Video report by ITV News Political Correspondent Daniel Hewitt
All adults in the UK aged 50 and over should receive a Covid vaccine by May, the government has pledged.
Ministers had previously refused to give a firm date, saying only that the first nine priority groups would be vaccinated by the spring.
But Downing Street then repeatedly declined to define when spring ends during a briefing for journalists on Thursday.
On Friday, however, the Cabinet Office announced that the government intended to vaccinate all nine cohorts by May.
It said the UK’s vaccination programme planned to have reached all nine priority cohorts by May – which it said gave the government confidence to commit to holding local elections that month.
According to the government’s vaccines delivery plan, some 27 million people in England, and 32 million people across the UK, are estimated to fall in the first nine groups.
ITV News Political Correspondent Daniel Hewitt explains how Boris Johnson hopes this lockdown will be the last for England
Who are the nine priority groups?
1 - Residents in a care home for older adults and their carers (800,000 people)
2 - Those aged 80 and over and frontline health and social care workers (a total of 7.1 million people in this group: 3.3m over 80s, 2.4m healthcare workers, 1.4m social care workers)
3 - Those aged 75 and over (2.3 million)
4 - Those aged 70 and over and clinically extremely vulnerable individuals (3.2 million)
5 - Those aged 65 and over (2.9 million)
6 - All individuals aged 16 to 64 with underlying health conditions which put them at higher risk of serious disease and mortality (7.3 million)
7 - Those aged 60 and over (1.8 million)
8 - Those aged 55 years and over (2.4 million)
9 - Those aged 50 years of age and over (2.8 million)
The government is on track to vaccinate the first four cohorts by February 15 – including NHS and care home staff, care home residents, and all those aged 70 and over.
So far, more than 10.4 million people in the UK have received their first dose of a coronavirus vaccine.
Categories five to nine include all those aged 50 and over, as well as adults aged 16-65 in an at-risk group.
Prioritisation for the rest of the population has yet to be determined.
The update comes as ministers were urged to "make decisions dependent on the circumstances, rather than being driven by a calendar of wanting to do things" with regards to easing of lockdown in England.
Government scientists have warned the government not to lift lockdown too quickly or risk a "yo yo" pattern of harsher and more relaxed measures.