control and Prevention.

"We might be seeing synergistic effects of people becoming less cautious in addition to the impacts of the Delta variant," Dr Lessler warned.

Currently, about half the US population is vaccinated, but the rate of vaccination has significantly slowed as vaccine scepticism - often fanned by right-wing conservative media and disinformation on social media - remains strong in pockets around the country, especially in heavily Republican states.

In the most likely scenario, Dr Lessler said, with the vaccination rate reaching 70 per cent, by the peak in mid-October there would be around 60,000 cases and around 850 deaths each day.

The vast majority of those hospitalised are unvaccinated. Children have had to be hospitalised in some cases. That the wave will persist through the autumn has parents worried as well, as schools are set to reopen in September after the summer break.

On Monday, New York City and the state of California announced that they would require hundreds of thousands of government workers to get inoculations or face weekly testing. The Department of Veterans Affairs became the first federal agency to require Covid-19 vaccinations. And the city of St Louis, in Missouri, has imposed an indoor mask mandate.

Fifty-seven medical organisations, including the American Medical Association, the American Nurses Association and the American Academy of Paediatrics, on Monday released a joint statement urging such vaccine requirements.

Meanwhile "breakthrough" cases - of people fully vaccinated yet contracting Covid-19 - are also causing concern.

An outbreak in Provincetown, Massachusetts, has seen at least 132 people infected since July 1. Most of those affected were vaccinated.

At one nursing home, as many as 33 residents and staff were infected - many of them reportedly vaccinated.

From Thursday, hundreds of bars in San Francisco will begin requiring proof of vaccination or negative Covid-19 tests from those who want to be indoors. The measure reportedly comes on the back of a rise in cases among fully vaccinated bar workers.

"We believe we are obligated to protect our workers and their families and to offer safe space for customers to relax and socialise," the San Francisco Bar Owner Alliance said in a statement.

"The floodgates have opened," Brown University School of Public Health dean Ashish Jha tweeted.

"More and more organisations are realising that there is no way back to safe offices, safe hospitals, and safe universities without vaccinations."

"For much of winter, spring, I was VERY optimistic we'd have a great summer with few infections, deaths," Dr Jha tweeted.

"Through June, things looked good," he added. "But (the) situation has clearly turned worse."

In Florida, Dr Victor Herrera, chief medical officer of AdventHealth, Central Florida's largest hospital chain, on Monday elevated the company's emergency status to red after months in green.

"We are approaching an all-time high, in terms of our in-patient number of Covid-19 cases, which is a stretch in our capacity," he told county officials.

The good news is that hospitals are far better resourced with essential items, such as personal protection equipment and ventilators, than they were when the pandemic hit the US last year.

But experts are warning that cases will continue to rise. And the peak expected in the autumn months could be sustained through winter when more people gather indoors because of cold weather.

"We're definitely seeing a very disturbing prediction… that we will see another major wave of cases peaking in the autumn, late-September to October, that looks to be roughly two-thirds of the size of the worst surge that we had this past winter, January through March," Dr Alan Williamson, chief medical officer at Eisenhower Health, in Riverside County in southern California, told local TV channel KESQ.

Dr Aaron Wendelboe, associate professor of biostatistics and epidemiology at the University of Oklahoma's Health Sciences Centre, told local station KFOR: "We don't have to experience the hospitalisation, we don't to have to shut down society… we can just utilise what's essentially right in front of us, the vaccine and masks."

Some countries with mandatory Covid-19 vaccination

Tajikistan

The Central Asian country became the first in the world to announce mandatory vaccination for citizens aged 18 and above on July 3. Citizens can book an appointment with their doctors or their local clinics to receive free jabs.

Turkmenistan

Turkmenistan followed Tajikistan's move days later - requiring those aged 18 and over to get Covid-19 jabs. Exceptions are made only for those with any medical reason against vaccination.

France

It is mandatory for all health workers and carers to get vaccinated. It is also compulsory for people to show proof of vaccination or a negative Covid-19 test via a health pass for trips to museums, swimming pools or clubs. From Aug 1, the pass will be needed for long distance train and plane journeys as well.

Australia

Last month, Australia decided to mandate vaccinations for aged-care workers and employees working in quarantine hotels. Those working in aged care must receive their first dose of a Covid-19 vaccine by mid-September. The government has allocated A$11 million (S$11 million) to allow affected workers to take time off to be vaccinated or deal with any side-effects.

Greece

Greece has mandated Covid-19 vaccination for all healthcare workers immediately and for nursing home staff by Sept 1. Those who refuse to do so will be temporarily suspended from work. The country has also allowed only fully vaccinated people to dine indoors at restaurants, bars and cafes.

Britain

From October, home-care workers must be fully inoculated, unless they have a medical exemption. Those who fail to do so risk being redeployed away from front-line care or potentially losing their jobs. It is also mandatory for people to show proof of full vaccination to enter crowded places from the end of September.

Saudi Arabia

Vaccinations are mandatory for employees who want to return to their offices. Citizens who want to travel abroad will also have to complete their two-dose vaccine regime. Additionally, proof of vaccination will be required from next month for those who want to use public transport as well as enter any government or private spaces.

Italy

From Aug 6, a green pass - an extension of the European Union's digital Covid-19 certificate - will be required for indoor dining and to enter crowded venues. The pass serves as proof that the holder has either been vaccinated, recently tested negative for the coronavirus or recovered from Covid-19.