Coronavirus | Why must COVID-19 clusters be identified?

Can infected individuals be tracked and quarantined quickly with the help of contact tracing?

Updated - December 03, 2021 06:37 am IST

Published - April 05, 2020 12:02 am IST

The Nizamuddin area, where around 200 persons with coronavirus symptoms were found and taken to different hospitals, in New Delhi on March 30, 2020.

The Nizamuddin area, where around 200 persons with coronavirus symptoms were found and taken to different hospitals, in New Delhi on March 30, 2020.

The story so far: On March 30, the Delhi Police cordoned off the area around a masjid located around 100 metres from the Nizamuddin dargah. Around 9,000 people from across the country, Indonesia, Malaysia, Thailand and Saudi Arabia had attended a gathering at the Alami Markaz Banglewali Masjid, the headquarters of the Tablighi Jamaat , in the Nizamuddin area of Delhi in March. Many returned home, and the government was alerted about a disease cluster when cases of COVID-19 surfaced in several States in people who had attended or were linked to the Nizamuddin gathering. Tamil Nadu, for instance, has identified all 1,103 people (from the State) who attended the conference in Delhi, helped by cooperation of the participants themselves. Several State governments have struggled to identify the participants as some of them switched off their phones or have simply gone off the radar.

What is a disease cluster?

A disease cluster is defined as “an aggregation of cases in an identifiable subpopulation.” Dr. Daniel Wartenberg, who spent more than a quarter century investigating such clusters in the United States, coined this definition in a research paper he wrote for the Journal of the Royal Statistical Society .


The word ‘cases’ in the definition stands for people with similar symptoms or a medical condition and ‘subpopulation’ points to those who share or belong to the same space, time, family, workplaces, etc.

Reports show that the novel coronavirus can travel about six feet from a diseased person and cause infection between two and four individuals. Thus, when people congregate in a place — typically for worship/shopping/commute — the chance of a disease spread multiplies, resulting in a cluster of cases. The size of a disease cluster could vary widely from just four cases to as many as 5,000 depending on the place visited by the infected individuals.

All patients who belong to a disease cluster need not have shared space and time. For instance, a person who picks up the infection from a co-passenger during a flight may infect a taxi driver who picks him up from the airport. Such secondary transmissions also belong to the same cluster with the primary source being the passenger. The cluster keeps growing as the driver could infect a family member, the passenger could infect a nurse in a clinic after developing symptoms and so on.

How are clusters identified?

Health workers often stumble upon clusters accidentally. In a paper published in The Lancet journal by the Singapore 2019 Novel Coronavirus Outbreak Research Team, the process of discovering clusters is described in detail ( see graphic ).

Plotting the cluster

Part A of the graphic maps the first set of patients who were infected at a conference in Singapore between January 20 and 22.

Part A: Patients infected at a Singapore conference


Part B shows the activity trails of their primary contacts during the conference and the secondary contacts outside the conference

Part B: Activity trails of primary and secondary contacts


(If graphs don't appear in full, click here )

The data and the graph have been sourced from the Lancet journal. Rachael Pung, Calvin J Chiew, et al. Investigation of three clusters of COVID-19 in Singapore: implications for surveillance and response measures. Lancet 2020; 395: 1039-46.

The graphic shows one such cluster of cases which originated in a company conference in Singapore. The conference that happened between January 20 and 22 was attended by 111 participants from 19 countries and at least one of them was from Wuhan, China, the epicentre of the novel coronavirus outbreak. On February 4, Malaysia declared a person who had been to the conference as a COVID-19 case. Singapore authorities were alerted. The health workers contacted the other participants and quarantined them. Then they mapped their secondary and tertiary level of contacts (all those not infected by the primary case) — as shown in Part A of the graphic . Then they were tested and isolated if necessary. Once they found a set of cases among those who attended the conference, it was identified as a cluster. In parallel, the activity trails of the primary infected during the conference and also of others outside the conference were pieced together like a puzzle. Those who had come in contact with the infected during such activities, for example the hotel workers, were called in for testing. This is called contact tracing.

Part B of the graphic shows detailed activity trails of all the primary, secondary and tertiary contacts. Such mapping also helps in calculating the time taken by each individual to show symptoms, or to be declared as suffering from COVID-19, from the time he or she was infected. This information impacts how future cases are handled.

Coronavirus | Interactive map of confirmed coronavirus cases in India

What are the challenges after identifying a cluster?

Discovering a cluster is akin to a fire alarm going off. Every moment wasted will spread the fire further. The faster the infected individuals are identified and quarantined, the lesser the number of future contacts. But this is easier said than done. As of Saturday at least 1,023 positive cases in 17 States/Union Territories have been linked to the recently discovered Nizamuddin cluster in India. The cluster which was discovered in late March originated in a religious congregation in the Nizamuddin area of New Delhi. As of April 2, 9,000 people linked to the event which happened in mid-March have been traced, according to the Joint Secretary, Health Ministry, Lav Agarwal. Close to 1,300 of them were foreigners, he said. In such large clusters, identifying the participants will be challenging. Reports from Tamil Nadu show that the attendees came forward in numbers after the State’s Chief Minister issued an appeal. On Friday, Maharashtra Health Minister Rajesh Tope said that around 1,400 people from the State, who had attended the Delhi event, had been traced. Mr. Tope said these people are being isolated or quarantined by the district administrations.

Sources said Andhra Pradesh is tracing the digital footprints of those who tested positive for COVID-19. As those who were infected respond differently in each State, a common protocol cannot be followed and this has pushed the States to adopt various uncharted methods making the process challenging.

Coronavirus | State-wise tracker for coronavirus cases, deaths and testing rates |   Helpline numbers

Why must the activity trail be followed?

A group of attendees took an early morning flight from Delhi on March 24 and landed in Port Blair, Andaman & Nicobar Islands. Another group boarded a train to Erode, Tamil Nadu. One of the attendees, Navi Mumbai’s index patient, had visited Noor Masjid in Vashi , Maharashtra. This led to a secondary transmission to six people, including the secretary of the mosque, his friend, son, grandson and maid and another person who was present at the mosque. This shows that the attendees fanned out to various locations after the conference and engaged in a variety of activities. This leads to the possibility of a wide range of secondary and tertiary transmissions across many States of India.

Coronavirus Who are most likely to get affected by COVID-19?

How have clusters functioned in other countries?

Almost all countries have discovered a large cluster and in most of them, the number of COVID-19 cases shot up after identification of the infected and contact tracing began. As of April 4, South Korea has mapped 83% of its cases to some clusters. More than 51% of the country’s cases originated from the Shincheonji Church of Jesus. And most of the infections were allegedly brought to the church by a 61-year-old woman who ignored her symptoms and attended the church.

In Austria, the Ischgl Ski Resort is said to have been linked to 600 cases. A funeral in George Town, Albany, U.S. may have resulted in more than 600 infections. The Osaka Live Music Venue in Japan is directly linked to 80 cases. A dinner party in Singapore is linked to 43 cases.


0 / 0
Sign in to unlock member-only benefits!
  • Access 10 free stories every month
  • Save stories to read later
  • Access to comment on every story
  • Sign-up/manage your newsletter subscriptions with a single click
  • Get notified by email for early access to discounts & offers on our products
Sign in


Comments have to be in English, and in full sentences. They cannot be abusive or personal. Please abide by our community guidelines for posting your comments.

We have migrated to a new commenting platform. If you are already a registered user of The Hindu and logged in, you may continue to engage with our articles. If you do not have an account please register and login to post comments. Users can access their older comments by logging into their accounts on Vuukle.