The AstraZeneca vaccine, which was slated to be the "workhorse" of Australia's vaccine rollout, was last week scrapped as the preferred option for people under 50 due to updated medical advice.

Three Australians have now developed a rare blood clotting disorder linked with the jab, with authorities investigating the latest case - the death of a NSW woman who had received the vaccine days earlier.

The Therapeutic Goods Administration's Vaccine Safety Investigation Group on Friday night concluded that her death "is likely to be linked to vaccination".

However a fresh Oxford University study, which is yet to be peer reviewed, found the virus itself is still far more dangerous.

Researchers examined and compared over 500,000 coronavirus diagnoses and more than 480,000 recipients of a mRNA vaccine - either the Pfizer or Moderna jab.

Based on US data, it found blood clotting in the brain - cerebral venous thrombosis or CVT - occurs in 39 people per million diagnosed with COVID-19, compared with 4.1 people per million who have received the Pfizer or Moderna vaccine.

The study notes the European Medicines Agency estimates that 5 per million people who receive the AstraZeneca vaccine develop CVT.

On those numbers, a person is 10 times more likely to develop a blood clot in the brain after contracting COVID-19 than after receiving a mRNA vaccine.

The risk posed by COVID-19 infection is eight times that of the AstraZeneca vaccine.

Australian Chief Medical Officer Professor Paul Kelly said the study was a reminder of why take up of the vaccine in Australia is so important.

"Clotting is a feature of COVID," he said on Friday.

"It also happens to be a feature, very rarely, of the AstraZeneca vaccine.

"But the benefit absolutely, and particularly for those over the age of 50, outweighs significantly the risk."

However Dr Daryl Cheng, the medical lead of the Melbourne Vaccine Education Centre, pointed out the study focused on CVT, which is only part of the syndrome linked to the AstraZeneca vaccine.

The thrombotic thrombocytopenia syndrome is characterised not just by clots in the brain and abdomen but in other areas of the body and also causes low platelet counts.

"Comparing isolated CVT with ... the syndrome of interest following COVID-19 AstraZeneca vaccine means we actually may be comparing two separate entities and drawing conclusions that are not entirely accurate," he said.

"With a low incidence of COVID-19 disease in Australia, and hence an absence of this increased risk of thrombosis, this study does not directly impact or should not change our approach to a COVID-19 vaccine rollout," he said.

Infectious disease doctor Michelle Ananda-Rajah agreed the scope of the study was a concern.

"Vaccine induced thrombotic thrombocytopenia has a higher published mortality of 55 to 60 per cent ... compared to the 18 to 20 per cent described in this report," she said.

However Griffith University Associate Professor of Haematology Dr Indu Singh said she believed vaccinations should continue in earnest in regions with community transmission.

"More evidence is required, and quickly, to make a conclusive statement but in the meantime, some protection is better than nothing," she said.

Australian Associated Press