LONDON (AP) — British Prime Minister Boris Johnson expressed his gratitude to the staff of the National Health Service for saving his life when his treatment for the coronavirus could have “gone either way" as the U.K. on Sunday became the fourth European country to surpass 10,000 virus-related deaths.

Dressed in a suit, and looking and sounding relatively assured, Johnson said in a video posted on Twitter after his discharge from St. Thomas’ Hospital in London that it was “hard to find the words” to express his debt of gratitude to the NHS for saving his life “no question.”

He listed a number of the frontline staff members who cared for him during his week-long stay at St. Thomas’ Hospital in London but singled out two nurses who stood by his bedside for 48 hours “when things could have gone either way.”

The prime minister said the nurses he identified as Jenny from Invercargill on New Zealand's South Island and Luis from Portugal, near Porto, were the reason that “in the end, my body did start to get enough oxygen.”

“Because for every second of the night they were watching and they were thinking and they were caring and making the interventions I needed,” he said. “So that is how I also know that across this country, 24 hours a day, for every second of every hour, there are hundreds of thousands of NHS staff who are acting with the same care and thought and precision as Jenny and Luis.”

After his release from the hospital, Johnson made his way to Chequers, the prime minister’s country retreat northwest of London, and on the advice of his medical team won't be returning to work immediately, his office said in statement.

It’s unclear what involvement Johnson will have in this week’s anticipated extension to the nationwide lockdown the prime minister announced on March 23 in response to the worldwide virus pandemic.

Johnson, 55, was the first world leader confirmed to have the virus. His COVID-19 symptoms, including a cough and a fever, at first were described as mild, and he worked from home during the first few days of self-isolation.

But he was admitted to St. Thomas’ on April 5 after his condition worsened and transferred the following day to the intensive care unit, where he received oxygen but was not put onto a ventilator. Johnson spent three nights in the ICU before he was moved back to a regular hospital ward on Thursday.

Johnson’s pregnant partner, Carrie Symonds, cheered the prime minister's improved health in a series of tweets, saying she “cannot thank our magnificent NHS enough.”

There “were times last week that were very dark indeed," Symonds wrote. "My heart goes out to all those in similar situations, worried sick about their loved ones.”

The government confirmed Sunday that the U.K. became the fourth European country after Italy, Spain and France to reach the grim milestone of 10,000 virus-related deaths. It said 737 more people who tested positive for the coronavirus had died, taking the total recorded in the U.K. to 10,612.

The figure reported Sunday represented a second straight daily decline in number of deaths, although the lower figures may be due to delays related with the Easter weekend.

Health Secretary Matt Hancock said this is a “somber day” for the country in its battle against this “invisible killer.”

With the day-to-day death tolls in Italy and Spain on a downward slope, there were growing fears the U.K. might end up as the country with the most virus deaths in Europe. However, the pace of new confirmed cases and hospitalizations in the U.K. appears to be plateauing, a trend that officials hope will show up in fewer deaths in the near future.

Wellcome Trust director Jeremy Farrar, a scientific adviser to the British government, said the U.K. was likely to be “one of the worst, if not the worst-affected country in Europe.”

He told the BBC that Britain has “lessons to learn” from Germany, where much more widespread early testing for the virus and aggressive contact tracing have been accompanied by many fewer coronavirus deaths.

Hancock on Sunday announced a new NHS contact tracing app that will anonymously alert users if someone they were in significant contact with in the previous few days tests positive for the virus.

While Johnson convalesces, Foreign Secretary Dominic Raab is handling the nation's response to the pandemic that has infected at least 1.81 million worldwide and killed more than 112,000 people. Experts say those numbers seriously understate the impact of the pandemic, due to limited testing and different ways of counting the dead.

Johnson's Conservative government has come under fire for its slow response to confronting the pandemic — allowing tens of thousands to gather at the Cheltenham horse racing festival in mid-March, for example. It's also faced criticism for its slow roll out of a coronavirus testing program.

In the past few days, the government also faced acute criticism over a lack of personal protective equipment for frontline hospital workers amid reports that some nurses had resorted to cutting up garbage bags to cover themselves. The Royal College of Nursing has piled more pressure on the government, urging members to refuse to treat patients as a “last resort” if adequate protections are not provided.

Hancock said he did not have an update on how many NHS workers with the virus have died following the 19 he confirmed on Saturday. He said efforts to procure more PPE are“moving in the right direction .... but until everyone gets the PPE they need, then we won’t rest.”

The prime minister also thanked the British people for the sacrifices they are making to get on top of the pandemic.

“I want you to know that this Easter Sunday I do believe that your efforts are worth it, and are daily proving their worth,” Johnson said.

“Because although we mourn every day those who are taken from us in such numbers, and though the struggle is by no means over, we are now making progress in this incredible national battle against coronavirus.”