Queensland could have a COVID-19 outbreak peaking at more than 1800 cases per day if vaccine immunity wanes without new restrictions and booster shots.
Queensland Health commissioned the QIMR-Berghofer Institute to model what would happen if the state reopened at various vaccination levels with only testing, tracing, isolation and quarantine measures in place.
The state will scrap quarantine for fully vaccinated domestic travellers from December 17, or when 80 per cent of eligible Queenslanders are fully vaccinated, whichever comes earlier.
Chief Health Officer Jeannette Young says the worst-case scenario will be 1200 new cases a day and 400 people in ICU by August, but she says hospitals are unlikely to face that.
“I don’t think we will because we’ll put in place those mitigating factors,” she told reporters.
“We will make sure that we put in place the restrictions to limit the spread when we start getting outbreaks, but if we did have worst case we’ve got the capacity to manage it.”
However, QIMR researchers also warns that waning vaccine immunity could pose “a serious threat” and the modelling may be “optimistic”.
The model is based on the assumption that everyone will continue to have the same level of immunity they had on October 10 for the next 10 months, regardless of when they were vaccinated.
“There is considerable uncertainty around waning of vaccine-derived immunity, information on waning is still emerging,” the report said.
“We modelled past vaccinations (up to 10 Oct) as still being at peak immunity on 10 Oct (sic), it is likely that people early in the rollout already have waning immunity.”
If immunity were to start waning, the modelling shows, cases could spike above 1800 cases per day by March-May 2022 without restrictions being imposed and a booster shot program being rolled out.
“Waning immunity may pose a serious threat in the absence of additional control measures or boosters,” the researchers wrote.
It said boosters shots, and jabs for children aged five to 11 when vaccines are approved for them, could help reduce the risk.
“Waning of vaccine-derived immunity may mean the above is optimistic, and boosters and/or vaccinating more of the population may be needed,” the report said.
Health Minister Yvette D’Ath has doused concerns about booster shots, saying there will be enough supply for anyone who needs them.
She said immunocompromised Queenslanders have already started getting their third jab.
The modelling shows that the state’s plan to open without any restrictions other than testing, tracing, isolation and quarantine measures is “likely to trigger an outbreak”.
But the researchers said the pressure on hospitals will be manageable with 80 per cent of the eligible population immune.
“The health system would be strained but not to breaking point,” the report said.
The young and the old are most likely to be infected, the report said, with severe symptoms most likely for the elderly.
Another issue is potential waning compliance, particularly among the fully vaccinated, affecting testing and tracing.
“High vaccination rates may also dramatically reduce the population’s willingness to test and isolate,” the report said.
“Likewise vaccinated people may be less compliant with QR sign in or responding to public notices of exposure sites.”
However, the QIMR-Berghofer report stressed that the modelling was preliminary and ongoing, and would be updated in coming months.
As of Monday, 71.81 per cent of eligible Queenslanders had received one dose of a vaccine and 57.45 per cent were fully vaccinated.