As the coronavirus spreads across the world, governments have started to identify clusters - multiple confirmed cases that emerge in one place around the same time.

In December, the Huanan Seafood Wholesale Market in Wuhan, a city in the central Chinese province of Hubei, was identified as the site of China’s first cluster, alerting authorities to the new coronavirus, which causes the disease known as Covid-19.

Although Wuhan and its 11 million residents went into lockdown to stop the virus from spreading, more than 90,000 people in at least 74 countries have contracted the pathogen, with more than 3,000 killed.

Clusters have emerged outside China, including towns in Lombardy in northern Italy, and among members of the Shincheonji church in South Korea.

While these clusters can cause panic among the public, associate professor Hsu Li Yang, programme leader for infectious diseases at the National University of Singapore's Saw Swee Hock School of Public Health, said clusters were a “natural part of virtually all outbreaks”.

“These events - which have been termed ‘superspreading events’ - capture the attention of the media and the public. But they generally result from a set of circumstances where an infected person comes into contact with a large number of uninfected persons within a short period of time and passes on the pathogen to them,” Hsu said.

Identifying clusters was an important part of the battle against any infectious disease, he said.

It helped authorities recognise an outbreak, as in the case of the seafood market, and “helps [governments] direct resources towards contact tracing, monitoring or quarantining of close contacts”, he said.

Here, we take a look at some of the major clusters in mainland China, Hong Kong, Singapore, Japan, South Korea, Iran and Italy.

MAINLAND CHINA

Huanan Seafood Wholesale Market

Vendors, delivery staff and neighbours became the first known cluster of the outbreak. The market is near a busy railway station and sold seafood, livestock and wild game. It was shut down by health authorities.

On December 31, Wuhan said 27 cases of an unknown form of pneumonia were diagnosed and linked to the market.

Ai Fen, a doctor from Wuhan Central Hospital, was quoted by China News Weekly as saying that one of the earliest Covid-19 cases at her hospital was a man with a fever who visited on December 16. He was a market deliveryman.

On January 1, the owner of a health clinic close to the market was admitted to Wuhan Central Hospital. Ai suspected that the man caught the virus from the wave of fever patients who attended his clinic.

The origin of the new strain coronavirus has not yet been established.

Hospitals

As early as the end of December, hospital staff in Wuhan began getting sick as supplies such as goggles, surgical masks and gowns started to run short.

There were 2,055 infections among medical workers at 476 hospitals across the country as of February 20, and 88 per cent of those were in Hubei, according to a report from the WHO-China joint mission.

Most of the cases came in the early stages of the Wuhan outbreak, when protective gear was in short supply and staff lacked experience of such infections, the report said.

Caixin news website said that, as of March 1, 26 medical staff had died in the line of duty during the outbreak, and 13 of them were killed by the coronavirus.

In Beijing, at least two hospitals in downtown Xicheng district reported Covid-19 clusters.

Peking University People’s Hospital treated 22 people who had become infected after an elderly patient caught the virus from her daughter and son-in-law.

Fuxing Hospital reported 36 cases on February 20 - eight were medical staff, nine were janitors and 19 were patients and family members, the municipal health commission said.

Prisons

Reports of Covid-19 outbreaks in prisons began at the end of January, when a document leaked from Shilifeng Prison in the eastern Zhejiang province said that a guard surnamed Luo was diagnosed with the virus on January 28.

He had been to Wuhan weeks before, according to website Hongxing News. On February 21, authorities revealed that more than 500 people - most of them inmates - had been infected in five prisons in Hubei, Zhejiang and Shandong provinces.

Rencheng Prison in Jining, Shandong province, and Wuhan Women’s Prison each had more than 200 cases.

The first patient in the outbreak at Rencheng was a guard. He began showing symptoms on February 12 and was confirmed as having the virus the next day.

SINGAPORE
Grand Hyatt Hotel

During a business meeting in January, 109 people gathered at the Grand Hyatt Singapore. Seven of them - two South Koreans, three Singaporeans, one Malaysian and a Briton - later fell ill and tested positive for Covid-19. Of the 109 participants, 94 left Singapore, including the Malaysian man who returned home, after which his sister and mother-in-law were found to have caught the virus.

The Briton left Singapore and visited a ski resort in France with a group of friends, before heading home. He showed no symptoms at the time but contacted British health authorities after learning he had been exposed to a confirmed case. Five of his companions in the French ski resort were later diagnosed with the new coronavirus. Other cases were later linked to the British traveller, including four in Britain and a man from Spain who had travelled to France.

Yong Thai Hang

The Chinese health product shop shut after five people linked to it tested positive for the virus. Yong Thai Hang catered to Chinese tourists and was visited by a group from southern Guangxi Zhuang autonomous region. Two of those cases spread the infection to four other people.

Grace Assembly of God

When members of the Grace Assembly of God church - 23 cases to date - were diagnosed with the disease, Singapore health authorities traced the cluster to two travellers from Wuhan. They had arrived in Singapore in January and visited Life Church and Missions, where six people later tested positive for coronavirus. Two of the Life Church and Missions cases, a married couple, attended a Lunar New Year gathering where a Grace Assembly of God staff caught the virus. It then spread to staff and members of his congregation.

Wizlearn Technologies

The company that provides internet and intranet-based learning was identified as a cluster at the end of February. Seven employees were diagnosed with Covid-19, while seven others linked to one of the employees also tested positive.

HONG KONG
Hotpot dinner

Eleven people who shared a hotpot meal at the Lento Party Room in Kwun Tong were confirmed as infected. They were part of a family gathering of 19 which included two relatives from mainland China. Two close contacts of the family were also confirmed. Soon after the cluster emerged, most restaurants in Hong Kong removed hotpot from their menus.

Fook Wai Ching She Temple

Eleven Covid-19 patients were found to have visited the Fook Wai Ching She Buddhist temple. A total of 18 cases have been linked to the site, including the family members of infected people who visited the temple. Fook Wai Ching She is located in North Point, which was found to have been visited by at least 27 confirmed patients - about a quarter of Hong Kong’s total. Six of the confirmed cases also dined at a North Point seafood restaurant.

JAPAN
Diamond Princess cruise

More than 3,000 people were quarantined aboard the Diamond Princess cruise ship that docked in Yokohama on February 5. An elderly passenger who had disembarked in Hong Kong a week earlier tested positive for coronavirus, triggering the lockdown. At least 600 people on the ship have been diagnosed with the virus and two have died. The decision to keep passengers on the ship was criticised by medical experts, who said the cabins were not equipped for isolation purposes and passengers should have been isolated onshore.

New Year's Taxi Association party boat

Ninety people were on board a traditional yakatabune river boat in Tokyo’s Jonan district, where a party for taxi drivers was held on January 18. A driver in his 70s was found to have contracted the coronavirus. His mother-in-law, in her late 80s, was infected and became Japan’s first coronavirus fatality on February 13. Ten others - including a driver who had taken a fare from passengers from Wuhan - were also found to have caught the infection. It spread to a female employee of the drivers’ association who was not at the party, and a doctor in his 60s who dined with the wife of a taxi driver who had been at the gathering.

Hokkaido three-day trade fair

Authorities suspect a cluster started at the event, which began on February 13. About 700 people took part in the trade fair, and at least 72 infections have been confirmed in the region. Hokkaido has declared a state of emergency and officials were trying to trace people who were in close contact with the visitors. Researchers doing statistical modeling say there could be as many as 940 infections as of last month.

Qom

Authorities confirmed Iran’s first cases with the the death of two elderly patients in the city of Qom - a popular destination for Shiite pilgrims and scholars about 150km (93 miles) south of Tehran - on February 19. Since then, the virus has spread to nearly all the country’s provinces, with the number of cases soaring to more than 3,500 as of March 6, including over 100 deaths. Iran’s vice-president of women’s affairs, its deputy health minister, and at least 20 members of parliament were among those to test positive for the virus. The rapid spike, after repeated government denials that the virus was present in Iran, sparked widespread distrust of the official figures and authorities’ handling of the outbreak.

SOUTH KOREA
Shincheonji church

From dozens of confirmed cases in mid-February, South Korea watched as thousands emerged in a matter of days when the virus spread among members of a controversial Christian sect in the southeastern city of Daegu. By March 3, South Korea had confirmed more than 4,800 cases, about 60 per cent of those traced to Shincheonji, whose founder Lee Man-hee is seen by followers as the Second Coming of Jesus Christ. The cases that have been linked to the sect include a cluster at the psychiatric ward of a hospital in Cheongdo county, near Daegu. All but two of 103 patients on the ward were infected and seven have died. As of March 6, South Korea confirmed nearly 6,300 cases, including 42 deaths.

Shincheonji faced a backlash in South Korea after reports that a 61-year-old member who became a “super spreader” of the virus had refused to be tested despite showing symptoms. Hundreds of other followers have been incommunicado as authorities try to track and test the congregation.

ITALY
Lombardy and Veneto regions

The outbreak in northern Italy emerged about 60km (37 miles) from Italy’s financial capital, Milan, in late February. Authorities identified a cluster of towns in Lombardy and a smaller zone in neighbouring Veneto as the epicentre and placed 50,000 inhabitants under quarantine.

Lombardy’s regional government urged the elderly to remain at home, because they were by far the most vulnerable to the contagion.

“All those who have died [in Lombardy] are people over 65 with underlying health conditions, especially cardiovascular problems,” said Giulio Gallera, the region’s councillor responsible for welfare policy.

Last week, Lombardy said that on average each patient was infecting two others. It said that in the worst-affected area, about 4 per cent of the population had contracted the disease.

In total, more than 100 people in Italy have died of the disease.

Rome

A police officer and firefighter tested positive for coronavirus in the Italian capital on March 2. The Lazzaro Spallanzani National Institute for Infectious Diseases, where the policeman was being treated, said that his wife, two sons and sister-in-law were also infected.

The school attended by a son in nearby Pomezia, a town of about 60,000 people, was closed. Lessons were suspended at Sapienza University of Rome, where the policeman’s other son is a student.

Rome, which has a population of 3 million people and is Italy’s biggest city, earlier reported that a Chinese couple on holiday and an Italian repatriated from Wuhan had contracted the virus. All three have recovered.