British Prime Minister Boris Johnson hailed a "significant milestone" on Sunday as data showed 15 million first doses of COVID-19 vaccinations had been delivered, fuelling calls for the government to start relaxing stringent lockdown measures.

The vaccine programme is seen as one of few successes in the government's handling of a pandemic that has left the country with a higher death toll and worse economic damage than its peers. With a population of around 67 million, the United Kingdom's vaccination progress is among the fastest in the world.

The British government set a Feb. 15 target date to reach 15 million people in priority categories: care home residents and staff, frontline health and care workers, all those aged 70 or over and the clinically extremely vulnerable.