South Australia has cleared the way for younger adults to book for the Pfizer COVID-19 vaccine from next week, but says demand is expected to outstrip supply.

All South Australians aged between 16 and 39 will be eligible to book for the Pfizer COVID-19 vaccine from next week, but authorities warn initial demand is expected to outstrip supply.

SA will become the first state to include people under 40 in the Pfizer rollout with bookings to be taken at state-run clinics from Monday for appointments starting in September.

It means all people over 16 will be eligible for either the Pfizer or AstraZeneca jabs.

"This nation-leading move is a huge milestone in our vaccine rollout, with South Australia leading the way as the first state in the nation to open up to everyone aged 16 and older," Premier Steven Marshall said on Thursday.

"Vaccination is our pathway out of the pandemic and we are determined to ensure everyone has access to the vaccine as soon as possible to keep our state safe and economy strong."

From Monday an extra 127,000 slots will be provided in the booking system for September and October.

Mr Marshall said demand for the Pfizer vaccine was initially expected to exceed the available doses.

"The constraint right around the country is with the mRNA vaccines," he said.

"We've got more Pfizer coming, we've got the Moderna vaccine which is coming. We are getting a significant increase coming to South Australia.

"But we would expect in the early days that demand would significantly outstrip supply."

The total number of vaccine doses administered in SA has just topped one million with 640,000 people now having one dose or more.

About 78 per cent of people aged over 70 have had at least one dose.

Chief Public Health Officer Nicola Spurrier said it was important to start vaccinating younger people who were more likely to spread the virus in the community.

She said the people coming into most contact with others were in the 20-to-40 age group.

"So it's really critical that we open it up. I'm very pleased with this decision," she said.

Also on Thursday, SA eased some local virus restrictions with density rules for seated activities, including those in pubs, cafes and restaurants, moving to three people for every four square metres or 75 per cent.

Any venues involved in standing activity, including general retail outlets, remain at one to every two square metres or 50 per cent.

Gyms and other fitness centres will stay at one to every four square metres.

Masks are still required in most public places, including shopping centres and there's no change to SA's borders which currently lock out people from Queensland, NSW and Victoria.

SA reported no new virus cases on Thursday. It has six active infections including three people being treated in hospital.

One of those is a man in his 30s who remains in a stable condition in intensive care.