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.
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."
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."