A Tasmanian man is stable in hospital after developing blood clots a week after getting the AstraZeneca coronavirus vaccine.
The 70-year-old reported symptoms seven days after getting the jab.
Australia's medical regulator has found the man's case was likely linked to the vaccine.
Tasmania's Department of Health has put together an expert advisory panel to review the case.
Acting director of public health Scott McKeown says that for people over the age of 70 the benefit of getting the AstraZeneca vaccine outweighed the risks.
"For people over 50 as they get older, the risk of severe outcomes from COVID becomes much much greater," Dr McKeown told reporters in Hobart.
Thrombosis with thrombocytopaenia syndrome is rare but can be deadly.
Australian immunisation experts say it occurs in about six per million people vaccinated.
In its latest data release, the Therapeutic Goods Administration has said there have been 11 cases of the syndrome as of May 2.
There had been 1.4 million AstraZeneca doses administered at that point.
Of the 11 cases, five were revealed by the TGA on Thursday.
They include the 70-year-old Tasmanian man, a 74-year-old man and a 51-year-old woman from Victoria, a 66-year-old man from Queensland and a 64-year-old woman from Western Australia.
Specialist medical advisor with Public Health Tasmania Shannon Melody says the syndrome was clotting with low platelets.
"It's the immune system in overdrive," Dr Melody said.
Symptoms are related to where blood clots occur in the body.
"It's not something that someone would shrug off normally, it's something that would alert you to see your usual healthcare provider."
Treatment involves moderating the immune system by administering medicine through a drip in the arm and treating the clot as well.
AstraZeneca is aware of the new cases.
"Patient safety remains AstraZeneca's highest priority and we continue to support regulators in Australia and overseas," the company said in a statement.
"Our global commitment remains to play an important role in addressing the current global health emergency posed by COVID-19 by providing a safe and effective vaccine, at no profit during the pandemic."
Australian Associated Press