Another year, another variant. Even before the omicron wave is over, the rising number of cases caused by a variant of omicron known as BA.2 is causing concern. Here is what we know about BA.2.
What is BA.2?
It is basically another flavour of omicron that has been around right from the start. The term omicron is used to describe a whole family of variants that appeared suddenly in November 2021. Most omicron cases have been caused by one of these variants known to scientists as BA.1. BA.2 has 32 of the same mutations as BA.1, but it also has 28 that are different. The first BA.2 sample was collected in South Africa on 17 November 2021.
Why are we hearing about it now?
In several countries, including the UK, Germany, India and Denmark, the proportion of cases caused by BA.2 is increasing rapidly. In other words, BA.2 is replacing BA.1, which suggests it is even more transmissible.
Should I be worried about BA.2?
If you are unvaccinated and haven’t been infected by omicron, then yes. If you get infected, even if you don’t become severely ill, you could start a chain of infections that does result in deaths. If you are fully vaccinated and boosted, or have had omicron already, these risks are much lower.
Can I get infected with BA.2 if I have had omicron already?
We don’t yet know, but many researchers expect that if people have recently been infected by BA.1, they are unlikely to get BA.2, especially if they have also been vaccinated. That isn’t to say this won’t happen, but the numbers are expected to be low. “Antibodies elicited by BA.1 will still probably react reasonably well against BA.2, certainly much better than delta antibodies,” says Jesse Bloom at the Fred Hutchinson Cancer Research Center in Seattle, Washington.
What if I have been vaccinated but haven’t had omicron?
The existing vaccines are actually even better at protecting against BA.2 than BA.1. According the UK Health Security Agency, three vaccine doses are 70 per cent effective at preventing symptomatic infections by BA.2 two weeks after the booster, and 63 per effective against BA.1. For people with two vaccines doses, the efficacy after 25 weeks is 13 per cent against BA.2 and 9 per cent against BA.1. These are the combined numbers for all vaccines used in the UK.
Will BA.2 cause yet another wave of cases around the world?
Hopefully not, though it might prolong the current omicron waves in many countries. In South Africa, whose omicron wave is pretty much over already, there is no sign of a resurgence despite a high proportion of cases now being BA.2. “I would be surprised if BA.2 caused major waves in places that have just had BA.1,” says Bloom. However, countries that have largely succeeded in preventing the spread of the SARS-CoV-2 coronavirus so far, such as Japan, might find it even more difficult to suppress BA.2 than BA.1. In other words, a few countries might have BA.2 waves.
Does all this mean that BA.2 is less dangerous than BA.1?
In practice, probably. The factors that determine the threat a virus poses include its inherent severity, how many people it infects and how much immune protection those people have. As far as we know, BA.2 is no more likely to cause severe illness than BA.1, but it may be 50 per cent more transmissible. More transmissible viruses can cause more hospitalisations and deaths by infecting more people, even if they are no more severe. However, in many countries, a large proportion of people now have good immunity from boosters and infections, which should greatly reduce both the number and severity of BA.2 infections.
Is BA.2 a “stealth variant”?
Some people have claimed that BA.2 is a “stealth variant” that isn’t picked up by tests, but, in fact, it is detected by both PCR and lateral flow (rapid) tests like all other variants.
PCR tests for SARS-CoV-2 look for three short sequences specific to the virus, and give a positive result if at least two are detected. One of the sequences is usually part of the spike protein gene, or S gene. BA.1 omicron – like some earlier variants – has mutations that mean the S gene sequence isn’t detected, though the PCR test will still be positive. This is called an S-gene dropout.
Normally, the only way to detect variants is by sequencing, but by looking for S-gene dropouts, many labs could see how BA.1 omicron was taking over from delta just from PCR tests. BA.2 doesn’t cause S-gene dropouts, so it couldn’t be distinguished from delta using PCR – hence the “stealth” misnomer. However, now that delta has largely disappeared in many places, a falling proportion of S-gene dropouts reveals the spread of BA.2, so it is anything but stealthy.
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