BA.2 explained: the Omicron sub-variant outpacing its predecessor

Scientists who track coronavirus mutations are closely monitoring an uptick in cases of an Omicron sub-variant named BA.2, just as countries were beginning to hope the worst of the latest wave of Covid-19 infections had passed.

The strain has been detected in 57 countries, according to the World Health Organization, and now accounts for about a tenth of Sars-Cov-2 genome sequences uploaded to the global repository Gisaid.

The original version of Omicron, designated as BA.1, is still responsible for most cases. Nevertheless, BA.2 has become the dominant variant in Denmark, India and South Africa.

How do Omicron sub-variants differ from the original version?

Scientists became aware of the offshoots of Omicron less than two weeks after South African health authorities first raised the alarm about the original strain in late November.

There are three officially designated subtypes of Omicron: BA.1, BA.2 and BA.3 – the last of which has been linked to fewer than 400 cases worldwide.

BA.2 possesses up to 27 mutations that are not found in the original version of Omicron, BA.1.

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Separate research from public health bodies in Denmark and the UK looking at household transmission indicate that these mutations make BA.2 between 30 and 34 per cent more infectious than BA.1.

The UK Health Security Agency designated BA.2 a “variant under investigation” in mid-January. The WHO treats all Omicron sub-lineages as one “variant of concern” and they are yet to assign BA.2 its own risk ranking.

Can BA.2 evade the body’s immune defenses from vaccination or prior infection?

The Danish study, published on Monday, found BA.2 “possesses immune-evasive properties” that “further reduce the protective effect of vaccination against infection”, compared with BA.1. But researchers added BA.2 did “not increase” the likelihood of vaccinated people passing on the virus once infected.

However, UKHSA data, released last week, found vaccines may be slightly more effective against the Omicron subtype than the original.

Booster shots were 63 per cent effective against symptomatic infection from BA.1, compared with 70 per cent against BA.2, at least two weeks after the dose was administered. But data on BA.2 are limited so the estimates are less certain.

The latest data suggest that, “at most”, BA.2 was “slightly more immune evasive” than BA.1, said Rafael Bayarri Olmos, an immunologist and researcher at Copenhagen University Hospital’s Laboratory of Molecular Medicine.

Richard Lessells, an infectious diseases physician at the University of KwaZulu-Natal in Durban, predicted BA.2 would not cause “a huge wave of reinfections” in South Africa, the epicenter of the original Omicron outbreak, where BA.2 now accounts for about half of cases.

“We might expect that there would be some reinfections because in many cases the immune response to Omicron may not be a particularly strong one,” said Lessells. “But I think it’s very unlikely that it’s going to break through. . . the immunity from BA.1. It’s definitely not a game-changer. ”

How worried should we be about BA.2?

Ravi Gupta, professor of clinical microbiology at Cambridge university, said despite BA.2’s transmission advantage it “does not warrant” public health measures “over and above” what governments have put in place in response to the original Omicron.

“[BA.2] may be more transmissible by a certain margin but Omicron is already so transmissible; the incremental increase is unlikely to throw us off course, ”said Gupta.

He explained that “not all mutations are equal” and some do little “to change the way the virus behaves”, adding that there were “no signals” that BA.2 was causing more severe illness.

Boris Pavlin, a member of the WHO’s Covid-19 response team, on Tuesday stressed BA.2 did not appear to be changing the picture in hospitals in countries where it is spreading fast.

“Looking at other countries where BA.2 is now overtaking, we’re not seeing any higher bumps in hospitalization than expected,” he said.

How are countries responding to BA.2?

Despite early data suggesting BA.2 is even more transmissible than its sister strain, several countries where it is dominant have pressed ahead with dropping Covid restrictions put in place during the original Omicron outbreak, citing limited hospital pressures.

Denmark, where BA.2 makes up more than 60 per cent of cases, ended all coronavirus restrictions on Tuesday, including dropping the legal requirement to self-isolate.

There are nearly 1,100 Covid patients in hospital in Denmark at the latest count, the highest level recorded during the pandemic, but about a quarter of them are not being treated primarily for the virus, according to the country’s Statens Serum Institut.

In South Africa, the government announced this week that people without symptoms would no longer have to self-isolate after testing positive.

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India’s business capital, Mumbai, also eased some Covid curbs this week despite BA.2 making up the majority of cases nationwide.

“In India, we know what it’s like to have a variant that causes major problems – BA.2 is not that,” said Prof Sunit Singh, head of the molecular biology unit at Banaras Hindu University, referring to the Delta outbreak that ripped through the country last spring.

Meanwhile, in Hong Kong, the Omicron offshoot is testing the city’s zero-covid strategy. A traveler infected with BA.2 in their third week of hotel quarantine sparked an outbreak, which has resulted in hundreds of cases, the highest level recorded throughout the pandemic.

“The equation has changed as what has worked for us in the past might not be as effective as it’s so much more transmissible,” Dr Siddharth Sridhar, a clinical virologist at Hong Kong University, remarking on what BA.2 means for Hong Kong’s zero -Covid strategy.

Is BA.2 likely to become the dominant variant worldwide?

Bayarri Olmos said BA.2’s increased infectiousness meant it would “likely become the dominant strain” in other countries beyond its native Denmark but it could also “fizzle out” elsewhere if it is outcompeted by another variant or comes up against “effective countermeasures”.

However, he predicted countries with high rates of vaccination would avoid “a wave of hospitalizations” regardless of infection levels. “I do not think this is going to prolong the pandemic significantly,” he said.

David Stuart, professor of structural biology at Oxford University, said “it would not be surprising if [BA.2] takes over ”to become the dominant variant in the UK.

Prof Tulio de Oliveira, director of the Center for Epidemic Response and Innovation in South Africa who first alerted the world to Omicron, said focusing too much on infection figures is what “people got really wrong” after the strain first emerged.

“Everybody started freaking out with the number of infections, but our hospitals were empty,” said de Oliveira. “If people start crying wolf for variants, the world will not take it seriously when the real one emerges.”

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