Vaccine hesitancy and variants mean Australia may not reach Covid herd immunity – researchers
Australia is unlikely to achieve herd immunity with current levels of Covid-19 vaccine hesitancy and the higher infectiousness of new variants, modelling from Melbourne medical research laboratory the Burnet Institute has found.
The scenarios modelled by researchers assume a vaccine rollout speed of either 150,000 or 250,000 doses per week, and that on detection of the first case, symptomatic testing increases, isolation of positive cases continues, masks become recommended but not mandatory, and contact tracing continues for up to 250 diagnoses per day.
The projections represent hypothetical near-worst-case scenarios, and model outbreaks that occur once Australia is Covid-free, without lockdowns and other public health measures in place.
The models reveal that public health initiatives will remain vital even in vaccinated populations and that without those measures, “thousands of Victorians would be hospitalised and die if an initially small outbreak was left to spread through the community unchecked,” the research published on Friday says.
Burnet Institute’s head of modelling, Dr Nick Scott, said public health measures, such as lockdowns, social distancing, mask wearing and use of QR codes, would need to continue to prevent a projected 4,800 deaths in Victoria alone within 12 months if the virus were to spread without a public health response beyond vaccination.
“Those who are vaccinated would be protected and may only experience mild or no symptoms,” Scott said. “But among those not vaccinated – possibly up to 30 per cent of the community – we could see a large number of hospitalisations and deaths, as well as many cases of ‘long Covid’.”
The modelling presents scenarios projecting Covid-19 infections, hospitalisations and deaths one year after new infections enter the community, even when people are vaccinated. Parameters around vaccine efficacy, viral infectiousness, vaccine coverage and the speed of the vaccine rollout can be modified under the model.
One scenario created by the researchers assumed a 50% vaccine efficacy in preventing infections and a 93% efficacy at preventing deaths among people who did become infected; a virus which was 1.5 times as infectious as the one in Victoria’s second wave in June to November 2020; and that 80% of people aged above 60, and 70% of people younger than 60, were vaccinated.
“We found that if the virus enters the community when 60 per cent vaccine coverage has been reached and is left unchecked, we could see 4,885 deaths in Victoria within a year if no public health responses are introduced,” Scott said. “If we get peak vaccination coverage up to 95 per cent, the number of deaths reduces to 1346.”
If the virus was more infectious, deaths would remain at very high levels, even if the vaccine was highly efficacious.
A survey conducted earlier in June by the Melbourne Institute found 29% of Australians are either unwilling to be vaccinated (16%) or don’t know whether they want to be vaccinated (13%).
Prof Tony Blakely, an epidemiologist and public health medicine specialist, said it was essential to start examining how to reopen Australia and what measures might be needed once international borders reopen.
The Burnet modelling was an important contribution to the conversation, he said, though he added modelling, including that being developed by his team, was always difficult because the situation kept changing.
“For example, the Burnet modelling is on one hand too optimistic, as variants such as the Delta variant are actually more infectious than their modelling allowed for, and on the other hand is probably a bit pessimistic, as we have learned the vaccine is probably a bit better at preventing transmission of Covid than they allowed for,” Blakely, who also heads the University of Melbourne’s Centre for Epidemiology and Biostatistics population interventions division, said.
“However, what they are trying to do is important as it shows how essential vaccination is, and the message from the Institute that we require higher vaccine coverage and that Australia is unlikely to ever achieve herd immunity are smack on the money.”
Blakely said even if Australia achieved 90% vaccine coverage, “it will be bumpy when we open the borders”.
“There will be morbidity and mortality,” he said. “Yes, there will be less than if we had no vaccine, but what the Australian public really needs to process is that we will never get to herd immunity completely.
“The sheer reality is we can’t stay locked up for the next five years, so we need to go through this process and perhaps understand that at some point once vaccination is higher, we can’t keep resorting to lockdowns and get Covid cases to zero every time there is an outbreak. We will have to let Covid wash through the community, so we must have a discussion about what the health system is able to manage to allow that.”