Super-spreaders
Andy Chi
In the spread of an infectious disease, it has been observed that a minority of infected individuals may infect a disproportionately larger number of contacts than other infected individuals in what is termed a ‘super - spreading event’ (SSE) when compared t o the specific reproduction number (R number) for the disease, which gives an average number of people that an infected individual will likely pass the pathogen or infectious agent to. Infected individuals of this characteristic are referred to as ‘super - s preaders’. Throughout history, such super -spreaders have been documented to heavily contribute to the spread of an infectious disease within communities, an early example being ‘Typhoid Mary’: in 1907, Mary Mallon was an asymptomatic carrier of the bacteria Salmonella typhi and responsible for infecting many with typhoid fever in New York (Marineli et al., 2013). Recent virus outbreaks such as the 2003 Severe Acute Respiratory Syndrome coronavirus (SARS-CoV) and 2015 Middle East Respiratory Syndrome coronavirus (MERS-CoV) outbreaks are also believed to have been driven by super-spreaders (G. Wong et al., 2015). This essay, focusing on epidemics in human populations, will look at factors that influence why some individuals become super-spreaders, and the effects of super- spreaders during an epidemic. Humans show transmission heterogeneity among different infected individuals. The reasons for these differences in disease transmission may explain the emergence of super-spreaders. Some may be explained by behavioural differences: one outstanding factor is that if the infected individual has, for differing reasons such as work or environment, somehow come into close contact with a large number of people. To account for the vast array of infectious diseases and modes of transmissions of pathogens yet to be understood, this assumes in general that the pathogen is transmitted when in close proximity with an infected individual, or that surfaces touched by such individuals remain infectious (social exposure). A number of SSEs of this nature have been documented. During the aforementioned 1907 New York typhoid fever epidemic, Mary Mallon worked as a cook in a number of different households and as such is believed to have infected well over 50 residents and workers. 1 At the start of the SARS- CoV outbreak in southeast China during early 2003, a physician who had likely contracted SARS-CoV while treating SARS patients in Guangzhou, travelled and stayed overnight at the Metropole Hotel in Hong Kong. He fell ill with SARS and died a few days later after being admitted into hospital. 13 other hotel guests staying on the same floor soon tested positive for SARS, 2 but not before some of these guests had already travelled overseas; notably, one of the imported cases in Canada resulted in over 100 consequent cases at a Toronto hospital. 3 One suggested explanation for this SSE, after a sweep of the hotel conducted by a World Health Organization investigative team at the hotel finding high levels of the virus on the hallway floor, was that the sick physician had vomited on the carpet of the hallway outside his room, perhaps felt embarrassed and so cleaned it up himself without notifying hotel staff, but as a result exposed many of the hotel guests on the same floor to the virus. 4 Another SSE of note during the same SARS-CoV outbreak occurred at the Amoy Gardens in Hong Kong, a high-rise
1 Marineli et al. 2013. 2 Wong et al. 2015. 3 Braden et al. 2013. 4 Normile 2013.
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