Prime Minister Scott Morrison has announced urgent changes to the federal government's rollout of the AstraZeneca vaccine.
The late announcement comes after consultation with the Australian Technical Advisory Group on Immunisation (ATAGI), which recommended people under the age of 50 avoid the AstraZeneca vaccine and instead be given the Pfizer vaccine.
European authorities have identified the link, prompting the United Kingdom to offer people aged under 30 an alternative vaccine due to the risk.
The updated advice is expected to impact Australia's vaccine rollout.
"ATAGI has met to consider the medical evidence regarding unforeseen but yet rare and serious side effects mostly associated with younger people from the AstraZeneca vaccine," Mr Morrison said.
The changes were due to "rare, but serious, blood clots" due to the AstraZeneca vaccine, Professor Paul Kelly said, adding he was at the meeting with ATAGI.
"The use of the Pfizer vaccine is preferred over the AstraZeneca vaccine in adults aged less than 50 years who have not already received a first dose of AstraZeneca vaccine," Prof Kelly said.
"This is based both on the increased risk of complications from COVID-19 with increasing age, and thus increased benefit of the vaccination, and the potentially lower, but not zero risk, of this rare event with increasing age."
Prof Kelly said the adverse reaction to the vaccine is less common in older people.
The PM stressed the urgency for elderly Australians to get vaccinated, which is why it is encouraged they take the AstraZeneca vaccine.
"It's important for them to be vaccinated, because the vaccine protects against very serious illness, and we have seen in the more than 900 deaths that we've had in Australia - they have predominantly been with older Australians," Mr Morrison said.
"So that would mean that the health advice would encourage them taking that vaccine to protect them from what is a global pandemic."
It was reiterated throughout the press conference this was a "precaution" based on the available evidence.
The prime minister said Australia is getting a regular supply of the Pfizer vaccine and that supply will be prioritised for individuals whom it would be more appropriate for.
ATAGI's new AstraZeneca recommendations
The first recommendation was for the Pfizer vaccine to be used for those aged under 50-years-old.
"The second recommendation is that immunisation providers should only give a first dose of AstraZeneca COVID-19 vaccine to adults under 50 years of age where benefit clearly outweighs the risk for that individual's circumstances," Prof Kelly said.
Health care workers aged under 50 who were due to receive the AstraZeneca vaccine will now be prioritised for the Pfizer vaccine, which will likely delay phase 1b of the rollout.
The third being if someone has had their first dose of the AstraZeneca vaccine and did not experience any adverse side effects, they can safely receive their second dose, Prof Kelly said.
The ATAGI's fourth recommendation is that the Department of Health will refine resources that convey the benefits and the risks of the vaccine for consumers of all ages.
More than 996,000 coronavirus vaccine doses have now been administered across Australia.
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