Is Omicron less deadly? Don't bet on it, says science

The official numbers, however, undoubtedly underestimate the true scale of infection, given the inability of testing sites to cope with the increased demand and the community's rising dependence on rapid antigen tests.

Though Australian health authorities insist Omicron is less severe than earlier strains, the most recent research has cast doubt on that assumption.

Here's everything you need to know about this latest iteration of COVID-19 and the threat it poses to you, your loved ones and the health system.

First identified in South Africa and Botswana in late November, the Omicron variant carries more than 50 mutations, with over 35 detected in the virus's spike or surface protein alone.

From the outset, global health authorities, including the World Health Organisation, were alarmed at the extent of spike mutation, because it suggested the variant would have little difficulty side-stepping the potency of existing vaccines.

Yes, a mere whiff of infected breath is enough to cause infection. Research suggests the variant is between two to four times as transmissible as Delta, which itself was more than twice as contagious as the original Covid-19 strain. On these numbers, Omicron infections tend to double every two to four days - figures backed by the soaring rates of infection witnessed in Australia alone.

In Victoria, one in five people who receive a PCR test currently returns a positive result, pointing to a tenfold increase in the state's positivity rate in a matter of mere weeks. Meanwhile, health authorities have confirmed that over three quarters of covid infections in the state were now Omicron.

To be clear, there's no scientific basis to the view you can only become infected with Omicron if you've been in the presence of a positive case for over four hours.

No, not really. All research to date shows two jabs offers little to no protection against Omicron infection three to five months after the second dose.

According to the latest data released by the United Kingdom Health Security Agency, the combined protection provided by two doses of AstraZeneca against symptomatic infection is close to non-existent, while the protection afforded by mRNA vaccines - such as Pfizer and Moderna - falls to 10 per cent fewer than five months on from the second dose.

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That said, both vaccines are still thought to provide at least some protection against hospitalisation or death, though nowhere near to the same level as against Delta.

The reason Omicron can easily by-pass the immune defences provided by vaccines owes the extent of its spike mutation, which enable the virus to neutralise or by-pass the protective antibodies released by the vaccines.

Monash University epidemiologist James Trauer said while data on Omicron was still new and emerging, there was no doubt the variant "is less affected by the past immunity generated by vaccines".

"We know Omicron can transmit very easily through vaccinated populations," Associate Professor Trauer said.

"With a lot of people only covered with just two doses of AstraZeneca [in Australia], we know that won't be enough to protect against a dangerous surge in transmission."

Happily, the chances of breakthrough infection are significantly reduced once a person has received an mRNA booster shot. A number of studies have found a booster shot restores protection against symptomatic disease to about 75 per cent two to four weeks after the jab, though this appears to fall to between 40 to 50 per cent some six to eight weeks later.

University of Melbourne epidemiologist Nancy Baxter said one of central challenges of the current outbreak in Australia was the relatively low proportion of people who'd received a booster shot.

"It's much, much safer to catch Omicron after you have received your booster," Professor Baxter said.

"And similar considerations apply to children [aged 5-11 years], who aren't yet vaccinated."

"The government decided not to follow extensive US evidence, which showed vaccination in children to be safe, and we know that wasn't a wise decision at the end of the day because now children are unprotected."

The short answer is that it depends. Certainly, the weight of evidence from the United Kingdom and South Africa appears to support early indications Omicron might be less virulent.

But Professor Trauer, who heads the epidemiological modelling unit at Monash University's school of public health, said it couldn't safely be assumed the same results would be replicated in the Omicron wave sweeping Australia.

"We have to be very cautious about importing international data into the Australian setting," Professor Trauer said.

"We've got very high vaccination coverage through our adult population, which is very different from the UK and South Africa, but we also have much lower levels of natural immunity because we've had far less transmission here.

"Very high proportions of people infected with Omicron in the UK and South Africa have already been exposed to covid, and so have natural immunity."

At least two recent studies have confirmed high rates of previous covid infection produce a natural immunity in people equivalent or superior to the protection afforded by a booster shot.

In other words, higher rates of previous infection clearly result in lower numbers of hospitalisations and deaths, which is why it can't be assumed Omicron will prove less deadly or dangerous for the Australian population.

Even if Omicron did prove to be less virulent than previous strains, experts say that will not necessarily avert a health crisis.

Professor Trauer said the chances of the hospital system being overwhelmed remained due to highly contagious nature of the variant.

"You can have a much more severe epidemic with a strain that is less severe on average for the individual and that's very much what we're looking at," he said.

Similarly, Professor Baxter said she was convinced the standard of care provided by hospitals would markedly fall under the weight of sky-rocketing infections.

"I don't doubt that throughout Australia we will see those tents erected outside of hospitals and it will become much more challenging to be seen by anyone in the emergency department for any reason," she said.

"Will we get to the point of total collapse? I hope not."