Another resident at Edmonton Meadows Care Home has been hospitalised after testing positive for Covid-19, management at the West Auckland facility has confirmed.
Fifteen residents and four staff have tested positive since Friday, when the first case - a fully vaccinated staff member - was discovered.
Four residents are now receiving care at North Shore Hospital, up one from the Ministry of Health's update yesterday afternoon.
"Edmonton Meadows acknowledges this is an extremely stressful time for residents, their families and our staff," the facility's management team said in a statement to the Herald.
"All our staff and the majority of our residents are fully vaccinated.
"We are having regular meetings with our GP, the Public Health Unit and the DHB, and all are supporting us to manage this difficult situation."
The Ministry of Health has said it has not yet traced the origin of the cluster.
The home's 52 residents and 40 staff have been tested, health officials said last weekend. A mobile testing unit was taken to the Henderson facility this week for repeat tests.
"Aged residential care facilities are well versed in infection prevention and control measures and have processes in place to care for Covid-19 positive residents while ensuring the safety of other residents and staff," a Ministry of Health spokesperson previously said of the facility.
Edmonton Meadows appears to be the first cluster at a care home amid the current Delta outbreak, which began in August.
Six rest homes had coronavirus cases in 2020 and the majority of them did not result in deadly clusters. But four people died after infections spread at CHT St Margaret's residential aged-care home in Auckland's Te Atatū and Rosewood rest home in Christchurch had 12 deaths - making it the nation's most deadly Covid cluster to date.
The two facilities accounted for 16 of the nation's 22 Covid-linked deaths at the time.
New Zealand isn't the same as it was a year ago, in the "pre-vaccine era", said Otago University epidemiology professor Michael Baker.
The latest outbreak is concerning, he said, and it could easily lead to more deaths. But high vaccination rates among New Zealand's older demographic this year have made rest homes more resilient to the virus, he said.
"Obviously, it's not what you want because even highly vaccinated older people are more likely to get the virus and are more likely to get ill and die from it," he said. "That is a concern."
But, he added, "it's nothing like a year ago". Vaccinated residents have a "vastly better chance of surviving it", he said.
Even as New Zealand continues its push towards 90 per cent vaccination rates, outbreaks will continue to occur at such facilities as we learn to live with the virus, University of Melbourne epidemiology professor Tony Blakely predicts.
"Even if all residents and staff are vaccinated, infection may still occasionally get in as Pfizer is only 80 per cent effective at stopping one get infected, and this wanes to about 50 per cent effectiveness at six months for elderly," he said. "But protection against serious illness and hospitalisations should remain high at something like 90 per cent."
Requiring masks for guests will be among the important preventive measures rest homes will have to continue into the future to reduce the vulnerability of their residents, he said. It will also be important, Blakely said, that staff and residents receive Covid-19 booster jabs at six months.