It is listed as a “variant under monitoring” by the World Health Organization, indicating there are genetic changes that could affect virus characteristics and “early signals of growth advantage”, with enhanced surveillance required.

According to the WHO, Arcturus has shown increasing trends globally, though there were no reports of increased severity. A recent laboratory study on Arcturus showed an increased growth rate compared to subvariants XBB and XBB.1.5, but their immune evasion characteristics were similar.

2. Why did Arcturus gain attention?

The WHO said that it was monitoring XBB.1.16 as lab studies showed that it had one additional mutation, which could increase infectivity and pathogenicity, meaning the ability to cause disease.

Dr Maria Van Kerkhove, the global health agency’s technical lead on Covid-19, said last month that XBB.1.16 “has potential changes that we need to keep a good eye out on”.

While medical experts described the subvariant as highly transmissive, it had not yet caused more severe cases, according to real-world data.

Since the emergence of Arcturus, India has experienced a rise in Covid-19 cases, which were believed to be fuelled by the subvariant. The increase forced India to bring back the mask mandate in public places across some parts of the country.

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The subvariant has also made up 7.2 per cent of the Covid-19 samples collected in the United States, contributing to the second most cases after another subvariant, XBB.1.5, according to an update by the Centres for Disease Control and Prevention last week.

But respiratory disease expert Dr Leung Chi-chiu said the current situation in India was less severe than in the past when previous strains of Omicron had triggered an exponential increase in infections.

“The fluctuation in the number of infections and the replacement of variants are natural phenomena,” he said.

Leung said as Covid-19 would not disappear after the pandemic had turned into an endemic, fluctuations would emerge from time to time.

3. How is it different from other subvariants?

Dr Vipin Vashishtha, a paediatrician from India, reported earlier this month that he had been treating more children with Covid-19 carrying the symptoms of high fever, cold and cough, as well as “itchy conjunctivitis with sticky eyes”. He described it as a situation he had not seen in earlier Covid-19 waves.

Hong Kong had not reported any children infected with Arcturus yet, according to Dr Mike Kwan Yat-wah, an honorary clinical associate professor from the University of Hong Kong’s department of paediatrics and adolescent medicine.

The reports of conjunctivitis among children with Arcturus in other countries might be related to the receptor molecules in the conjunctiva that allowed infection of the viral subvariant, he said.

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He warned that healthcare professionals should be aware the virus could be spread through the eyes, such as when contaminated fingers rub them. Eye protection should also be worn when caring for people suspected of carrying Arcturus, he said.

Kwan said that overseas medical literature showed that children infected with the subvariant would display milder symptoms when compared to other Omicron infections.

Further observation would be needed to see whether they would develop croup, an infection of the upper airway that could cause a barking-like cough and inflammation of the brain. He said these two symptoms were the most worrying among children.

“Different variants had different symptoms, but we should not neglect other important symptoms,” Kwan said.

He said the public need not be too worried, and vaccination remained the most effective way of protecting against the Covid-19 infection.

4. How prevalent is Arcturus in Hong Kong and other countries?

The Department of Health revealed on Monday that there were at least seven cases of Arcturus in Hong Kong.

According to the WHO, cases of Arcturus have been reported in at least 29 countries.

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5. What is the Covid-19 situation in Hong Kong?

The Centre for Health Protection on Tuesday warned that local data, including sewage surveillance and cases at public and private clinics, showed there had been a recent uptick in Covid-19 in Hong Kong.

The daily number of new infections identified through polymerase chain reaction tests has also surged from about 100 in early April to 200 since late last week.

The weekly number of severe cases had also increased from around 20 in mid-March to 30 early this month, though the level was still low compared to two months ago when there were nearly 130 such infections.

Additional reporting by Sammy Heung