Johnson made the pledge on Saturday, which his Health Secretary Matt Hancock elaborated on on Sunday.
“We’re now aiming to offer a vaccine to everybody in categories 1-9, that’s those who are most vulnerable, and the over 50s, and health and social care workers. And to do that by April 15,” Hancock told Sky News on Sunday.
“And then, all adults should be offered a jab by the end of July.”
Hancock said the ministers now think the UK has enough supplies to achieve the goal.
When asked about plans to vaccinate children. Hancock said clinical trials are underway to determine whether or not vaccines are “safe specifically for children,” and that more evidence is needed to show the effectiveness of the vaccines on stopping the transmission of the CCP (Chinese Communist Party) virus, which causes the disease COVID-19.
“Because children very, very rarely get symptoms or serious illness from the disease, and the value, the importance of vaccinating children, is to try to stop the spread of the disease [as opposed to reducing mortality],” Hancock said.
“It looks like the first jab reduces the impact of transmitting the disease by about two thirds, but we need more evidence on that as well,” he added.
So far, Britain has given the first dose of a COVID-19 vaccine to 17.2 million people, over a quarter of its 67 million population and behind only Israel and the United Arab Emirates in vaccines per head of population.
But despite the improving picture, Hancock and leading epidemiologist John Edmunds said the restrictions must be eased gently and in stages, to see what impact the increased movement of people has on the virus.
Hancock suggested each easing could require a couple of weeks to detect the impact, before another part of the economy can reopen. Schools are expected to return first in early March.
Edmunds said it was difficult to say how widespread the South African variant was but that, like the rest of the pandemic, it was being held in place by the lockdown.
“The risk comes when we release the lockdown,” he said, adding that allowing the virus to circulate in younger healthier people could lead to further mutations that undermine the vaccine programme.
Johnson is due to announce his “cautious and phased” plan for England to exit the lockdown on Monday.
Hancock said the details were yet to be signed off, but getting children back to school is at the top of the cabinet’s priority list.
Reuters contributed to this report.