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 variants and subvariants of COVID, and are more capable of evading vaccine immunity and previous infections. Therefore, we are likely to see an increase in the number of cases.
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 as three subvariants (i.e., 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, that didn’t happen, and the World Health Organization designated them as Omicron subvariants.
BA.1 quickly took over from Delta in Australia in early January this year, forming a massive wave of cases, with a maximum of 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 with more than 60,000 cases a day.
Omicron’s first and second waves peaked in early January and early April. (Source: Covid19data.com.au)
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 branches of BA.2, which share many identical mutations. They also have many additional mutations that can affect transmission.
They talk about it together because the mutations in their spike protein (the piece that sticks to human cells) are identical. (For brevity, I refer to them as BA.4 / 5.)
However, they do differ in some of the mutations in the body of the virus.
How transmissible are BA.4 / 5?
We measure how contagious a disease is by the basic number of reproduction (R0). This is the average number of people who infect an initial case in a population without immunity (from vaccines or previous infection).
New mutations give the virus an advantage if they can increase transmissibility:
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the original Wuhan strain has an R0 of 3.3
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Delta has an R0 of 5.1
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Omicron BA.1 has an R0 of 9.5
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BA.2, which is currently the dominant subvariant in Australia, is 1.4 times more transmissible than BA.1 and therefore has an R0 of approximately 13.3
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a pre-print publication from South Africa suggests that BA.4 / 5 has a growth advantage over BA.2 similar to BA.2’s growth advantage over BA.1. This would give you an R0 of 18.6.
This is similar to measles, which until now was our most infectious viral disease.
Read more about the spread of COVID-19:
What is the probability of reinfection?
BA.4 / BA.5 seem 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 have a new infection in 12 weeks and therefore do not count.
There are probably tens of thousands of Australians now in their second or third infection, and that number will only increase with BA.4 / 5.
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What is the probability of increasing the number of cases?
Around Australia, we are starting to see a third wave of cases due to BA.4 / 5.
The number of effective reproductions — or Reff — tells us, on average, how many people will be transmitted by an infected person, given the immunity of the population.
All Australian states and territories now have more than one ref, which means that even with current levels of immunity, we are seeing exponential growth in the number of cases. This will inevitably lead to an increase in hospitalizations and deaths.
The second wave of Omicron due to BA.2 was not as high as the first caused by BA.1, probably because there were so many people infected with BA.1 that the consequent immunity decreased the second wave.
This third wave may not be as high as the second for the same reason.
What is the severity of BA.4 / 5 disease?
A recent pre-print publication (a publication that has so far not been peer-reviewed) from a Japanese research group found that in laboratory-based cell culture experiments, BA.4 / 5 was able to replicate -is more efficiently in the lungs. that BA.2. In experiments with hamsters, it became a more serious disease.
However, data from South Africa and the UK found that their BA.4 / 5 wave did not see a significant increase in serious illness and death.
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This is possibly due to high rates of immunity due to previous infections.
Our high vaccine-induced immunity rates could have a similar protective effect here.
Will BA.4 / 5 change the long COVID?
At this stage, we do not know if any of Omicron’s subvariants differ in their ability to cause long COVID.
However, we know that full vaccination (three doses for most people) provides some protection against long-term COVID-19.
What protection are our vaccines against BA.4 / 5?
Each new Omicron subvariant has been more able to evade vaccination immunity than its predecessor.
Although current Wuhan strain-based vaccines will still provide some protection against serious illness and death from BA.4 / 5, they are unlikely to provide much, if any, protection against infection or symptomatic disease.
Existing vaccines are unlikely to provide much protection against infection (Reuters: David W Cerny)
What about the new vaccines?
The good news is that second-generation vaccines are in clinical trials. Modern is testing a vaccine containing mRNA against the original Wuhan and Omicron BA.1 strain.
The first results are very promising, and will probably offer much better protection against BA.4 / 5.
But this third wave of Omicron, along with a very severe flu season, will likely see our hospitals struggle even harder over the next few weeks.
If things go wrong enough, state and territorial governments could be forced to reintroduce facial mask mandates in many settings, in my opinion, it’s not that bad.
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Adrian Esterman is Professor of Biostatistics and Epidemiology at the University of South Australia. This piece first appeared in The Conversation.