Cases are falling in Victoria. What happens next?
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Victoria’s COVID-19 outbreak has reached an inflection point, with government modelling suggesting the state’s cases should now start to track up, while other experts point to NSW’s declining caseload as evidence something unexpected is going on.
Burnet modelling, commissioned by the state government and released on October 17, predicts a dip in new cases between mid-October and mid-November, followed by a steady increase in new cases and a peak mid-December.
Victoria has largely followed those forecasts so far, the state’s five-day average of new cases dipping from 2002 on October 18 to 1158 by November 3.
Ending lockdown should give the virus the conditions it needs to spread more rapidly. But NSW has seen the opposite happen: cases have continued to fall since it reopened on October 11, the five-day average dropping from 551 to 189 on November 4.
“NSW seems to be virtually at herd immunity currently, which most modellers weren’t expecting,” said Professor James Trauer, head of epidemiological modelling at Monash University’s School of Public Health.
Professor Trauer, along with several other experts, said NSW and Victoria may be enjoying an unexpected bonus boost from the vaccines right now.
Evidence suggests vaccines from AstraZeneca and Pfizer are much more effective within one to two months of receiving the second dose.
“We’re at the stage where we’re nearing the optimum vaccine-induced protection,” said Professor Greg Dore, a leading epidemiologist at the Kirby Institute and one of the few to predict NSW cases would fall after lockdown ended. “The greatest impact is in the first few months.
“That has enabled us to continue to have declining case numbers despite opening up.”
The Burnet Institute’s modelling assumes Pfizer’s vaccine effectiveness against infection is 80 per cent.
However, Pfizer’s effectiveness changes over time, evidence suggests. Within a month of full vaccination, it is around 88 per cent effective at preventing infection – and more effective still for younger age groups, who do most of the transmission of the virus. It is only after three months that vaccine effectiveness falls to 80 per cent.
AstraZeneca’s vaccine also offers the strongest protection in the month after being vaccinated.
Some 56 per cent of people in Victoria, and 54 per cent in NSW, who are fully vaccinated had their second jab within the last two months, meaning they are enjoying boosted protection.
“We are in the sweet spot at the moment,” said University of Melbourne epidemiologist Professor Tony Blakely.
In addition, because the vaccine rollout initially prioritised the elderly, people who were fully-vaccinated in the last couple of months are more likely to be young. Adults aged 20 to 49 are much more likely to spread the virus than older groups and children; boosting their protection may have an even greater effect on virus transmission, said Professor Dore.
Associate Professor James Wood, a vaccine epidemiologist at the University of NSW, said he expected cases to continue to fall in NSW.
“I think it’s fairly clear that there is waning of immunity over time and your antibody levels will be highest in the month or so after that second dose,” he said. “The evidence is still evolving as to exactly what effect that has on risk of infection or onwards transmission.”
Burnet Institute head of modelling Dr Nick Scott said we still did not know if vaccination produced a “spike in protection” immediately after the second jab.
“Right now is one of the most uncertain times – models show that even by varying input parameters within their confidence intervals, model scenarios can have very different outcomes,” he said.
After five months, Pfizer’s effectiveness at preventing infections from Delta falls to 47 per cent, according to a large study published in the Lancet in October. AstraZeneca declines by about 7 per cent per month.
“It’s unlikely that numbers will continue to decline. There is likely to be some surge of cases at some stage, over the coming months,” said Professor Dore.
“However, even with a surge of cases in NSW, back over 1000 cases a day, the burden on the hospital system should be less than the previous Delta wave.”
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