Case numbers in Ballarat are the highest they have been since the end of last year with the 3350 postcode recording 190 new cases last week alone - the second highest number of any postcode across the state.

The past two weeks have each seen more than 300 new reported cases but the real figure is likely to be much higher with many people not reporting their positive test or not testing at all.

According to the Department of Health there were 319 reported COVID cases in Ballarat last week, up from 306 the previous week.

And it appears schools are again bearing the brunt.

"Recent case notifications have increased most notably among school-aged children, as has been occurring for other respiratory viruses such as influenza and respiratory syncytial virus (RSV)," the office of the Chief Health Officer reported in a statement. Transmission of the virus is being driven through a combination of waning immunity, and continual evolution of Omicron variants.

Across Victoria there were 10,642 COVID-19 cases reported last week, an increase of 15 per cent on the previous week.

Of concern the increase of hospitalisations, with 431 COVID patients in Victorian hospitals last week, an increase of 23 per cent on the previous week. Of those, 15 are in intensive care with three patients on a ventilator.

Deakin University epidemiologist Professor Catherine Bennett said the number of cases would likely continue to climb over winter, and the next two months were a "critical time" as immunity waned from the previous peak in December/January.

"We're seeing numbers push up again which is what you expect as you get into colder weather," Professor Bennett said.

"We did see a bit of a shift going back to March/April, a shift in the variants when XBB became the dominant family of variants. It didn't really take off, but we did see it push up then and now there's a slightly different mix in the community which can accelerate with immunity waning," she said.

"You might have had an infection of a different variant and if you come across this one it might increase your chance of an infection."

Professor Bennett said many people who had avoided COVID through most of the pandemic had caught it during the December/January wave, meaning it was coming up to six months post-infection and they would have no remaining immunity.

"It's time for people to think ... when they had their last booster dose and for people eligible for a booster, who haven't had COVID in the past six months, to have that dose now.

"The next two months are going to be a critical time and the booster almost halves the chance of having the infection."

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With flu, RSV, COVID and colds circulating at higher levels during winter, Professor Bennett urged people to be cautious around groups of people while indoors.

"It's the time where everybody just needs to be that bit more mindful of respiratory infections," Professor Bennett said.

Of Victorians aged 18 and over, 18 per cent have recorded a vaccination or COVID diagnosis in the past six months. This means 4.2 million Victorians are eligible for a 2023 booster dose.

According to latest data, just 17.4 per cent of Ballarat residents aged 18+ have had a vaccination booster shot in the past six months, although that number does not take into account those who have had COVID in the past six months and are waiting until the six months has passed before getting their next booster.

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