A more transmissible subvariant of the coronavirus called Arcturus is being blamed for India’s surge in Covid-19 cases.
It appears to spread even more easily between people than earlier forms of the virus and may be able to evade immune protection from vaccination or previous infection.
Here we look at Arcturus and consider what threats it might pose.
What is Arcturus?
Arcturus, which also has the name XBB.1.16, is a subvariant of Omicron which by the end of last month had become the dominant form of the coronavirus in India, although it has also been recorded in other countries.
It closely resembles Kraken, another Omicron subvariant that has outcompeted other strains of the virus in many other parts of the world.
Arcturus has two mutations in the virus spike protein, the part that binds to human cells, that distinguish it from Kraken. In a recent paper, Japanese researchers calculated that it spreads 1.17 times more quickly.
Scientists say Arcturus’s ability to cause more cases than Kraken is either because it is not recognised as easily by the immune system or because the mutations it contains "may contribute to increased viral growth efficiency".
Because of this rapid growth rate, researchers suggest "that XBB.1.16 will spread worldwide in the near future", as the new subvariant is likely to be able to spread faster than other existing forms of the virus.
This has been the pattern since the coronavirus emerged more than three years ago: new, more easily transmissible forms have displaced others that do not pass between people quite as readily.
Is Arcturus able to infect people who have already been vaccinated?
Laboratory experiments in Japan indicate that Arcturus, like Kraken and some other subvariants that have emerged, is "robustly resistant" to a variety of antibodies against the virus.
This suggests that Arcturus is more likely than some other forms of the virus to infect people who have been vaccinated or who have previously had Covid-19. These are known as "breakthrough infections".
However, Ian Jones, a professor of virology at the University of Reading in the UK, said the data indicated that people who have had a relatively recent infection, perhaps in the past few months with one of the other recent subvariants, were likely to be reasonably well protected.
Like some other subvariants that have emerged during the course of the pandemic, Arcturus also tends to resist the effects of most monoclonal antibody treatments.
Monoclonal antibodies consist of large numbers of identical lab-made antibodies and they can be given to patients who are seriously ill to aid their immune system’s fight against the virus.
While this is a concern, the Japanese scientists indicate that Arcturus’ ability to evade the immune response is comparable to that of Kraken.
Prof Jones said that while the virus was continuing to evolve and become more transmissible, it did not appear to be becoming more dangerous to those infected.
"Virus evolution will drive it towards transmission," he said. "In this case … there’s no indication that hospitalisation numbers have gone up.
"Transmission doesn’t mean hospitalisation. It may mean people will get a cold but it doesn’t necessarily mean it’s going to put more people into hospital.
"There doesn’t appear to be any more danger from this variant than from the previous variants."
He warned that people who are more at risk in the event of being infected, such as because of other medical conditions, may face increased danger if case numbers grow.
"The risk of a global spread or big spread is that it will find the small percentage of people who are vulnerable. These people may be at higher risk," he said.
What problems is it causing in India?
India has seen a significant rise in cases in recent weeks and the spread of Arcturus is thought to be behind this.
The country has now broken through the 10,000 cases a day barrier, a significant number but still well down on previous peaks. But numbers are rising rapidly — India reported 5,676 new coronavirus cases and 21 deaths in 24 hours, according to Union Health Ministry data released on Tuesday.
As a result of the surge in cases, hospitals have held drills to ensure equipment needed to treat Covid-19 patients is working properly. Some states in India have reintroduced requirements to wear masks.
There is some indication that children infected with Arcturus show certain symptoms not typically seen with other forms of the virus.
Dr Vipin Vashishtha, a paediatrician in India who has been following the emergence of new variants closely, reported on Twitter that some children with Covid-19 now have itchy conjunctivitis, a symptom not seen until now.
While much attention is focused on Arcturus, there are other emerging forms of the virus that are also sparking interest.
One is called a Deltacron (XBC.1.6) variant because it has been created by the sharing of genetic material between Delta and Omicron variants. It is becoming more common in countries, including Australia.
