In a national address on Monday, U.K. Prime Minister Boris Johnson became the latest world leader to order a nationwide lockdown in an effort to stop the spread of the coronavirus.

Why it matters: Johnson warned on Sunday that a surge of coronavirus cases over the next two weeks could cripple the country's National Health Service, and that the U.K. is only "a matter of weeks" behind Italy — now the site of the worst coronavirus outbreak in the world.

"I urge you, at this moment of national emergency, to stay at home, protect our NHS, and save lives," Johnson said.

The government will reassess the lockdown in three weeks.

Details: Johnson ordered all non-essential shops, playgrounds, libraries, gyms and places of worship to close, and banned public meetings of more than two people, which he said will be enforced by police. He said citizens can leave their homes for the following "very limited" reasons:

Shopping for basic necessities as "infrequently as possible"

One form of exercise a day

Medical need, to provide care or to help a vulnerable person

Traveling to and from essential work

Between the lines: This is a huge reversal from Johnson's original coronavirus strategy, which eschewed lockdowns and closures like those being implemented around Europe in favor of allowing millions to be infected to establish "herd immunity."

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