Epidemiology of SARS-CoV-2 transmission and superspreading in Salt Lake County, Utah, March-May 2020

Abstract

Background: Understanding the drivers of SARS-CoV-2 transmission can inform the development of interventions. We evaluated transmission identified by contact tracing investigations between March-May 2020 in Salt Lake County, Utah, to quantify the impact of this intervention and identify risk factors for transmission.

Methods: RT-PCR positive and untested symptomatic contacts were classified as confirmed and probable secondary case-patients, respectively. We compared the number of case-patients and close contacts generated by different groups, and used logistic regression to evaluate factors associated with transmission.

Results: Data were collected on 184 index case-patients and up to six generations of contacts. Of 1,499 close contacts, 374 (25%) were classified as secondary case-patients. Decreased transmission odds were observed for contacts aged <18 years (OR = 0.55 [95% CI: 0.38-0.79]), versus 18-44 years, and for workplace (OR = 0.36 [95% CI: 0.23-0.55]) and social (OR = 0.44 [95% CI: 0.28-0.66]) contacts, versus household contacts. Higher transmission odds were observed for case-patient's spouses than other household contacts (OR = 2.25 [95% CI: 1.52-3.35]). Compared to index case-patients identified in the community, secondary case-patients identified through contract-tracing generated significantly fewer close contacts and secondary case-patients of their own. Transmission was heterogeneous, with 41% of index case-patients generating 81% of directly-linked secondary case-patients.

Conclusions: Given sufficient resources and complementary public health measures, contact tracing can contain known chains of SARS-CoV-2 transmission. Transmission is associated with age and exposure setting, and can be highly variable, with a few infections generating a disproportionately high share of onward transmission.

Fig 1. Secondary* case-patients generated by index case-patients.

Each pair of bars corresponds to the group of index case-patients who each generated X confirmed or probable secondary case-patients (0 to 6+). Green bars indicate the relative size of each group, as a percent of the total number of index case-patients (n = 184), while the orange bars represent the total percent of secondary case-patients directly generated by each group (n = 286). A plurality of index case-patients (30%, 56/184) were not linked to any secondary case-patients at all, while the small group of index-case patients that generated at least 6 secondary case-patients each (4%, 7/184) produced 16% (47/184) of secondary case-patients in the first contact-tracing generation, a highly disproportionate contribution. More broadly, 81% (233/286) of first-generation secondary case-patients were linked to just 41% (75/184) of index case-patient, demonstrating the unevenness of SARS-CoV-2 transmission and important role of superspreading in this population. *Includes secondary case-patients (confirmed and probable) in the first generation of contact tracing (i.e., individuals directly exposed to one of the 184 index case-patients).