Superspreading Event

A superspreading event (SSEV) is an event in which an infectious disease is spread much more than usual, while an unusually contagious organism infected with a disease is known as a superspreader. In the context of a human-borne illness, a superspreader is an individual who is more likely to infect others, compared with a typical infected person. Such superspreaders are of particular concern in epidemiology.

Some cases of superspreading conform to the 80/20 rule, where approximately 20% of infected individuals are responsible for 80% of transmissions, although superspreading can still be said to occur when superspreaders account for a higher or lower percentage of transmissions. In epidemics with such superspreader events, the majority of individuals infect relatively few secondary contacts.

SSEVs are shaped by multiple factors including a decline in herd immunity, nosocomial infections, virulence, viral load, misdiagnosis, airflow dynamics, immune suppression, and co-infection with another pathogen.

Definition

Although loose definitions of superspreader events exist, some effort has been made at defining what qualifies as a superspreader event (SSEV). Lloyd-Smith et al. (2005) define a protocol to identify a superspreader event as follows:
# estimate the effective reproductive number, ''R'', for the disease and population in question;
# construct a Poisson distribution with mean ''R'', representing the expected range of ''Z'' due to stochasticity without individual variation;
# define an SSEV as any infected person who infects more than ''Z⁽ⁿ⁾ others'', where ''Z⁽ⁿ⁾'' is the nth percentile of the Poisson(''R'') distribution.

This protocol defines a 99th-percentile SSEV as a case which causes more infections than would occur in 99% of infectious histories in a homogeneous population.

During the SARS-CoV-1 2002–2004 SARS outbreak from China, epidemiologists defined a superspreader as an individual with at least eight transmissions of the disease.

Superspreaders may or may not show any symptoms of the disease.

SSEVs can further be classified into 'societal' and 'isolated' events.

In April 2020 Jonathan Kay reported in relation to the COVID-19 pandemic:

Factors in transmission

Superspreaders have been identified who excrete a higher than normal number of pathogens during the time they are infectious. This causes their contacts to be exposed to higher viral/bacterial loads than would be seen in the contacts of non-superspreaders with the same duration of exposure.Kenneth J. Rothman, Sander Greenland, and Timothy L. Lash. ''Modern Epidemiology'', 3rd Edition. 2008. p. 561. Lippincott, Williams & Wilkins. Philadelphia.

Basic reproductive number

The basic reproduction number R₀ is the average number of secondary infections caused by a typical infective person in a totally susceptible population. The basic reproductive number is found by multiplying the average number of contacts by the average probability that a susceptible individual will become infected, which is called the shedding potential.

Individual reproductive number

The individual reproductive number represents the number of secondary infections caused by a specific individual during the time that individual is infectious. Some individuals have significantly higher than average individual reproductive numbers and are known as superspreaders. Through contact tracing, epidemiologists have identified superspreaders in measles, tuberculosis, rubella, monkeypox, smallpox, Ebola hemorrhagic fever and SARS.

Co-infections with other pathogens

Studies have shown that men with HIV who are co-infected with at least one other sexually transmitted disease, such as gonorrhea, hepatitis C, and herpes simplex 2 virus, have a higher HIV shedding rate than men without co-infection. This shedding rate was calculated in men with similar HIV viral loads. Once treatment for the co-infection has been completed, the HIV shedding rate returns to levels comparable to men without co-infection.

Lack of herd immunity

Herd immunity, or herd effect, refers to the indirect protection that immunized community members provide to non-immunized members in preventing the spread of contagious disease. The greater the number of immunized individuals, the less likely an outbreak can occur because there are fewer susceptible contacts. In epidemiology, herd immunity is known as a ''dependent happening'' because it influences transmission over time. As a pathogen that confers immunity to the survivors moves through a susceptible population, the number of susceptible contacts declines. Even if susceptible individuals remain, their contacts are likely to be immunized, preventing any further spread of the infection. The proportion of immune individuals in a population above which a disease may no longer persist is the ''herd immunity threshold''. Its value varies with the virulence of the disease, the efficacy of the vaccine, and the contact parameter for the population. That is not to say that an outbreak can't occur, but it will be limited.

Superspreaders during outbreaks or pandemics

COVID-19 pandemic: 2020–present

Several factors are identified as contributing to superspreading events with COVID-19: closed spaces with poor ventilation, crowds, and close contact settings ("three Cs").

The South Korean spread of confirmed cases of SARS-CoV-2 infection jumped suddenly starting on 19–20 February 2020. On 19 February, the number of confirmed cases increased by 20. On 20 February, 58 or 70 new cases were confirmed, giving a total of 104 confirmed cases, according to the Centers for Disease Control and Prevention Korea (KCDC). According to ''Reuters'', KCDC attributed the sudden jump to 70 cases linked to "Patient 31", who had participated in a gathering in Daegu at the Shincheonji Church of Jesus the Temple of the Tabernacle of the Testimony. On 20 February, the streets of Daegu were empty in reaction to the Shincheonji outbreak. A resident described the reaction, stating "It's like someone dropped a bomb in the middle of the city. It looks like a zombie apocalypse." On 21 February, the first death was reported. According to the mayor of Daegu, the number of suspected cases as of 21 February is 544 among 4,400 examined followers of the church. Later in the outbreak, in May, a 29-year-old man visited several Seoul nightclubs in one night and resulted in accumulated infections of at least 79 other people.

A two-day leadership conference for