Severe Acute Respiratory Syndrome Coronavirus 2 Transmission in Georgia, USA, February 1–July 13, 2020

Yuke Wang; Casey Siesel; Yangping Chen; Ben Lopman; Laura Edison; Michael Thomas; Carly Adams; Max Lau; Peter F.M. Teunis


Emerging Infectious Diseases. 2021;27(10):2578-2587. 

In This Article

Abstract and Introduction


The serial interval and effective reproduction number for coronavirus disease (COVID-19) are heterogenous, varying by demographic characteristics, region, and period. During February 1–July 13, 2020, we identified 4,080 transmission pairs in Georgia, USA, by using contact tracing information from COVID-19 cases reported to the Georgia Department of Public Health. We examined how various transmission characteristics were affected by symptoms, demographics, and period (during shelter-in-place and after subsequent reopening) and estimated the time course of reproduction numbers for all 159 Georgia counties. Transmission varied by time and place but also by persons' sex and race. The mean serial interval decreased from 5.97 days in February–April to 4.40 days in June–July. Younger adults (20–50 years of age) were involved in most transmission events occurring during or after reopening. The shelter-in-place period was not long enough to prevent sustained virus transmission in densely populated urban areas connected by major transportation links.


Coronavirus disease (COVID-19) is an infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). After it was first reported in Wuhan, China, in December 2019, COVID-19 spread rapidly across the world as an ongoing global pandemic. As of July 9, 2021, most confirmed COVID-19 cases (33,792,898 cases) and deaths (606,487) in the world were in the United States,[1] and 906,136 confirmed cases and 18,544 deaths were in the state of Georgia.[2]

Transmission of COVID-19 varies by region,[3,4] setting (long-term care facilities, prisons, and factories),[5] population demographics (age, sex, and race), and even among individual persons (physiologic and behavioral differences).[6] During the early phases of transmission in the United States, new cases were mainly imported by travelers and transmission was associated with human mobility.[7] Local transmission was more intense in regions with high population density and in populations with frequent social contacts.[3,8,9] When SARS-CoV-2 was introduced into high-risk settings (e.g., long-term care facilities), transmission rates were intense, and the outcomes were often fatal.[10]

To study transmission of SARS-CoV-2, we examined the serial interval for symptom onset (defined as the time interval between symptom onset in a primary case-patient and symptom onset in a secondary case-patient infected by the primary case-patient) and the effective reproduction number Rt (the expected number of cases directly caused by any single infectious person). Rt has been shown to vary strongly; some case-patients have caused superspreading events.[11,12] Such heterogeneity influences the spread as well as the control of COVID-19, as documented by studies of nonpharmaceutical interventions in China[13,14] and Europe[15] at the province and country levels.

After the first case of COVID-19 was reported in the state of Georgia on March 2, 2020, a series of events and interventions followed (Appendix Table 1, On April 3, state officials announced a shelter-in-place order, requiring all residents and visitors to remain in their residence and take every possible precaution to limit social interactions. On April 24, officials allowed some businesses to reopen, and on April 30 the shelter-in-place order was lifted. On June 1, state officials further relaxed restrictions. During June–July 2020, as new COVID-19 cases continued to surge in Georgia and other states, knowing how shelter-in-place and the subsequent reopening events affected the transmission of SARS-CoV-2 in different regions became crucial.

Identifying a large number of the primary and secondary case-patient pairs enabled us to estimate the distribution of the serial interval for symptom onset. Using the serial interval distribution, we can estimate the time-varying Rt.[16] With Rts over time, we can study the spatial distribution of transmission across all 159 Georgia counties as well as the effects of shelter-in-place and subsequent gradual reopening.

The Georgia Department of Public Health (GDPH) Institutional Review Board determined that this analysis was exempt from the requirement for review and approval, and informed consent was not required. This activity was reviewed by the Centers for Disease Control and Prevention and was consistent with their applicable policy and with federal law.