Almost all of the guests at a birthday party near Sydney later tested positive for the coronavirus.
The six fully vaccinated guests remained healthy, a health official said.
Only 5% of Australians are fully vaccinated, leaving the population vulnerable to the Delta variant.
A birthday party in Australia has been called a "superspreader" event after 24 guests tested positive for the coronavirus.
The highly infectious Delta variant could have infected every guest at the party had there not been six fully vaccinated healthcare workers in attendance, New South Wales Health Minister Brad Hazzard said on Monday.
Hazzard told reporters that all 24 guests who tested positive were unvaccinated, demonstrating "just how important vaccinations are," per the Australian Broadcasting Corporation.
He also said an attendee who had received only a single dose of the vaccine did not get infected.
The Delta variant is driving Australia's recent outbreaks
The birthday party took place in the West Hoxton area of Sydney on June 19. Another seven cases have been linked to the party since household contacts were exposed to the infected guests.
The 31 cases linked to the party are just one cluster in the outbreak affecting Sydney, with 170 active cases recorded in New South Wales. The state is under a stay-at-home order in an attempt to curb the spread.
Australia was among the best in the world at controlling its coronavirus outbreak in the first year of the pandemic. But the country's relatively low case and death counts meant it was not as quick to procure COVID-19 vaccines.
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