Meanwhile, a health board in Wales has set out plans to offer its entire population aged over 18 a first dose of vaccine by 30 May - two months ahead of the government deadline - and in Northern Ireland people aged 25-29 can now book COVID-19 jabs, although appointments available each week are limited.
Almost 37m people UK-wide had received a first dose of vaccine by the end of 18 May - around 70% of the adult population. Nearly 21m people - 40% of the UK adult population - have now received both doses of vaccine.
The government has said that vaccinations in England will open up to more people in their early 30s in 'the next few days and weeks'.
A government-backed trial of booster COVID-19 vaccination launched this week will look at seven vaccines - all of which the UK government has ordered significant stocks of - for potential use in a booster campaign that could begin later this year.
Booster doses will be given to participants who received first doses early in the vaccination campaign, and will be administered at least 10 to 12 weeks after a second dose. Volunteers may receive booster doses of a different type to the first two doses they received.
More than 1.5m appointments for COVID-19 vaccination were booked in the week beginning 10 May in England, the government has said - and 440,000 bookings came in a single day earlier this week after text message invites to patients aged 36 and 37 years old.
Widening rollout to patients aged under 40 comes as the MHRA considers an overhaul of storage rules for the Pfizer/BioNTech vaccine that could allow it to be stored for up to a month at normal fridge temperatures.
The move could prove a logistical boost to vaccination sites delivering jabs to younger patients, for whom the Pfizer or Moderna vaccines are recommended as a first choice instead of the Oxford/AstraZeneca vaccine.
Nearly three quarters of the 22.1m COVID-19 vaccinations delivered UK-wide since the start of April have been second doses, as millions of people given a first dose early in the campaign became eligible for their second jabs.
In a bid to check the spread of the Indian variant of coronavirus, B.1.617.2, the Joint Committee on Vaccination and Immunisation has recommended that people aged 50 and over and the clinically vulnerable should have their second doses brought forward to eight weeks after their first dose. To date second doses have been offered after 11 to 12 weeks.
NHS England medical director Professor Stephen Powis said: 'The success of the NHS COVID-19 vaccination programme, the biggest in history, is not by accident but down to careful planning and precision by NHS staff who have now delivered 48.5m doses across England in less than six months.'
We have arranged appointments for our population aged 18 and over to receive their first vaccine by 30th May. Appointment letters should arrive by the end of this week ✉️— Cardiff & Vale UHB (@CV_UHB) May 18, 2021
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