Hong Kong is “very likely” to tighten quarantine measures as health experts will discuss the issue on Monday, a government pandemic adviser revealed after a woman who completed her seven-day isolation period was later found to be carrying a more infectious coronavirus variant.

The 38-year-old domestic helper, who works in Stanley and carried the L452R mutant strain, was one of two new imported cases confirmed on Saturday.

Both patients, who arrived separately from the United States, were fully vaccinated with the German-made BioNTech jab and asymptomatic. The L452R mutation is linked to several variants, including the highly infectious Delta version.

A scientific committee under the Centre for Health Protection will on Monday study various data such as the number of imported cases involving the Delta variant and L452R strain in recent months and make recommendations on quarantine policies, according to Professor David Hui Shu-cheong.

“They are very likely to be tightened,” Hui said on Saturday.

The government recently revamped its anti-pandemic rules and from last Monday started using a streamlined process to assess the Covid-19 risk countries posed, allowing more fully vaccinated residents stranded overseas to return home along with foreign domestic helpers and businesspeople waiting to come in.

But Hui warned that the helper’s case showed the seven-day isolation period was not enough, suggesting quarantine stays for fully vaccinated travellers from medium-risk countries should be extended to two weeks even when they obtained positive antibody test results.

The policy currently allows such travellers to quarantine at a designated hotel for just seven days.

Hui said at least 11 imported cases over the past two weeks were linked to the US, so authorities might need to consider moving it to the list of high-risk countries from the existing medium-risk category.

The helper – who Hui said had a high viral load – tested negative for the coronavirus and positive for antibodies when she arrived at the airport from the US on August 1.

After completing quarantine, she travelled to several places in Wan Chai, Causeway Bay and Stanley before her infection was identified on Thursday through compulsory testing on day 12 of her return.

The other imported infection involved a 56-year-old man, bringing the city’s overall tally to 12,032 cases, with 212 related deaths. Fewer than five preliminary-positive cases were reported.

While Hui said the helper’s case could pose a risk of a community outbreak, he pointed out that previous community-detected infections over the past few months had not triggered such a spread.

“Hongkongers are accustomed to wearing masks and frequently wash their hands, so it seems these helped to prevent an outbreak in the community in the previous cases,” he said. “I hope it will be the same this time.”

Respiratory medicine specialist Dr Leung Chi-chiu said the new community-detected infection showed the city had failed to guard against imported cases, urging health officers to implement contact-tracing work quickly.

Leung said vaccination could not be regarded as the main tool to stem the spread of the virus after the emergence of the Delta variant. He said the efficacy rate of vaccines would also drop over time.

“When you only require people to quarantine for the first week of a 14-day incubation period but not the second week, it is equivalent to guarding the left side of the goal and allowing people to freely shoot the ball into the right side,” he said.

Leung said authorities should tighten quarantine measures and apply them to those who were undergoing seven-day isolation right now.

Separately, Professor Wallace Lau Chak-sing, convenor of an advisory panel on the jabs, conceded administrative issues with the Hospital Authority’s new vaccine allergy assessment service had resulted in overly long waiting times for appointments.

The authority announced plans earlier this week to expand the programme across the city after a “rapid and continuous surge” in interest was observed at Grantham Hospital, where the scheme – designed to determine if people are suitable candidates for Covid-19 vaccines – was initiated.

The authority revealed there was a backlog of about 4,000 people awaiting appointments, with some residents reportedly saying they were told no slots were available until 2029.

Speaking on a radio show, Lau admitted the programme could be more effectively administered.

“For those who [said they] had to wait for years, we are still calling them up to understand more about their situation and will try to make appointments earlier,” he told a radio show.

University of Hong Kong clinical assistant professor Philip Li Hei, speaking on the same show, sought to allay concerns about potential allergic reactions to the coronavirus vaccines.

Only three types of residents needed to be particularly cautious, he said – those with a suspected reaction to their first vaccine dose, people with a history of anaphylaxis, and those with very serious allergies to various types of food or medicine.