Vaccination centres across most of Wales will be open for people to walk in with no appointment from Saturday.
It comes as three-quarters of people aged under 50 in Wales have now had their first Covid-19 jab, the Welsh government said.
Over-18s without an appointment will be able to get a first dose at many centres, and young adults in particular are being encouraged to attend.
Professor of public health Linda Bauld welcomed the plan as "a good idea."
"If you make it easy for people so that they do not have to plan in advance, they don't have to cancel anything and can fit it in their day, particularly for younger adults who have a lot of different commitments - working, studying et cetera - I think this will be a good idea," said Prof Bauld, of Edinburgh University.
All adults should already have received an offer of a vaccine.
But Health Minister Eluned Morgan said in some cases people may have had to cancel or missed their appointments, and the government is keen not to leave anyone behind.
Ms Morgan said: "We want to make it as easy as possible for people to get their vaccine, particularly younger adults who have other demands on their time.
"In Wales we have a 'no-one left behind' policy and making walk-in appointments available is another way in which we can ensure all adults have access to a vaccination at a time and place convenient for them."
Medical students are among those who have been trained to give vaccines at the Bayside centre in Cardiff Bay.
Third year Cardiff University medical student Cameron Fleming said: "There tends to be quite a few people who missed appointments for various reasons, be it isolation or nerves, or whatever it might have been at the time."
He said some people who decided "at first that they weren't so keen to have it" have since used walk-in appointments to get their first dose.
"A lot have been changing their minds since we have managed to roll it out to more and more people, which has been a very positive result of the whole thing."
Mr Fleming said the experience had been a welcome addition to his time as a medical student.
"Many of us are in our third or second year, where we haven't had so much of the public exposure of being on the wards and actually dealing with patients yet. So having the opportunity to meet so much of the population from every background has been a very positive experience."
Fellow Cardiff University second year medical student Abbie Carter said she enjoyed seeing the programme progress.
"We started with the older generations, coming right down to the young people," she said.
"It has generally been a really positive response to having the vaccine. Even with people who have been a bit unsure - it has been great to chat to them, hear their concerns and to tackle those."
Since it started offering a first dose of the vaccine without an appointment, about 5,000 people have walked in to have a jab in Cardiff Bay.
The vaccination centre, which is housed in a former toy superstore, has been nicknamed "Jabs R Us" and is administering tens of thousands of first and second doses each week.
"It is really important, as a population effort, for everyone to get the vaccine," Ms Carter said.
Where can I get a walk-in vaccine?
Walk-in vaccination appointments will be available at the following locations:
Aneurin Bevan University Health Board
- Newport Leisure Centre, Saturday from 08:30 BST to 16:00 (first doses for over-18s)
- Newbridge Leisure Centre, Saturday from 08:30 to 13:30 (first doses for over-18s)
Betsi Cadwaladr University Health Board
- All mass vaccination centres will accept walk-ins at all times (first doses for over-18s)
Cardiff and Vale University Health Board
- Bayside Mass Vaccination Centre, 08:00 to 16:00 every Saturday and Sunday (first doses for over-18s)
- Holme View Barry, this Saturday and Sunday, 08:30 to 19:30 (first doses for over-18s)
Cwm Taf Morgannwg University Health Board
- All mass vaccination centres will accept walk-ins at all times from 5 July (first doses for over-18s)
Hywel Dda University Health Board
- All sites will accept walk-ins and drive throughs at all times (first and second doses for over-18s)
Powys Teaching Health Board
- Walk in appointments are not currently offered, but this will be kept under review
Swansea Bay University Health Board
- The Immbulance at City Church, Swansea, Saturday from 0900 to 1600, offering Oxford AstraZeneca to over-40s
- Bay Field Hospital, Sunday from 10:00 to 18:00 (first doses for 18 to 39-year-olds)
Tracy Meredith, head of testing and mass immunisation at Cardiff and Vale health board, said: "We're working really hard across vaccination centres in Wales to make getting the jab as quick and convenient for people as possible.
"It's a great opportunity to drop in this weekend, get your vaccine and be protected."
Ms Morgan added: "The vaccine remains the best way we can protect ourselves and we need everyone to keep saying 'yes' to the vaccine and to remember that one dose isn't enough.
"We all need two doses to complete the course and to have the best chance of reducing our risk of serious illness."
Prof Bauld added that worries young people may not get jabbed had not materialised.
"We have seen this in a number of parts of the UK now. We were concerned young people might not be as keen to take up the vaccine... but that has not panned out."
She said "the bigger challenge" might be getting people to take a second dose, and that although booster jabs could be a difficult, most people would have them.