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Australia is heading for its third Omicron wave

Australia is heading for its third Omicron wave in the coming weeks, as BA.4 and BA.5 become the dominant COVID strains. BA.4 and BA.5 are more infectious than previous COVID variants and subvariants, and are better able to evade immunity from vaccines and previous infections, so we’re likely to see a rise in case numbers.

So what are BA.4 and BA.5? And what can we expect in this next phase of the pandemic?

How did it start? BA.1, BA.2 and BA.3

Omicron started off as three subvariants (that is, a group of viruses from the same parent virus), all appearing in late November 2021 in South Africa: BA.1, BA.2, and BA.3.

The three are genetically different enough that they could have had their own Greek names. But for some reason, this did not happen, and the World Health Organization designated them as subvariants of Omicron.

BA.1 rapidly took over from Delta in Australia in early January this year, forming a massive wave of cases, peaking at more than 100,000 a day.

However, BA.2 is even more transmissible than BA.1, and Australia saw a second wave of cases, this time caused by BA.2. This wave peaked in early April at more than 60,000 cases a day.

The first and second Omicron waves peaked in early January and early April.

When were BA.4 and BA.5 detected?

BA.4 was first detected in January 2022 in South Africa. BA.5 was also detected in South Africa, in February 2022. Both appear to be offshoots of BA.2, sharing many identical mutations. They also have many additional mutations likely to impact transmission. They are talked about together because mutations in their spike protein (the bit that latches on to human cells) are identical. (For brevity, I refer to them as BA.4/5.) However, they do differ in some of the mutations on the body of the virus.

How transmissible are BA.4/5?

We measure how contagious a disease is by the basic reproduction number (R0). This is the average number of people an initial case infects in a population with no immunity (from vaccines or previous infection).

New mutations give the virus an advantage if they can increase transmissibility:

  • the original Wuhan strain has an R0 of 3.3
  • Delta has an R0 of 5.1
  • Omicron BA.1 has an R0 of 9.5
  • BA.2, which is the dominant subvariant in Australia at the moment, is 1.4 times more transmissible than BA.1, and so has an R0 of about 13.3
  • a pre-print publication from South Africa suggests BA.4/5 has a growth advantage over BA.2 similar to the growth advantage of BA.2 over BA.1. That would give it an R0 of 18.6.

This is similar to measles, which was until now was our most infectious viral disease.

How likely is reinfection?

BA.4/BA.5 appear to be masters at evading immunity. This increases the chance of reinfection. Reinfection is defined as a new infection at least 12 weeks after the first. This gap is in place because many infected people still shed virus particles many weeks after recovery. However, some unfortunate people get a new infection within the 12 weeks, and therefore are not counted. Likely, there are now tens of thousands of Australians into their second or third infections, and this number will only get bigger with BA.4/5.

How high are case numbers likely to rise?

Around Australia, we are starting to see a third wave of cases because of BA.4/5.

The effective reproduction number, or Reff tells us, on average, how many people an infected person will pass it on to, given the immunity in the population. All Australian states and territories now have a Reff greater than 1, meaning that even with the current levels of immunity, we are seeing an exponential growth in case numbers. This will inevitably lead to an increase in hospitalisation and deaths.

The second Omicron wave due to BA.2 was not as high as the first one caused by BA.1, probably because there were so many people infected with BA.1, that the ensuing immunity dampened the second wave down.

This third wave may not be as high as the second for the same reason.

How severe is the disease from BA.4/5?

A recent pre-print publication (a publication that has so far not been peer-reviewed) from a Japanese research group found that in lab-based, cell-culture experiments, BA.4/5 was able to replicate more efficiently in the lungs than BA.2. In hamster experiments, it developed into more serious illness.

However, data from South Africa and the United Kingdom found that their BA.4/5 wave didn’t see a major increase in severe disease and death.

This is possibly because of the high rates of immunity due to previous infections. Our high rates of vaccine-induced immunity might have a similar protective effect here.

At this stage, we do not know whether any of the Omicron subvariants differ in their ability to cause long COVID.

However, we do know that full vaccination (three doses for most people) does provide some protection against long COVID.

How protective are our vaccines against BA.4/5?

Each new subvariant of Omicron has been better able to evade immunity from vaccination than its predecessor.

Although current vaccines based on the Wuhan strain will still provide some protection against serious illness and death against BA.4/5, they are unlikely to provide much, if any, protection against infection or symptomatic disease.

Nurse prepares COVID vaccine
Existing vaccines are unlikely to provide much protection against infection. Shutterstock

What about new vaccines?

The good news is second-generation vaccines are in clinical trials. Moderna is trialling a vaccine containing mRNA against the original Wuhan strain and Omicron BA.1. Early results are very promising, and likely to give much better protection against BA.4/5. But this third Omicron wave – along with a very severe flu season – will likely see our hospitals struggling even more over the next few weeks.

If things get bad enough, state and territory governments might be forced to reintroduce face mask mandates in many settings – in my opinion, not such a bad thing.The Conversation


Adrian Esterman, Professor of Biostatistics and Epidemiology, University of South Australia. This article is republished from The Conversation under a Creative Commons license.


July 21, 2022

There are so many selfish "Chris's" out there. Thankfully, there are thoughtful intelligent people who have responded to him and also many more that haven't. All mRNA viruses can cause DNA damage in the infected host, irrespective of the severity of symptoms during the infection itself. The logical offshoot of this is that this may manifest in coming years as bizarre and possibly fatal cancer(s). The cost to families and the health system has the potential to send this country into a third world spiral. Now, if you think this is hyperbole, good for you. But for goodness sake, for those who value their "personal freedoms" above community safety, just keep your rotten virus to yourself!

July 16, 2022

I often think Epidemiologists are missing so badly their day in the sun, when everyone wanted their opinion and they were so valued.
Since when do we run the country to suit hospital emergency departments ?
I know a number of covid hysterics, petrified of it, ruining their lives hermit like whilst flu, a nasty one this year too, doesn't seem to be a worry, and they aren't getting the flu vac.
Wear your mask if you want to but don't prescribe to me what I should do. the country is in my opinion totally sick of being dictated to.

July 16, 2022

A very selfish attitude . When you carry a virus it's not just about you, but about all the other people you give it to who might have to stop working or going to school or end up in hospital or even die. Contagious disease is not an individual matter !! Don't complain if you have to attend hospital for a noncovid matter but have to wait 10 hrs to see a doctor . That's what's happening.

July 16, 2022

Agree with C. After seeing lines of ambulances in street outside a hospital this w/end waiting hours to drop patients at A & E for whatever ailment, I hope Chris doesn't break a leg & wait in agony for an ambulance if it is stuck in that queue or perhaps no bed for him, even worse - a heart attack or stroke when a paramedic may keep him alive to actually reach a hospital. Luckily most of us help by wearing masks, such a minor consideration for greater good plus taking responsibility for one's health & of our country's financial health/recovery if we continue covid payments to those with no sick pay. I'd wager those poor devils won't agree with Chris either.

July 14, 2022

hospitals will struggle in coming weeks ??
boy, you are sadly behind the times.
have you visited a hospital Emergency department in the past few months ??

July 13, 2022

I can't understand why so many people don't want to wear masks. They say we should 'live with the virus', well, pretty soon we are going to die with it too.

July 13, 2022

It seems a good idea to wear a face mask anywhere we come across other people, especially indoors. It doesn't seem such a high price to pay to wear a face mask (especially in winter, more uncomfortable in summer).


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