CNA Explains: What we know about the new COVID-19 variant BA.2.75 or 'Centaurus'
Is the BA.2.75 subvariant more transmissible and does the COVID-19 vaccine protect against it? CNA looks into the new variant and the concerns surrounding it.
SINGAPORE: As Singapore grapples with its latest wave of COVID-19 infections due to the BA.4 and BA.5 Omicron variants, an even newer mutant could be on the horizon – BA.2.75.
First detected in India in May, the subvariant has since spread to countries in Asia, Europe, North America and Australia. Singapore reported its first two imported cases of the variant last Thursday (Jul 14).
In a tweet last month, a virologist at Imperial College London said the subvariant was worth keeping a “close eye” on, due to its number of spike mutations, apparent rapid growth and wide geographical spread.
But while the variant appears to be spreading quickly, scientists have said it is still too soon to know for certain if BA.2.75 is really better at evading immunity from vaccines and prior infection.
Here’s what we know about the new subvariant so far:
What is BA.2.75 or “Centaurus”?
BA.2.75 is a sub-lineage strain of Omicron subvariant BA.2, which was responsible for triggering outbreaks in places including the US and Hong Kong earlier this year.
If it sounds complicated, think of a tree.
COVID-19 is the trunk of the tree and one of the branches is Omicron. From the Omicron branch, there are a few branches including the BA.2 subvariant. BA.2.75 branches out from BA.2.
The subvariant has been nicknamed “Centaurus” but this is not its official name. It is believed to have stemmed from a Twitter user, who often uses his account to share news about COVID-19.
The nickname is a reference to a galaxy which is also the name of the father of centaurs in Greek mythology.
The WHO has not designated BA.2.75 its own name, similar to its treatment of other subvariants such as BA.4 and BA.5.
Despite this, the nickname appears to be sticking, with media outlets around the world using it interchangeably with BA.2.75.
Where is it spreading?
BA.2.75 has been spreading quickly since it was detected in India, with reports saying that the subvariant is competing with the more prominent BA.5 as the dominant strain.
Cases have been reported in at least 10 other countries, including the United Kingdom, United States, Australia, Germany and Canada.
The two imported cases reported in Singapore caught the virus after travelling to India. Both immediately self-isolated after testing positive for COVID-19 and have since recovered, the Ministry of Health said.
Is it more transmissible or severe?
While there are some indications that BA.2.75 could be more transmissible or associated with more severe disease compared to its Omicron predecessors, the evidence is weak or has not been assessed yet.
As such, the European Centre for Disease Prevention and Control designated the BA.2.75 variant as a "variant under monitoring" on Jul 7.
According to statistics provided by the GISAID database and A*STAR, BA.2.75 has a total of nine new mutations to its spike protein compared to its parent BA.2. This refers to the part of the virus that attaches to receptors on human cells.
“Although there are a few more spike changes compared to the current dominant and other variants, they are still from the same lineage family,” said Dr Sebastian Maurer-Stroh, executive director of A*STAR’s Bioinformatics Institute.
“Appearance of new lineages with this amount of mutations are expected as the virus adapts to evade immune responses.”
In a tweet last month, the Bloom Lab at the Fred Hutch research institute said the variant was worth tracking due to “appreciable antigenic change” compared to its parent, BA.2. In particular, it flagged two mutations - G446S and R493Q.
The former has the potential to help the virus escape from antibodies elicited by current vaccines that still are effective against BA.2 while the latter, R493Q, seems to increase the viruses’ ability to attach to human cells.
“If (the mutations) make the (spike protein) stickier, then that would mean you need less virus to cause an infection, which makes it more transmissible,” said Professor Dale Fisher, a senior consultant at the National University Hospital’s Division of Infectious Diseases.
Based on figures from the Worldometer website, infectious diseases expert Paul Tambyah observed that the subvariant did not have much of an impact on the number of cases and deaths in India.
“There is no indication based on the data out of India (which has the capacity to test as evidenced by their reporting of the earlier delta and omicron waves) to suggest significantly increased transmissibility although that is what one would expect at this stage of the pandemic,” said Prof Tambyah in an email interview with CNA.
Drawing parallels to the 1918 Spanish flu, which slowly mutated into the most common form of seasonal influenza from 1920 to 1957, he said: “Although the two viruses are from different families, it is about the time for the SARS CoV2 to have adapted to be very highly transmissible and much less virulent.”
Will vaccines and boosters provide protection?
With current vaccines and boosters mainly based on the original strain of the SARS-CoV-2 virus that emerged in Wuhan in late 2019, there is a concern that the large number of mutations to the new variant’s spike protein could help it to break through immune system defences and infect more people.
Despite this, experts say vaccines still offer strong protection against severe COVID-19 and death.
“We need to firstly monitor this new sublineage to see if it is going to have an advantage and see if it does become more dominant in terms of the number of cases and one of the explanations for that could be escaping immunity,” said Prof Fisher.
“We saw BA.4 and BA.5 escape and become more infectious because it could resist some of the immune response but ... it only caused more cases, it didn't cause more severe disease,” he added.
Prof Tambyah said some kind of “hybrid immunity”, which involves getting vaccinated and then getting a mild infection, would also provide the best long-term protection.
“BA.2.75 is also derived from BA.2 so it is possible that prior infection may continue to provide hybrid immunity. The reality is that these viruses are very hard to avoid,” he said.