Washington (AFP) - President Joe Biden warned Thursday that the Omicron variant of the coronavirus will start to spread much more rapidly in the United States and urged Americans to get vaccinated or boosted.

"The only real protection is to get your shot," he said, predicting "a winter of severe illness and death" for the unvaccinated.

He spoke as America is growing more and more nervous over the latest chapter of the nightmare that is the coronavirus pandemic.

As of December 1, the average of new daily cases of infection was 86,000. On December 14, it had shot up to 117,000 -- an increase of 35 percent.

Biden stressed the importance of vaccinated people getting a booster and of those who have yet to be vaccinated receiving their first shot.

Health ministers from the G7 on Thursday called for international cooperation in the face of the Omicron strain, which they called the "biggest current threat to global public health."

The outbreak linked to the new variant has spread globally, and more European nations are implementing travel restrictions.

The United States, the hardest-hit country in the world, is currently averaging 1,150 Covid-19 deaths per day, according to figures from the Centers for Disease Control and Prevention (CDC).

Americans do seem to be in for a rough winter.

Broadway shows are being canceled because of more and more positive tests among drama companies.

Tools 'are working'

Major US universities are reverting to classes and tests online to try to halt the spread of the virus.

The National Football League has introduced stricter health controls after 100 or so players tested positive since the beginning of the week. The NBA has also been hit with games being postponed.

At the White House, too, there has been a change in tone. 

A few weeks ago, Biden told Americans the new strain of the virus was no reason to panic.

On Thursday, he brought reporters in at the end of a meeting on the pandemic to, as he said in a no-nonsense voice, send a message directly to Americans: vaccinated people should get a booster and the non-vaccinated should get their shots.

Shortly before, deputy White House spokeswoman Karine Jean-Pierre had suggested the administration did not intend to take specific restrictive measures for the moment and to focus instead on vaccination.

"The tools that we have, we know are working," she said, adding, "We're just going to continue pushing forward... to get Americans vaccinated and boosted."

Biden, who was elected largely on a pledge to end the health crisis in America, unveiled on December 2 a plan to ward off a steep rise in infections, hospitalizations and death.

But he stopped short of taking more restrictive measures, knowing the issue is very sensitive.

Like people elsewhere, Americans are fed up with the pandemic, and Republicans regularly speak out against the idea of forcing people to get vaccinated or wear masks.

When the Omicron strain was identified weeks ago, the United States halted travel from several southern African countries but did not introduce any other restrictions, and has not tightened health measures on domestic flights.

Biden's maneuvering room is limited in any case, as seen in several lawsuits that have held up his order for larger companies to force their workers to get vaccinated.

The United States surpassed 800,000 Covid deaths Tuesday, according to the Johns Hopkins University tracker.

That is higher than the population of some states, such as Alaska or North Dakota.

This year, around 450,000 people have died of Covid, most of them non-vaccinated, even though effective and free vaccines became available in the spring.