The announcement follows a new vaccine milestone for the ACT, with more than 70 per cent of eligible people receiving their first dose.

The territory is still leading the country in immunisation rates, with a higher rate of fully vaccinated citizens than any other jurisdiction.

Of over-16s in the ACT, 45.9 per cent have received two doses. Chief Minister Andrew Barr said he believed half of eligible Canberrans will be fully immunised soon.

MORE COVID NEWS:

He said the decision to reduce times between AstraZeneca doses was based on advice from chief health officer Dr Kerryn Coleman, using ATAGI guidelines.

"The best vaccine is the one that you can get today. We want to see more Canberrans fully vaccinated faster to give them protection during this outbreak," he said.

Health minister Rachel Stephen-Smith recommends people bring forward their second AstraZeneca jab.

"People who have a second-dose AstraZeneca booking with their GP or pharmacist should contact them about bringing their appointment forward," she said.

"AstraZeneca appointments are widely available for people aged 18 and over."

A smaller interval period between AstraZeneca doses may reduce its efficacy. ATAGI advises a 12-week interval period between jabs in non-outbreak situations.

IN OTHER COVID NEWS:

The body also says that two doses of the AstraZeneca vaccine given four weeks apart are 55 per cent effective against symptomatic COVID-19.

When two doses are given 12 weeks apart, the vaccine is 81 per cent effective against symptomatic COVID.

One dose of AstraZeneca reduces the risk of symptomatic infection by about 30 per cent and hospitalisation by 71 per cent.

However, the body said that "shortening the interval from 12 weeks to no less than 4 weeks between doses is acceptable" in outbreak situations.

Two doses provides greater protection to an individual than one, despite the interval time between jabs. A higher full vaccination rate also provides greater protection to the broader community in an outbreak.

Someone who has received one dose of AstraZeneca cannot receive Pfizer or Moderna as their second vaccine.

Our coverage of the health and safety aspects of this outbreak of COVID-19 in the ACT and the lockdown is free for anyone to access. However, we depend on subscription revenue to support our journalism. If you are able, please subscribe here. If you are already a subscriber, thank you for your support. You can also sign up for our newsletters for regular updates.

Our journalists work hard to provide local, up-to-date news to the community. This is how you can continue to access our trusted content: