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SARS-CoV-2 clustering and superspreading in Hong Kong

Natalia Reoutova
03 Jul 2020
Twenty percent of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) cases are responsible for 80 percent of local transmission, according to estimates by the University of Hong Kong (HKU) WHO Collaborating Centre for Infectious Disease Epidemiology and Control.

The epidemiologists used contact tracing data to identify and describe clusters of SARS-CoV-2 infection in Hong Kong and estimate the degree of variance or overdispersion in transmissibility, and therefore the potential for SARS-CoV-2 superspreading events (SSEs). [Research Square 2020, doi: 10.21203/rs.3.rs-29548/v1]

“[As of 28 April 2020,] the majority of SARS-CoV-2 infections [51.9 percent; 539 cases out of 1,038] in Hong Kong have been associated with at least one of 135 known clusters,” reported the study authors. “The largest cluster involved 106 local cases, while the median cluster size was just two cases.”

The largest local cluster was traced back to multiple social exposures among a collection of bars in Lan Kwai Fong, where the infection was spread to additional venues by a number of band musicians. The 73 primary bar cases comprised 39 customers, 20 staff members and 14 musicians, while the remaining 33 infections were secondary, tertiary or quaternary contacts. “This single outbreak accounted for 10.2 percent of all cases in Hong Kong regardless of the source, and for 30.4 percent of all local cases acquired in Hong Kong,” noted the researchers.

SSEs are generally defined as outbreaks in which a small number of cases infect a large number of secondary cases well above the expected average. [Nature 2005;438:293-295] “With the global consensus of R0 for SARS-CoV-2 in the range of 2–3, we defined its superspreading threshold as 6–8 individual secondary cases,” wrote the researchers. “The largest established number of individual secondary cases [in Hong Kong] was 11.”

“Our results indicate 5–7 probable SSEs in Hong Kong and substantial individual heterogeneity in the transmissibility of SARS-CoV-2 infection [k=0.45], and therefore a potential for future superspreading, albeit a lower one than observed with SARS-CoV and Middle East respiratory syndrome coronavirus [MERS-CoV] [k=0.16 and k=0.26, respectively],” wrote the researchers. [Nature 2005;438:355-359; Euro Surveillance 2015;20:pii=21167] “In the absence of non-pharmaceutical interventions such as physical distancing implemented in Hong Kong, the potential for SSEs is likely greater than observed in our study.”

Two more known SSEs occurred at a wedding, with a cluster of 21 linked cases, and at a local temple, where 18 cases were recorded in total. “Our results showed that the number of individual secondary cases was significantly higher within social settings … compared with family or work exposures [p<0.001],” reported the researchers. “Social exposures are therefore associated with an increased risk of SARS-CoV-2 transmission and likely constitute the core behavioural risk factor for SSEs. Targeted interventions should therefore focus on reducing high numbers of social contacts at high-risk venues.”

Most clusters comprised only imported cases (60.7 percent), followed by clusters initiated by an imported case (22.2 percent) and clusters initiated by a local case (17 percent). Among the 499 sporadic cases not linked to any cluster, 90.0 percent of infections were acquired overseas. “Overall, 33 percent of all SARS-CoV-2 infections have been acquired within Hong Kong, either within clusters or as sporadic local cases occurring through limited community transmission that has been controlled through a range of public health measures,” commented the researchers. 
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