Speculation in Australian health circles might mean that Australians need a third vaccine in order to travel overseas.

It also means depending on the brand of vaccine they receive, they might be barred completely from entering specific countries.

Professor Mary-Louise McLaws said that due to Australia's slow vaccine rollout, other countries are now weighing up the pros and cons of a booster shot.

“With more research done and as we’re learning more about COVID-19 it would appear we need a third shot, preferably around eight months,” she told Seven’s The Latest.

“But that (time) would depend on what your vaccine efficacy was.”

However, she stressed that research was continuing into booster shots and whether recipients are able to "mix and match" different brands of the vaccine.

“I think everybody will have to have a booster,” she said.

“The Oxford University researchers are having a look at different types of doses - so they’re looking at different studies with the first dose being AstraZeneca or Pfizer and the second being a different one.

“And it looks like that mix and match could give you a better response.”

She confirmed that the "gossip" in health circles was that some countries might not accept travellers who have had two doses of the AstraZeneca vaccine.

“I haven’t seen anything absolutely definitive yet but this might fit in with everybody getting that third booster with Pfizer so that we can, of course, go into the US.”

This is due to the FDA, the US regulator, not recognising AstraZeneca as an approved vaccine.