Omicron subvariants BA.4 and BA.5 have triggered widespread outbreaks around the world in countries such as the United States, South Africa and the United Kingdom, leading to concerns that the more transmissive strains could also affect Hong Kong, which has just recovered from a fifth wave of infections and is about to negotiate reopening the border with mainland China.

Here is what we know about the subvariants so far.

1. Why have subvariants become a concern in Hong Kong?

Since the city began easing social-distancing curbs in May, there has been a rebound in Covid-19 infections, with the numbers rising to more than 1,000 in mid-June, while relaxed restrictions for overseas arrivals has led to an increase in imported cases. The number of local infections passed 3,000 a day on Thursday before dropping a little on Friday.

While Omicron subvariant BA.2 remains the dominant strain in the city, other ones, which are widespread overseas, have found their way into the community. Health authorities have reported dozens of cases per day involving subvariants BA.2.12.1, BA.4 and BA.5.

According to ​​science journal Nature, the three lineages carry mutations that allow immune escape, meaning they can evade protection offered by vaccines and previous infections.

Cases of BA.4 and BA.5 have been surging globally due to their higher transmissibility and ability to infect people who are immune to earlier forms of Omicron and other variants.

A source familiar with the government’s internal assessment told the Post on Tuesday that one of the benchmarks regarding the severity of the city’s pandemic situation was whether BA.4 and BA.5 would become dominant strains in the community.

2. Which subvariants are spreading fast in Hong Kong?

As of Thursday, a total of 561 local and 223 imported cases of BA.2.12.1 were recorded in the city, in addition to 147 and 483 cases respectively of BA. 4 or BA.5.

Three cases of BA.2.12.1 reinfection and a case of BA.4 or BA.5 reinfection were detected as of Wednesday.

“We can see the proportion of BA.4 and BA.5 local cases [among overall daily infections] has been rising. On June 21, the proportion was 0 per cent. It is almost 1 per cent now,” Dr Chuang Shuk-kwan of the Centre for Health Protection said on Wednesday.

“We are also worried about possible reinfections. We will continue to monitor the situation,” she said, adding that she was unsure if the subvariants would be as widespread as they were in other places.

“Whether the situation will be similar to that of overseas, it is difficult for us to predict as it is difficult to find a place that is similar to Hong Kong for comparison.”

Infectious disease expert Professor Ivan Hung Fan-ngai said he believed BA.4 and BA.5 could become the dominant strains in the city but the public need not be concerned.

Infectious disease expert Professor Ivan Hung.

“Four million or more people have been infected with BA.2 in the fifth wave of the pandemic. Studies show that the immunity provided by BA.2 infections is strong enough to fight against BA.4 or BA.5,” he said.

“In addition to the high proportion of vaccination, a hybrid immunity and a natural immunity barrier has been created, which is very effective in preventing outbreaks of BA.4 and BA.5.”

Government pandemic adviser Professor David Hui Shu-cheong said even if the subvariants became dominant, they would not be a great concern.

People getting tested for Covid-19 during the fifth wave outbreak in the city.

“When you have a high vaccination rate, even if people are infected, the healthcare system is able to handle it,” he said. “As long as we continue to boost the vaccination rate for the third dose and help people at high risk receive the fourth dose, we will be very safe.”

But he said there was limited information on whether a previous BA.2 infection would provide better protection from BA.4 and BA.5 compared with BA.1.

3. What are the symptoms? Can the strains cause severe infections?

Hung said symptoms of BA.4 and BA.5 were the same as Omicron BA.1 and BA.2, such as fever, dry cough, fatigue, sore throat, runny nose, headaches and muscle pain. He added that the strains’ fatality rate was similar to influenza.

Hui noted that the symptoms of BA.4 and BA.5 were as mild as those of BA.2 even though the former two had higher transmissibility.

Chuang said authorities had not seen a large number of serious cases caused by the subvariants, adding that there was no huge difference compared with BA.2.2 infections.

4. Will subvariants affect border reopening?

Gilman Siu Kit-hang, associate professor at Polytechnic University’s health, technology and informatics department, said efforts to reopen the border with the mainland would not be hindered by the potential takeover of BA.4 or BA.5.

Arrivals at Hong Kong International Airport.

“China has just recorded a few cases of BA.5 itself in Xian, so the threat is from the entire world but it has still cut quarantine recently to seven days in a government site plus three days at home,” Siu said.

He added that although there was a risk of reinfection, a booster shot or Omicron-targeting vaccines from BioNTech or Moderna, which were likely to be available later this year, could provide more long-lasting protection.

5. How are subvariants affecting other countries and regions?

The mainland city of Xian saw its first BA.5 cases this week. A local infectious disease expert said the strain was the cause of a recent outbreak there.

In mid-May, Singapore found its first three local BA.4 and BA.5 cases, which subsequently triggered a new wave of infections. The Covid-19 caseload reached more than 10,000 this week, with half of the new cases linked to the subvariants.

But the city state’s health minister said the epidemic would not be as severe as its previous BA.2 outbreaks because of stronger immunity in the populace resulting from past infections and booster shots, adding that the uptick in hospitalisations had not overwhelmed its healthcare system.

In the US, BA.4 and BA.5 recently replaced BA.2 as the dominant strains, leading to a surge in infections and making up more than 70 per cent of samples tested by local health authorities. The proportion of BA.2.12.1 has continued to drop in the country.

Hospital admissions also increased as more cases emerged, but there is currently no evidence that cases linked to dominant strains are more severe compared with other subvariants.

A new wave of the pandemic in the UK was recently set off by BA.4 and BA.5, which accounted for most new cases. An estimated 2.3 million people in the country were infected with the virus, a 32 per cent increase from a week earlier.

A health official told local media that Covid-19 hospital cases were expected to rise in the coming weeks and were likely to surpass a peak in April caused by BA.2.