With Victoria continuing to break daily new case records and the odds of infection correspondingly rising with every passing day, how should Omicron affect your planned New Year's Eve celebrations?

The question is one of risk versus reward, the weighing of which will, at minimum, vary according to your vaccination status and relative vulnerability, including age.

First, how is the Omicron variant different?

Though infectious disease experts haven't yet grasped the full complexity of Omicron, some of its defining characteristics appear to be settled.

To begin with, scientists agree Omicron is the most contagious variant yet.

Monash University epidemiologist James Trauer said Omicron appeared to be at least twice as transmissible as Delta, which itself was more than twice as infectious as the original strain.

"It seems clear [Omicron] has a very big transmission advantage over Delta, which was very transmissible in the first place," Associate Professor Trauer said. "So, there's something about it which allows it to transmit much more rapidly."

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The new variant also has a lot of mutations - some of which, recent research suggests, enables it to penetrate some of the immune defences afforded by both vaccines and natural immunity. This, Professor Trauer said, enables Omicron to "transmit in vaccinated populations you'd otherwise expect to be immune to covid".

On the other hand, the weight of evidence to date suggests Omicron might be less severe for the average individual than previous strains. But experts have cautioned against being blindsided by this.

The very transmissibility of Omicron, they say, means even just twice as many cases as Delta will result in the same number of hospitalisations and deaths, if not more.

So, am I protected if I've been vaccinated?

The short answer is not as well as you might think. Current research suggests the combined protection provided by two vaccine doses is markedly reduced in a person infected with Omicron.

In people who have received an mRNA vaccine - Pfizer or Moderna - protection against symptomatic disease was shown to wane to 35 per cent in less than four months. In people vaccinated with AstraZeneca, protection against Omicron drops to around zero in the same period, though it may still provide some protection against serious illness and death.

Burnet Institute Mike Toole said this meant people who'd only been vaccinated with AstraZeneca - particularly older people - were, at present, little more than "sitting ducks".

"What worries me and many others is there is a large number of older people vaccinated with AstraZeneca more than four months ago who are not yet eligible for their booster," Professor Toole said. "We know from UK data they have no protection against Omicron".

There's also no data to suggest Omicron does not, or will not, lead to Long Covid, even in the mildest of cases.

How does a booster vaccine alter the balance of risk?

The good news is a mRNA booster shot restores protection against symptomatic disease to around 75 per cent.

That said, Professor Trauer said it was important to factor in the time it takes for the booster shot to take effect when gauging whether to attend any New Year's gatherings.

"You have to remember that it takes at least a week after getting your booster before your immunity develops," he said.

"For Ballarat, I think elderly people and vulnerable people [who have only recently received their booster] should make plans to really limit their association with others as we head towards the peak of the Omicron wave.

"And for [everyone], a third dose is clearly really important to picking your immunity back up."

I'm going out. What precautions should I take?

Experts suggest social distancing, choosing smaller gatherings over larger ones, outdoors over indoors, wearing a well-fitted mask and rapid antigen testing would all help minimise the risk of transmission.

Weighing self-interest against the risk to others

For those who decide to ring in 2022 because they've weighed their personal risk as low, the risk New Year's Eve will be a mass superspreader event nonetheless remains.

At best, Omicron infection would likely only result in mild to moderate disease in the NYE reveller, and only attract a short period of mandatory isolation.

At worst, Omicron infection could, at an individual level, result in the reveller's hospitalisation, death or Long Covid.

But even if it none of those possibilities transpired, Professor Trauer said, the infection could still prove fatal to any person to whom the virus is unwittingly carried.

"Because Omicron is milder, there will be a greater proportion of cases that are asymptomatic," he said.

"It'd therefore be very sensible if you're a younger person that you ensure you limit your association with elderly or vulnerable relatives if you decide to go out."

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