Two of Hong Kong’s top infectious disease experts advising the government on the coronavirus pandemic warned on Thursday that the city’s fourth wave of Covid-19 had begun and called for urgent action, after health officials reported more than 30 confirmed and preliminary infections across various districts and sectors.

All non-essential indoor gatherings should be postponed with immediate effect and the public should be prepared to face school closures and to work from home again if caseloads doubled or tripled in the next few days, the experts said.

Health officials did not mince words either in warning of a “rapid and big rebound”, as they confirmed 12 cases and revealed more than 20 preliminary infections. Four of the confirmed cases were locally transmitted, including two without a known source.

Dr Wong Ka-hing, controller of the Centre for Health Protection, said: “Based on the CHP’s information, the situation is increasingly worrying.”

The top CHP official also publicly stated for the first time the government was “comprehensively reviewing” its social-distancing measures.

Among the latest cases were six patrons of a dance club, a taxi driver, a student at a disability centre and a hotel partygoer.

Ecuador and Germany would also be added to the city’s list of high-risk countries, joining 15 others. New arrangements for arrivals from the two countries will come into effect on November 28, meaning travellers need to have a negative Covid-19 test result before coming to the city on top of the current requirement of hotel quarantine for all arrivals from foreign countries.

Dr Chuang Shuk-kwan, head of the CHP’s communicable disease branch, also warned about the situation locally.

“There are now different sources of infection in various districts, suggesting that there are transmissions in the community,” she said. “If there is active transmission, together with a lowered guard on vigilance, there could be an exponential growth in cases. We are very worried.”

She said an infection cluster could be emerging at Starlight Dance Club, on Lockhart Road in Wan Chai, where one confirmed patient – a 75-year-old businesswoman – and five others who tested preliminary positive had visited on or around Saturday.

Initial investigations revealed the premises provided drinks and live music, and group lessons for two to four people per table, while two of its instructors also taught at Heavenly Dance in Sheung Wan, according to Chuang.

“Changing rooms could be a route of transmission … We are still investigating whether they wore masks, but the dancers would be close to each other, so they would also be close contacts,” she said, urging those who visited the club on November 14 to get in touch with the CHP.

One preliminary positive patient had joined an overnight birthday party with about eight people at the Olympian Hong Kong, a hotel in West Kowloon, on November 7. Chuang said the 27-year-old man worked on a construction site in Yuen Long and 12 of his colleagues would have to undergo quarantine. More than 100 would be tested for the virus.

A 37-year-old student with special learning needs and a 73-year-old man who lived with him were also among those who tested preliminary positive. Seven students and other staff members who had used the same room as the 37-year-old at the Fu Hong Society Leung King Adult Training Centre will be quarantined, while more than 40 other students and 20 staff members will be tested.

The facility will also be closed temporarily.

Other preliminary positive cases included a 63-year-old cabby who shared a taxi with three other drivers, a security guard who lived in Sha Tin, two housewives in Mong Kok and North Point, and a foreign national who had been in Hong Kong since January.

Hong Kong’s official Covid-19 tally stood at 5,491 as of Thursday, with 108 related deaths.

The city was hit by its first two waves of infection from late January until June, and a third one from July. While cases fell in September, there have been signs of a rising trend in recent weeks.

Chinese University respiratory medicine specialist Professor David Hui Shu-cheong, a government adviser on the coronavirus, said he believed Hong Kong had entered its fourth wave because of the spike in both overall and untraceable cases, which also happened in the second and third waves.

Hui said the government’s immediate response should be to regulate private venues such as Starlight Dance Club, which might not fall under existing rules.

He said if daily caseloads shot up exponentially in the next few days, harsher measures including school closures and work-from-home arrangements could be needed.

The University of Hong Kong’s Professor Yuen Kwok-yung also said the city had likely entered its fourth wave. “Now we are seeing cases around and they are not limited to one location, and there has been a sudden increase in infections,” Yuen said.

HKU faculty of medicine dean Professor Gabriel Leung went further, and said all non-essential indoor gatherings should be postponed “with immediate effect”.

“Given the extensive and diffuse pattern recently confirmed cases are distributed, Hong Kong would have to be incredibly lucky to dodge an imminent fourth wave,” he said. “We are in a race against time with the virus spreading out of control.”

Professor Keiji Fukuda, head of HKU’s school of public health, took a more cautious tone.

“I don’t think it’s possible to conclude we’ve entered into a fourth wave right now,” he said. “To know this convincingly, we’d need to see a clear, upward trend in new cases each day for at least a few days accompanied by an increase in the reproductive number.”

But he said it was fair to say there was “heightened worry” because of the global worsening situation and a continued mix of imported and locally infected cases in Hong Kong every day.

Meanwhile, more than 10,570 people have applied to return to the city between November 23 and 29 via Return2HK, a travel scheme for those returning from across the border without the need to quarantine for 14 days on arrival.

Online registration for the scheme opened on Wednesday.