(NEXSTAR) — Six vaccinated people at a recent party in a suburb of Sydney, Australia were the only ones who didn’t test positive for COVID-19, according to health officials.

The superspreader event is a cautionary tale, health officials in the state of New South Wales say, and proof that the country won’t be able to leave the pandemic behind until its single-digit vaccination rate increases.

The Australian Broadcasting Corporation reports that all of the people who tested positive after attending the birthday celebration in West Hoxton were infected with the delta strain of the coronavirus.

“To emphasize just how important vaccinations are … not one of those 24 people were vaccinated,” NSW Health Minister Brad Hazzard told reporters. “I can also advise that six health workers who attended the party, who were fully vaccinated, not one of those people has been infected.”

Hazzard added that one of the care workers was elderly and, despite only having received one dose by the time of the party, did not contract COVID-19.

Hazzard said Monday that the situation was still “unfolding,” but that the “early and strong indication from that party are if you’re vaccinated you are much more likely to not be infected with COVID-19.”

Chief Health Officer Dr. Kerry Chant told 7 News that an asymptomatic guest unknowingly spread the virus throughout the party. Had the six guests not been vaccinated, it’s possible that the infection rate at the party could have been 100%.

Chant said the infected partygoers are now isolating and have been tested.

“We got to this group very quickly,” Chant said adding that they likely had only 24 hours during which the could have spread the virus in the community.

Australia has done an effective job of tracing and containing COVID-19 cases during the pandemic compared to other countries – there have been fewer than 31,000 cases and 910 deaths, according to the Associated Press.

The country’s low vaccination rate, combined with delta strain case studies such as the West Hoxton birthday party, are now causing alarm, however.

Prime Minister Scott Morrison has been under fire since he declared in March that the vaccine rollout is “not a race.” Australia lags far behind dozens of other countries with a vaccination rate of roughly 5%.

In a bid to jumpstart vaccination numbers, the government agreed Monday to offer the AstraZeneca vaccine, which has been tied to two at least two deaths in Australia from a rare blood clot complication – one more death than the number of fatalities from the virus itself this year.

As part of the rollout, the government will indemnify doctors who give the AstraZeneca vaccine.