Britain has seen one of the world's quickest vaccine rollouts, behind only Israel in the proportion of its population receiving at least one dose of a COVID-19 shot. The government said it had now offered at least one shot to priority cohorts 1 to 9, which include all adults over 50, the clinically vulnerable, and health and social care workers, ahead of a target to do so by Thursday.

"We will now move forward with completing essential second doses and making progress towards our target of offering all adults a vaccine by the end of July," Prime Minister Boris Johnson said in a statement. Britain has slowed down the pace of first doses, to ensure people in high-priority groups receive a second dose despite lower vaccine supplies in April than March.

In a boost to vaccine supplies for first doses, the rollout of Moderna's shot in England begins on Tuesday, after the first doses were given in Wales last week. Already used in the United States and other parts of Europe, it becomes the third vaccine to be used in Britain after AstraZeneca and one from Pfizer-BioNTech, Moderna uses the same mRNA technology as Pfizer's shots but can be stored at normal fridge temperatures unlike its rival U.S. vaccine, which must be kept and shipped at ultra-low temperatures. Johnson's office said that the government remained on track to offer all adults a shot by July 31. On Tuesday, the National Health Service said that people aged 45 or over could now book appointments to receive a COVID-19 vaccine.

The success of the vaccine program has underpinned Johnson's roadmap out of lockdown, which on Monday saw all shops and outdoor hospitality settings in England reopen, with some celebrating on the street into the evening. Adding a note of caution to the optimism, the government announced an expansion of so-called surge testing in the south London boroughs of Lambeth and Wandsworth to detect cases of the variant first found in South Africa.

There have been 74 confirmed and probable cases of the coronavirus variant, known as B.1.351, in the boroughs, and there is concern that vaccines are less effective against it. "The important thing will be to watch: If the South African variant has really taken off, and we'll probably know in about two to three weeks, then we may need to pause re-opening a little bit," James Naismith, professor of structural biology at the University of Oxford and director of Rosalind Franklin Institute, told BBC Radio.

With more than 127,000 fatalities, the United Kingdom has the fifth-highest death toll in the world from COVID-19.