The Therapeutic Goods Administration is investigating three new cases of a rare blood clot syndrome "likely" linked to the AstraZeneca vaccine.
The cases are a 35-year-old woman, a 49-year-old man and an 80-year-old man.
It is the first time the syndrome has been reported in a person over 50 in Australia.
The TGA said in a statement all cases were likely linked to the AstraZeneca vaccine.
"All three patients are clinically stable, have responded well to treatment and are recovering," the statement read.
"Two of the three cases appear to be milder forms of the syndrome that were recognised very early by the treating health professionals and are responding well to treatment."
The TGA said one patient developed symptoms "unusually late" - 26 days after the vaccination.
There have now been six cases of the blood clot syndrome, or thrombosis with thrombocytopenia syndrome, in Australia.
The TGA confirmed last week the death of a NSW woman who developed blood clots was likely linked to the AstraZeneca vaccine she received days earlier.
The TGA said one of the new cases suffered a blood clot in the brain, which caused a headache, nosebleed, nausea and vomiting.
The other two cases involved more common sites for blood clots, one was deep vein thrombosis in the calf and the other deep vein thrombosis in the upper leg.
The TGA said the five people under 50 who have suffered the condition were vaccinated prior to the government changing advice around the AstraZeneca vaccine.
The Prime Minister announced on April 8, under 50s should get the Pfizer vaccine instead, unless the "benefit clearly outweighed the risk".
The TGA said the blood clot syndrome occurred in about 6 cases per one million doses of the vaccine but was estimated to be higher - 20-40 cases per million - in people under 50.
"However, Australian estimated age-specific incidence rates are imprecise due to small numbers and will be updated as further information become available," it said.
The Australian Technical Advisory Group on Immunisation said vaccination remained the best way to protect against COVID-19.
"ATAGI continues to recommend that people who have received a first dose of AstraZeneca vaccine without serious adverse events can be given a second dose."
It said the risk of developing the syndrome after the second dose was considerably lower than the first.
People under 50 are able to get the AstraZeneca vaccine and should consult their doctor about their individual situation.
About 1.1 million doses of AstraZeneca have been administered in Australia.
"People who have received COVID-19 vaccines should be aware of the common side effects, which include fever, sore muscles, tiredness and headache," the TGA said.
"These usually start within 24 hours of vaccination and last for 1-2 days. These side effects are expected and are not of concern unless severe or persistent.
"The reports of these rare clotting complications have occurred later (usually between day 4 and 20 after vaccination) and have generally been severe, requiring hospitalisation."
