UK PM says vaccine a welcome boost to Covid fight
But with most people not expected to get vaccinated until early 2021, he said the public still needed to be careful to stop the spread of the virus.
"We can't afford to relax," he said on a visit to a central London hospital.
Britain starts virus vaccinations as California locks down
Britain became the first Western country to start a mass coronavirus vaccine campaign with a pensioner receiving the initial jab on Tuesday as across the Atlantic millions of Californians went into lockdown to stop surging US infections.
The UK's rollout of the Pfizer-BioNTech jab -- the first country to start using the approved inoculations -- is one of several vaccines bringing hope for an end to the pandemic that has killed more than 1.5 million people worldwide and ravaged economies since first emerging in China late last year.
British pensioner Margaret Keenan, 90, said she felt "privileged" to be given the injection, the first of millions expected to be administered over the coming months in one of the world's worst-hit countries.
"My advice to anyone offered the vaccine is to take it," said Keenan, wearing a mask and a penguin T-shirt. "If I can have it at 90 then you can have it too."
British officials hailed the Pfizer-BioNTech jab as a turning point in the fight against the pandemic, and on what has been dubbed "V-Day", people rolled up their sleeves for an initial dose from early Tuesday morning.
The UK's second jab reportedly went to a man named William Shakespeare. The over-80s, care home workers, and at-risk health and social care staff will be at the front of the line.
The British public has been largely supportive of the rapid approval of the vaccine, but ministers and health professionals are aware they still need to combat mistrust over treatments that risks hampering global efforts to fight the disease.
Russia on Saturday began vaccinating high-risk workers with its own jab and Beijing has also begun an emergency inoculation campaign with a medicine made in China.
Britain's start to its vaccinations came as around the world the virus picture darkens, with more countries reimposing curbs and around 30 million people in California placed under stay-at-home orders to fight a deepening US crisis.
While the World Health Organization has warned that successful vaccines on their own will not immediately end the crisis, nations around the world are seeking safe candidates to administer to populations growing increasingly weary of restrictions and economic pain they cause.
The floundering efforts to quell the pandemic in the US have been widely criticised -- the nation is the world's worst-hit with known infections approaching 15 million and more than 283,000 deaths.
The extent of the crisis was illustrated by the lockdown in California, the most populous American state, where authorities forced most offices to close and banned gatherings among different households.
Bars and services such as hair salons were shut and restaurants were only allowed to serve takeaways. Non-essential travel was also temporarily restricted statewide as California experienced record new Covid-19 cases.
"The overwhelming majority of Californians are now in this new stay-at-home order protocol," said state Governor Gavin Newsom, who earlier warned that the state hospital system was at risk of being "overwhelmed."
The United States is expected to grant emergency authorisation for the Pfizer-BioNTech and another vaccine produced by Moderna, hoping to vaccinate millions by the end of the year.
Canadian Prime Minister Justin Trudeau said Pfizer and BioNTech will deliver their first doses to his nation within weeks.
In India, two pharmaceutical firms -- including Serum Institute, the world's biggest manufacturer of vaccines -- on Monday sought fast-track approval for coronavirus shots.
India is the second-worst hit nation having already recorded more than 140,000 deaths.
And in Brazil's Sao Paulo state -- Latin America's coronavirus epicentre -- authorities will launch a campaign from January providing the Chinese-developed vaccine CoronaVac to healthcare workers, seniors and other vulnerable groups first.
Even as hopes rise for vaccines and a resulting economic recovery, governments around the world are being forced to tighten restrictions to contain surges in infections.
German Chancellor Angela Merkel urged regions with high coronavirus rates to tighten curbs, while Denmark said it will close middle and high schools, bars, cafes and restaurants in half of the country.
But Austrians got a boost when the government lifted its measures.
"We couldn't wait with the shopping, even if it might be a bit crowded today," Robert Bauer told AFP as he shopped in Vienna.
Israel has announced a nationwide night-time curfew from December 9 following a steep increase in virus cases. And Hong Kong announced a ban on evening dining at restaurants and the closure of fitness centres to contain a new wave of cases.