Cluster Infection

A phenomenon in which many infected people occur in a concentrated manner within a short period due to the same pathogen at a specific facility or group, with epidemiological investigation and early c

Cluster Infection

Overview

Cluster infection (집단감염, 集團感染) is a phenomenon in which infected persons occur intensively within a short period at a specific facility, group, or region due to the same pathogen, greatly exceeding the usual expected level. Unlike sporadic cases in which individual cases appear scattered, cluster infection is characterized by sharing an epidemiological link, such as the same source of exposure or route of transmission. Accordingly, public health authorities regard cluster infections as a precursor signal of a large-scale epidemic and concentrate prevention capacity on breaking the links through epidemiological investigation.

Main Content

1. Concept and Classification of Stages

The scale of infectious disease outbreaks is generally classified into sporadic occurrence, cluster infection (cluster), epidemic, and pandemic. Cluster infection has temporal and spatial concentration, and epidemiological association between cases is confirmed. An epidemic is the stage in which cluster infections spread throughout the community and continuously exceed the expected incidence level, while a pandemic is the stage in which it spreads simultaneously across multiple continents. In other words, cluster infection often corresponds to the early phase of an epidemic, making early detection extremely valuable.

2. Mechanism of Occurrence and Routes of Transmission

The transmission routes of cluster infections are divided according to pathogen characteristics into droplet transmission, airborne (aerosol) transmission, contact transmission, secretion transmission, vector-borne (mosquito, tick) transmission, and blood/body fluid transmission. In particular, in environments where indoor crowding, insufficient ventilation, and long-term exposure overlap, the risk of airborne transmission rises sharply. The conditions commonly called the "3Cs"—closed spaces, crowded places, and close contact—are representative environmental factors that trigger cluster infections. When groups with low immunity, unvaccinated groups, and groups with poor sanitary environments are added to this, chain infections can occur even through small-scale contact.

3. Vulnerable Environments and Facilities

Representative environments where cluster infections occur include long-term care facilities such as nursing hospitals and nursing homes, correctional facilities, schools and daycare centers, religious facilities, call centers and offices, military bases, ships and cruise ships, hospital emergency rooms and intensive care units, and indoor public-use facilities (karaoke rooms, gyms, restaurants). The common features of these spaces are that many people stay together for long periods in enclosed spaces, high-risk groups are mixed in, and infection surveillance and isolation are structurally difficult. In the case of nursing facilities with many elderly people and patients with underlying diseases, cluster infections can directly lead to high fatality rates, making them priority management targets.

4. Key Epidemiological Indicators

Indicators such as attack rate, secondary attack rate, basic reproduction number (R0), effective reproduction number (Rt), case fatality rate (CFR), incubation period, and serial interval are used in the analysis of cluster infections. The attack rate refers to the proportion of cases among those exposed in a specific group, and the secondary attack rate refers to the proportion of infected people among those who contacted the index patient. If R0 exceeds 1, it is interpreted as a spreading phase; if below 1, as a shrinking phase, and Rt becomes key evidence for judging the effectiveness of prevention measures.

5. Epidemiological Investigation Procedure

Epidemiological investigation is at the center of the response to cluster infections. The general procedure proceeds in the order of ① receiving reports of suspected cases, ② specimen collection and diagnostic confirmation, ③ establishing a case definition, ④ contact tracing and classification, ⑤ assessment of exposure environments and movement routes, ⑥ identification of transmission routes and causes, ⑦ control measures such as isolation and disinfection, and ⑧ evaluation of the effectiveness of measures and feedback. Recently, methods of combining epidemiological investigation data with geographic information systems (GIS) to visualize spatial clusters and confirming viral lineages within the same group through genomic sequencing analysis are becoming standardized.

6. Representative Cases

In South Korea, the MERS cluster infection that spread centered on Pyeongtaek St. Mary's Hospital in 2015 is representative, and close contact within the hospital and the emergency room environment were cited as the main causes of the spread. In 2020, COVID-19 spread rapidly through religious facilities and nursing hospitals in the Daegu and Gyeongbuk regions, causing large-scale cluster infections, and afterward sporadic cluster infections were repeated in various facilities such as call centers, logistics centers, military bases, and ships. In addition, measles, pertussis, tuberculosis, norovirus, influenza, and mpox (monkeypox) in 2022 have been recorded as cluster outbreak cases.

7. Management and Response Strategies

Control of cluster infections consists of two axes: containment and mitigation. Containment strategies include patient isolation, cohort isolation, self-quarantine and active monitoring of contacts, facility closure, and movement restrictions. Mitigation strategies include mask wearing, hand hygiene, periodic ventilation and air purification, vaccination, administration of antiviral drugs, and protection of high-risk groups. In addition, it is emphasized that adjustment of crisis alert levels, securing on-site applicability of prevention guidelines, and building trust among members determine long-term control effectiveness.

Latest Trends

In 2024–2025, after the transition of COVID-19 to endemic status, the paradigm for managing cluster infections is being reorganized into "everyday disease prevention." First, indoor air quality management has been institutionalized, and ventilation and air purification standards have become standing infrastructure for infectious disease response. Second, wastewater-based epidemiology (wastewater surveillance) and genomic surveillance have expanded as key means of early warning systems. Third, with the spread of avian influenza (H5N1) in mammals and the mpox clade Ib outbreak, the importance of zoonotic surveillance at the animal-human interface has grown. Fourth, vaccine-preventable diseases such as measles and pertussis have reemerged along with declining vaccination rates, and managing vaccination blind spots has emerged as a major task. Fifth, discussions on the WHO pandemic treaty, integration of surveillance systems, and introduction of AI-based epidemic forecasting models are underway, and efforts to reorganize the legal and institutional foundations for infectious disease response are continuing.