More than 35 million people in the UK have received at least one dose of the Covid-19 vaccine.
A total of 16 million have already had a second jab.
British government aims to offer vaccines to all its people by summer.
Summer, which usually begins in July, is a season of holidays in UK and Europe.
The government says the continued success of the vaccine programme is one of the four conditions needed for further easing of restrictions, BBC reportes.
Almost everyone in the top priority groups, including all those over the age of 50, plus those in high-risk categories have received at least a first jab, before 15 April.
Priority has been given to these groups because they account for 99% of coronavirus deaths so far. Most of the fatalities were from these groups.
Now the government plans to vaccinate the rest of the adult population (young people), another 21 million people, by the end of July.