Experts attribute Victoria’s COVID-19 rate rise to change in weather, waning immunity

As Victorians emerged from two years largely defined by lockdowns, the start of 2022 was marked by the sharp rise and fall of cases as a wave of Omicron swept through.

That wave, which peaked in early January with tens of thousands of cases per day, subsided somewhat, but epidemiologists warned there would be more to come, particularly as the winter months approached.

This week, the number of new daily cases in Victoria rose above 10,000 for multiple consecutive days and reached numbers not seen since early February, with 14,333 new infections reported on Thursday.

Experts are still watching the data carefully as it emerges, but a definite “resurgence” in cases has been noted.

Cold weather, waning immunity likely behind change

University of Melbourne epidemiologist Nancy Baxter said it appeared multiple factors converging had resulted in the uptick of cases.

She said the relaxing of restrictions, including no longer requiring household contacts to stay at home was playing a role in higher transmission, but no one factor was solely to blame.

“The weather is getting colder, so people are socializing more indoors,” she said.

Nancy Baxter, head of the Melbourne School of Population and Global Health.(Supplied)

Professor Baxter said waning immunity was another key factor.

“You do have some immunity after having a COVID infection … usually people are protected pretty well for 60 days and have some protection for three months,” she said.

“We’re well past the three months for the people that got Omicron over the Christmas period, so those people are now at risk again.

“And people that got boosted, they had protection from transmission close to that Christmas time, but the protection from transmission does wane or decrease faster than the protection from serious illness or death.”

Professor Baxter said the past week had demonstrated the effect of those changes.

“We’d reached a plateau where we hadn’t gone down because we continued to relax our restrictions and protections,” she said.

New sub-variants not yet playing a role

Earlier this month, the Victorian government confirmed Omicron sub-variants, known as BA.2.12.1, BA.4 and BA.5, had been detected in Melbourne’s wastewater.

In total, wastewater detections have picked up the BA.4 and BA.5 sub-variants 15 times, and the BA.2.12.1 sub-variant has been detected five times.

Monash University’s head of epidemiological modeling, James Trauer, said tracking those sub-variants was important because they appear to be more transmissible than the current dominant sub-variants.

But he said it was “too early” for them to be leading to an increase in cases.

Jame Trauer sits on a chair in front of his computer.
Monash University’s James Trauer says simultaneous flu and COVID-19 waves should be avoided.(Supplied: James Trauer)

“There does seem to be a bit of a resurgence,” he said.

“We do need to look for things that could explain this change.”

He agreed with Professor Baxter that the change in weather, with people spending more time indoors, was likely an important factor behind the increase.

Just a week after Victoria marked the grim milestone of more than 3000 deaths from COVID throughout the pandemic, Professor Baxter warned more hospitalisations and deaths would inevitably follow the rise in cases.

“It’s not going to be tomorrow,” she said.

“We know that these things take seven to 10 days to become apparent in the hospital system, and even longer than that to become apparent in the number of people dying of COVID.”

She said there wasn’t an “appetite” to bring back restrictions like indoor mask-wearing, but encouraged people to take steps to protect themselves by wearing “high quality, well-ventilating” masks.

A sign reads 'You must wear a face mask while traveling' on the inside of a tram.
People are being encouraged to protect themselves in settings like public transport as cases rise again.(ABC News: Kyle Harley)

COVID-19 booster shots, flu vaccine key

Professor Baxter also said getting the 30 per cent of people yet to receive booster shots in Victoria jabbed would help mitigate the effects of the increase.

Despite 94.6 per cent of Victorians aged 12 and over having received two doses of a COVID vaccine, just 66.8 per cent of those aged 16 and over having received three doses – a figure which has barely budged for weeks.

She said the healthcare system had already been under strain for some time.

“It’s not ending, you have these new surges of COVID on top of a flu season and it’s just going to get to the point where the hospital system is at real risk,” Professor Baxter said.

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Dr Trauer said a renewed push to encourage eligible people to get COVID-19 booster vaccines would help curb the effects of the current uptick in cases.

“We’re still getting a substantial number of deaths and there’s a substantial strain on the health system as well.

“The best way to prevent that is to vaccinate people at high risk to prevent those severe outcomes after they’ve been infected.”

Federal government data from May 10 showed that 90.7 per cent of the 175,640 Australians in aged care had received three or more COVID vaccine doses.

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