13. Super Spreaders

They may sound like superheroes, but they’re more like the villains of these COVIDian times. We’re talking all about superspreaders today!

 

What is a superspreader?

Definition: an unusually contagious organism infected with a disease These are not new! Remember Typhoid Mary?1 Also described in recent pandemics and in other diseases: TB, measles, SARS, MERS, Ebola There is no numerical cutoff for what constitutes a superspreading event (SSE) Large enough to be considered epidemiologically noteworthy

 

Reminder: R0

R0: the basic reproductive number of a disease to how many other people does one infected person transmit the virus The R0 is a dynamic parameter and varies among individuals! Example: R0 on the Diamond Princess cruise ship went from 15 to 2 after strict isolation and quarantine measures were instituted2 The goal of many public health measures is to push the average R0 below 1 – which is a good thing! BUT outliers matter!! In many epidemics, the majority of individuals infect very few secondary contacts; BUT a small number of people have very high numbers of secondary contacts The statistical way of describing this properly is called the fat tail of the degree distribution; the lay term is superspreaders In one mathematical model, over 80% of secondary infections were caused by 10% of infected cases3 This group4 did some fancy and complicated statistical modelling – and came to the important conclusion that “even with an expectation of less than one new case per person, our model shows that exponential growth is still possible”

 

Factors affecting SSE1,5

Virus factors: binding sites, virulence, infectious dose, mode of transmission Interesting opinion piece theorizing that secondary infections caused by a superspreader are more likely themselves to be highly infectious (proposed mechanism due to high viral loads), and result in continued chain of superspreading events6 The author postulates that this may explain some of the heterogeneity of growth seen in different parts of the world Host factors: duration of infection, location and burden of infection, symptoms From previous pandemics: most superspreaders seem to have been symptomatic Environmental factors: population density, public health guidelines, PPE, airflow dynamics, misdiagnosis or delayed diagnosis; inter-hospital transfers Behavioural factors: cough/mask/hand hygiene, adherence to public health guidance Also: activities such as singing, coughing, sneezing, yelling The science of sneezing is incredibly complex, but one fascinating recent paper hypothesizes that large and small droplets may actually travel much further than we think: 7-8m!7 Response factors: timely intervention to prevent growth of outbreaks (both in community and in healthcare settings) – isolation, contact tracing, testing

 

 

SSEs in COVID

Fascinating lay press article8 breaks down the patterns of reported SSEs in COVID so far Most occurred between late February and late March – before a lot of travel restrictions and strict social distancing were in place Many linked to intercontinental travel; and therefore higher among groups with higher socioeconomic status Striking commonality of activities - over 70% involved one of: Religious gatherings Parties or “liquor-fueled mass attendance festivals” including weddings Funerals Conferences and face-to-face business networking Almost all were indoors in crowded, socially intimate settings High noise level seems to be a common feature – forcing close-range conversation or raised voices Interesting individual examples Skagit choir example9 61 individuals attended a 2.5hr choir practice with 1 symptomatic index patient Secondary cases: 32 confirmed, 20 probable – secondary attack rate 53-87% (!) Interesting because it took place under some precautions (some physical distancing of members), yet still resulted in a very high secondary attack rate Prolonged singing with high velocity transmission of respiratory droplets in an enclosed space The case of a symptomatic New Zealand flight attendant who spread infection primarily at a wedding reception he attended after a flight, NOT to passengers on the aircraft10 Seems to highlight social intimacy as a key in transmission, rather than shared airspace Small community of Gangelt in Germany had an SSE originating from a town carnival; 15% of the town ended up becoming infected, with 22% of these being asymptomatic11 One paper12 looked at “mini SSEs” – shared meals where one index patient spread to multiple secondary contacts. From their data set, they found a secondary attack rate of 35% among close contacts sharing a meal with an index case patient Religious gathering in Malaysia: SSE associated with attendees at the 4-day Tablighi Jama’at festival. Over 50% of the cases in Malaysia to date are linked to this single event13 Après-ski: SSE in Austria of 36 cases, where the barkeeper was later confirmed to be SARS-CoV-2 positive14 Nosocomial SSEs: case of an index patient who repeatedly visited a family member in hospital. The family member, as well as the other 3 people in the 4-person room, all became infected; as did 4 physicians and 1 nurse who had directed contact with the index patient. These secondary cases then went on to be the source of 11 tertiary cases15 Notable LACK of examples: movie theaters, symphony/opera, college lecture halls

 

 

 What SSEs tell us about mode of transmission

Analysis of SSEs can give clues as to the dominant mode of transmission: large ballistic droplets, persistent concentrations of tiny airborne particles, or fomite transmission? From the documented SSEs, we see that almost all involve close contact, social intimacy, raised voices – people getting really up close and personal This strongly supports large droplets as the primary mode of transmission This is hugely important as it will affect what public health measures to emphasize, and which ones may actually be causing harm For example: case cluster in China of a restaurant operating with air conditioning – which would theoretically help decrease transmission if the virus were predominantly airborne. However, the authors of this paper actually found that the direction of airflow in the restaurant was crucial – being downstream from the index infected case was the strongest predictor of secondary infection16

 

 

What this means going forward

Limiting SSEs is KEY à without this, other well-intentioned public health interventions may be of little value “As most infected individuals do not contribute to the expansion of an epidemic, the effective reproduction number could be drastically reduced by preventing relatively rare superspreading events”3 Controlling SSEs Early recognition is paramount Study in MERS showed that timely intervention (within 1 week) reduced size and duration of MERS transmission17 Rapid testing and contact tracing Universal precautions in healthcare areas Analysis of SSE can help us re-open as we determine which activities are the highest risk The highest risk activities seem to involve indoor spaces (primarily), close physical contact and social intimacy, multiple social interactions Things that may have to wait until a vaccine: traditional weddings, parties, conferences, funerals, networking events, clubs Activities that seem to pose less risk as may be safer to institute sooner: Movie theaters, the symphony or the opera: quiet attendance, many people in a shared space but limited social interaction Outdoor recreation activities like camping, hiking, swimming, tennis Playgrounds? Fomite spread does not seem significant based on analysis of these events, but there are other factors to consider Bottom line: listen to your local public health guidance!

 

 

 

Sources

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