What Is a Superspreader Event?
The FLARE Four
- Communicable diseases like COVID-19 may be characterized by a basic reproductive number, R0 or "R naught," that describes the number of secondary cases expected from a single infected person in a population
- Circumstances may occur, however, in which a single person infects many more people than would be expected from R0. These superspreader events may result from a combination of factors in the infected index case
- Currently available testing techniques do not obviate the need for physical distancing and masking. Even with regular testing, breaches of masking and physical distancing recommendations result in a risk of transmission
- Persons exposed to individuals infected with SARS-CoV-2 should quarantine and seek testing per public health guidance. A negative test during the exposure window does not change the duration and does not exclude the possibility of a later positive result
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Many people are asking...What is a superspreader event?
The primary mode of transmission of SARS-Cov-2 is close contact between persons (Wiersinga et al. 2020). Accordingly, there has been a heavy emphasis on effective non-pharmaceutical interventions such as social distancing (Koo et al. 2020) and mask-wearing (Klompas et al. 2020) to control the pandemic. Breakdowns in physical distancing and gatherings, whether in churches, restaurants or rose gardens, have been associated with significant numbers of infections. Today, we postpone our planned FLARE on open controversies in the management of COVID-19 to discuss the suddenly relevant topic of superspreader events.
A superspreading event is one in which a single infected individual infects an "unusually high" number of secondary cases (Lloyd-Smith et al. 2005); what constitutes unusually high varies by disease, but as an example, tracing of 77 SARS cases in Beijing in 2004 showed that four patients transmitted to eight or more other individuals, seven transmitted to there or fewer contacts, and the remaining patients had no evidence of onward transmission (Shen et al. 2004). Superspreading events have been documented for many infectious diseases. For example, during the 2003 SARS epidemic, based on mathematical modeling, it was estimated that nearly 75% of infections in Hong Kong and Singapore were the result of superspreading events (Li et al. 2004). Further, London researchers have estimated that around 10% of infectious individuals may be responsible for approximately 80% of secondary SARS-CoV-2 transmissions (Endo et al. 2020).
Within the U.S., certain SARS-CoV-2 superspreading events have made media headlines. A corporate conference in Boston at the end of February led to over 90 identified cases in people who attended the conference or their contacts. A preprint describing a phylogenetic analysis of the viral genome in these cases suggests that this event spread the virus throughout the Boston area and even to other domestic and international sites. The conference is thought to have seeded outbreaks among individuals experiencing homelessness (Lemieux et al. 2020) in Boston and is argued to have ultimately given rise to hundreds or, perhaps, thousands of cases, through onward transmission chains.
In March 2020 an individual with cold-like symptoms attended an indoor choir practice in Skagit County, Washington, and became the index case for 52 secondary cases (Hamner et al. 2020).
A wedding in northern Maine in August 2020 has been linked to at least 270 cases and 8 deaths including outbreaks at a nursing home and a jail. Most recently, President Trump and (as of October 14) 31 other people in his inner circle and recent contacts have tested positive for SARS-CoV-2. At least 11 of the infected, including the president, attended a ceremony on September 26 in the White House Rose Garden (followed by indoor receptions) to announce the Supreme Court nominee. As of October 6, the CDC had not been requested to conduct contact tracing of the Rose Garden attendees, so the number of infections traced to the event may be much larger. Reportedly the White House has limited its efforts to notifying—mostly by email—people who were in close contact with Trump in the two days prior to his diagnosis on Thursday, October 1.
What these events had in common are high numbers of people in close contact, indoors, for prolonged periods of time, often unmasked. In the choir practice case, the act of singing may have contributed to transmission, as respiratory droplets can be generated during speech, and is correlated with the loudness of vocalization (Hamner et al. 2020). In Maine, the wedding reception had 65 guests, exceeding the 50 person limit for indoor events. An employee of a rehabilitation and assisted living facility lived in the same household as one of the wedding guests, and that employee was then linked to multiple infections and seven deaths among residents in the facility who did not attend the wedding.
In contrast to these examples of superspreading events involving largely unmasked people in indoor spaces, there are also examples where masking policies have likely mitigated the spread of SARS-CoV-2. In Missouri, two symptomatic hairstylists, who later tested positive for COVID-19, worked with a total of 139 clients, indoors, with appointments ranging from 15-45 minutes in length (Hendrix et al. 2020). Following local ordinance and salon policies, the stylists and clients all reported wearing masks during their appointments. There were no reported secondary cases among the 139 clients, nor with the other stylists working at the salon.
Child care programs in Rhode Island began reopening on June 1, 2020, with reduced enrollments, consistent groups of staff and students, universal use of masks for adults, daily symptom screening and enhanced cleaning and disinfection. By July 31, 666 child care programs had reopened, with a capacity for 18,945 children. Between June 1 and July 31, 52 confirmed or probable COVID-19 cases were identified across 29 child care programs. Only nine programs reported more than one case, and based on the timing of symptom onset, child care-related transmission was ruled out for five of those programs (Link-Gelles et al. 2020). Although it can be difficult to ascertain cases in children, the apparent lack of secondary transmission in 662 child care programs suggests that adherence to guidelines such as masking, maximum class sizes and minimal mixing between student-teacher groups can minimize transmission in child care settings.
How SARS-CoV-2 Spreads: An Aside on Aerosols & Droplets
The existence of superspreading events has occasionally given rise to a discussion of the mode of viral transmission. Do large clusters of cases suggest airborne (as opposed to droplet) spread? We covered modes of transmission and the controversy surrounding the possibility of airborne spread in a prior FLARE (May 31). The majority of SARS-CoV-2 transmission events occur between persons in close contact with each other, consistent with spread via so-called respiratory droplets. However, it is now known (CDC 2020), that there are some instances where infection can be transmitted between individuals who are farther than six feet apart. Since we last addressed the issue of SARS-CoV-2 transmission there has been much discussion of the distinction between airborne and droplet spread and the possibility that SARS-CoV-2 may spread by the aerosol or airborne route. Some of this discussion was prompted by a letter to the WHO by a number of investigators arguing that the possibility exists of airborne spread.
Droplets have been historically defined by CDC as particles > 5 µm in size. Due to their larger size, they are believed to fall to the ground quickly via a ballistic trajectory (Wei and Li 2016). For this reason, diseases transmitted via the droplet route are spread through close person-to-person contact, typically at a distance < 6 feet.
Fine particle aerosols and droplet nuclei, on the other hand, are < 5 µm. This distinction matters for two important reasons:
- Fine particle aerosols may potentially remain suspended in the air
- These respirable particles are also just small enough to be inhaled deeper into the respiratory tract than larger droplets, which typically settle in the upper respiratory tract (CDC 2003)
More sophisticated treatment of the physics surrounding exhaled droplets and aerosols recognizes that such a binary distinction is not realistic—an infected individual likely exhales particles with a broad distribution of sizes (Morawska et al. 2009). The distance traveled by such particles, the likelihood that they result in disease transmission and the time and distance over which they remain potentially infectious is a function not just of the virus, but also of ambient environmental conditions (Xie et al. 2007), ventilation (Lu et al. 2020) and susceptibility of any persons who encounter the particles. That is to say: expelled viral particles contained in droplets/droplet nuclei have to survive in the environment long enough to find a susceptible host; that host must have a portal of entry through which the virus enters; the inoculum must be large enough to cause disease.
So Is It Airborne?
The term "airborne transmission" is generally reserved for infections that are transmitted over large distances or times (Roy and Milton 2004). This is a key distinction: airborne transmission requires more than the demonstration of small particles containing viruses. The significance of airborne transmission can only be established by the frequent occurrence of infections across distance and time. A small number of pathogens—Mycobacterium tuberculosis, varicella-zoster virus, rubeola virus and the measles virus—transmit efficiently over large distances and times. These pathogens are characterized by high attack rates and distinct epidemiology that has, so far, not been observed with SARS-CoV-2 (CDC 2020).
A number of examples have been cited in support of the existence of airborne transmission, including a now well-known outbreak at a restaurant in China (Lu et al. 2020). This example highlights the difficulty of making a rigid distinction between modes of transmission—all infections occurred in three adjacent tables (thus among people in close proximity), none of the wait staff were infected nor were any of the other patrons. Notably, the ventilation in the restaurant in question was relatively poor (0.6 to 0.8 air changes per hour). The same ambiguities apply to the types of spreading events discussed above—they are largely episodes in which people were interacting unmasked and could have been infected in multiple ways, including droplet and contact routes. From the existing epidemiology, it is clear that the primary mode of transmission of SARS-CoV-2 is by close contact and respiratory droplets. It is also clear, however, that transmission can occur over longer times and at distances greater than six feet in favorable circumstances, such as poor ventilation or aerosol-generating medical procedures (Yu et al. 2007).
Why Testing Is Not a Substitute for Masking and Physical Distancing
A number of attempts have been made to try to avoid masking and make unmasked gatherings feel safe by employing SARS-Cov-2 testing. The argument is that if everyone has tested negative within a specific time frame, the need for physical distancing and masking is less. Unfortunately, features of both the virus and the available tests make this strategy risky. Unlike the closely related SARS-CoV-1, in which viral load peaks after several days of symptoms (Peiris et al. 2003), viral load in SARS-CoV-2 peaks at or near symptom onset (Zou et al. 2020), and the virus can be detected by PCR as early as several days before symptom onset (Arons et al. 2020). As a result, it has been estimated that as many as 48% of infections involve transmission from presymptomatic persons (He et al. 2020).
While highly sensitive tests such as polymerase chain reaction (PCR)-based assays can detect the virus in this presymptomatic period, these tests require expert personnel and laboratory resources to run. Results may take hours to days to return (Guglielmi 2020). Moreover, these tests may not always be widely available. Antigen-based tests, such as the Abbott laboratory assay, which recently received an emergency use authorization from the FDA, return results rapidly and do not require laboratory personnel or instrumentation. However, unlike PCR-based assays, which amplify viral nucleic acid and can thus detect very low levels of virus, antigen-based assays depend on the detection of already present viral components and thus their sensitivity is dependent on viral load. The Abbot test, for example, is only approved for use within seven days of symptom onset (when viral load is likely high), and its sensitivity in the presymptomatic period is almost certainly much lower than advertised.
So You Just Found Out You Were in Close Contact with an Individual with COVID-19—What's Next?
It is best to avoid crowded places where it may be difficult to maintain distancing (at least six feet), and masks should be worn to help to reduce the spread of respiratory droplets. If however a person has been in such an environment such as in an SUV, rose garden or golf clubhouse for greater than 15 minutes within six feet with someone who has subsequently tested positive for SARS-CoV-2, then there are actions that they should take.
Current CDC guidelines state that people who have been in close contact for greater than 15 minutes with someone who has COVID-19 should quarantine for 14 days after the last contact with the person and monitor for symptoms, including fever, cough and shortness of breath This 14-day quarantine should be followed even if the exposed individual tests negative within the 14-day window or otherwise feels healthy, as symptoms may appear 2-14 days after virus exposure and testing is imperfect.
Testing is recommended for people with symptoms of COVID-19 or who have had close contact (within six feet for at least 15 minutes) with someone with confirmed COVID-19. Persons who test positive for COVID-19 should isolate for at least 10 days after the onset of symptoms (and 24 hours with no fever, without fever-reducing medications; other symptoms should also be improving, although the loss of taste and smell may persist for weeks). For those who test positive but remain asymptomatic, isolation is 10 days from the positive test. The duration of isolation may be extended in particular circumstances, including underlying immunocompromise or severe illness in the infected individual.
While the length of the current pandemic has led to an understandable desire to resume some activities that we have been avoiding, large indoor gatherings continue to be high risk. This risk is despite the more widespread availability of testing, as SARS-CoV-2 can be spread by asymptomatic people, and some tests may be poorly sensitive in the asymptomatic period. It is also true that while very large events increase the chance of at least one individual present being infected, smaller gatherings in which the basic principles of masking and distancing are not implemented can also result in transmission. Moreover, circumstances such as poor ventilation and lack of masks can substantially increase the risk of transmission. In some cases, single events where such factors have been present have led to a large number of cases and have been termed superspreading events. Tragically, the risk of such superspreading events is not just to the participants but also to the wider community because of their ability to generate multiple secondary transmissions. For the time being, continued consistent attention to mask-wearing and physical distancing is extremely important to protect each other.
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