Pope Francis holds a palm branch as he celebrates Palm Sunday Mass behind closed doors in St. Peter's Basilica, at the Vatican, Sunday, April 5, 2020, during the lockdown aimed at curbing the spread of the COVID-19 infection, caused by the novel coronavirus.

NEW YORK — Americans braced for what the nation’s top doctor warned Sunday would be “the hardest and saddest week” of their lives while Britain assumed the unwelcome mantle of deadliest coronavirus hotspot in Europe after a record 24-hour jump in deaths that surpassed even hard-hit Italy.

British Prime Minister Boris Johnson was hospitalized, 10 days after testing positive for COVID-19. His office described the move as a “precautionary step” and said the 55-year-old Conservative would undergo tests.

U.S. Surgeon General Jerome Adams offered a stark warning about the expected wave of virus deaths.

“This is going to be our Pearl Harbor moment, our 9/11 moment,” he told “Fox News Sunday.”

New York City, the U.S. epicenter of the pandemic, saw a glimmer of hope, with Gov. Andrew Cuomo saying that daily deaths had dropped slightly, along with intensive care admissions and the number of patients who needed breathing tubes inserted. Still, he warned that it was “too early to tell” the significance of those numbers.

Italy and Spain also got some encouraging news. Italy registered its lowest day-to-day increase in deaths in more than two weeks — 525, said Angelo Borrelli, the head of the national Civil Protection agency.

The pace of infection also seemed to be slowing. The country recorded 4,316 new cases Sunday. Earlier in the outbreak, daily increases topped 6,000.

Even so, Borrelli warned, “This good news shouldn’t make us drop our guard.”

Confirmed infections fell in Spain too, and new deaths declined for the third straight day, dropping to 674 — the first time daily deaths have fallen below 800 in the past week.

“We are starting to see the light at the end of the tunnel,” Spanish Prime Minister Pedro Sánchez said.

The outlook remained bleak in Britain, which recorded 708 new coronavirus deaths Saturday compared with Italy’s 631 deaths. With 600-plus more deaths reported Sunday, Britain has recorded more than 4,900 virus deaths overall among nearly 48,000 cases. Italy still has by far the world’s highest coronavirus death toll — almost 16,000 out of nearly 130,000 confirmed cases.

There are wide fears that Johnson’s government did not take the virus seriously enough at first and that spring weather will tempt Britons and others to break social distancing rules.

In a rare televised address to the nation, Queen Elizabeth II appealed to Britons to exercise self-discipline in “an increasingly challenging time.” The 93-year-old monarch said the pandemic had caused enormous disruptions, bringing grief, financial difficulties and daunting challenges to everybody. It was only the fourth time since her reign began in 1953 that she has given such an address.

“I hope in the years to come, everyone will be able to take pride in how they responded to this challenge,” she said. “And those who come after us will say that the Britons of this generation were as strong as any.”

Health Secretary Matt Hancock said sunbathing in public places was not allowed and the U.K. might even ban outdoor exercise if people still “flout the rules.”

“The vast majority of people are following the public health advice, which is absolutely critical, and staying at home,” Hancock told Sky TV. “But there are a small minority of people who are still not doing that — it’s quite unbelievable, frankly, to see that.”

As the numbers of infections rose, the deputy head of Britain’s National Health Service Providers said the agency needed to focus on quickly increasing ventilator capacity and getting more protective equipment for health care workers.