Numbers of cases in the age group have surged over the Christmas-New Year period, with reports of many local infections stemming from trips to the coast over the new year period then transmitted to others on return.

People in their 20s currently account for one in five active cases of the virus across the state, and almost one in four cases recorded since the pandemic began despite making up just over 14 per cent of the population.

Deakin University epidemiologist Professor Catherine Bennett said a combination of work, social and vaccine-timing factors meant 20-somethings were at greater risk of contracting and transmitting COVID-19.

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"They mix more and they do it socially, often in higher risk settings like pubs and nightclubs ... are more likely to be in jobs where they can't work from home, and part of the essential workforce," Professor Bennett said.

And with most not yet eligible for their booster vaccination, which provides protection against the Omicron variant which is now most prevalent in Victoria, it is likely to be at least a month before case numbers start to fall in the vulnerable age group.

"Mixing through work, socially and at home ... that's how the virus moves through the community," Professor Bennett said.

She said people in their 20s who develop symptoms, even if they cannot get a rapid antigen test or PCR to confirm, should assume they have the virus because of its prevalence within their cohort.

Ballarat currently has the second highest number of COVID cases in regional Victoria with 824 active cases, including 263 new cases reported on Tuesday, behind only Greater Geelong with 1268 active cases.

Professor Bennett said people in their 20s were more likely to be in public-facing jobs like hospitality, part of the essential workforce in fields like nursing and distribution centres, in more manual jobs, and more casualised professional jobs putting them at greater risk of being exposed to the virus.

"They are the core group. They have higher infection risk than other parts of the community, and as soon as we have an outbreak they have higher infection rates," she said.

While Omicron is the dominant variant circulating throughout Victoria, Delta is still present and Professor Bennett said those in their 20s were better protected than other age groups against the more severe Delta strain because of their recent vaccinations.

"Because their second dose was more recent, they're better protected from Delta ... but primarily not really protected from Omicron beyond the first few weeks (after a dose). The vaccine works against Omicron if they've had the (third dose) booster and most are not eligible unless they are essential workers."

At the end of January, when the waiting time for a booster reduces to three months after the second-dose, many of those aged under 30 will become eligible.

"That will help them catch up and put tough downward pressure on the transmission risk," she said.

"It reduces your risk of any symptomatic infection by about 50 per cent ... which will translate in to fewer case numbers."

As of Tuesday there were 33,478 active COVID-19 cases in Victorians aged 20 to 29 out of a total of 171,369 across the state and throughout the pandemic there have been 108,650 cases in the age group out of a total 439,865.

From midnight Wednesday, workers of all ages in key sectors who are already required to be fully vaccinated with two doses, ust get their third dose before being allowed to work.

The new pandemic order applies to workers in healthcare, aged care, disability, emergency services, correctional facility, quarantine accommodation and food processing and distribution workers.

Workers eligible for a third dose on or before January 12 have a month to get their third dose, and workers not yet eligible for a third dose must have it within three months and two weeks of the deadline to receive their second mandatory dose.

Professor Bennett said there was plenty that people in their 20s could do to protect themselves and their families against contracting COVID-19, including being aware of the increased risk for this age group.

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"We're not going to change what they do, they just have to be conscious and do it more safely. At this stage, these coming weeks when they haven't had the booster and Omicron is more dominant, that's the real concern," she said.

"When they are mixing they go back to everyone they live with, or visit family and we want them to be aware and really conscious that when you go back home you could be bringing the virus home."

Professor Bennett said those in their 20s should ensure they keep socially distanced when catching up with friends, make sure there is good ventilation and follow other public health measures - and if they develop symptoms do not visit those who might be more vulnerable.

"Whether or not you get a rapid antigen test is not the issue, if you've got symptoms you've probably got the virus at the moment. It's better to pull yourself out of circulation and fully recover before going out to socialise."

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